Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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development. However, concentration-dependent ad-

verse effects of many glucosinolate hydrolysis prod-

ucts must be considered: dose–effect relationships are

not clinically proven and tissue bio-availability

remains unknown. It is also unclear if, to what extent,

and in which chemical form, they are absorbed, dis-

tributed, and metabolized, and also whether active

metabolites reach possible target organs. Given these

factors, and the absence of information derived from

preclinical studies or studies involving animal pharma-

cology and toxicology, much work is needed before

any overall recommendations can be offered regarding

their likely beneficial effects on human health.

Antinutritional Effects

0026 In contrast to the above putative beneficial proper-

ties, many glucosinolate-derived hydrolysis products

are considered as natural toxicants, thereby being the

focus of breeding and processing strategies to reduce/

remove them from animal feeding stuffs. These com-

pounds elicit a variety of adverse effects on the

morphology and function of different cells and organs

(See Goitrogens and Antithyroid Compounds) and

antioxidant status. Liver, kidney, thyroid gland, and

pancreas are the main target organs; in rats, toxic

effects were observed with daily doses of 10–

50 mg kg

1

body weight. In high concentration, cer-

tain isothiocyanates and nitriles may initiate muta-

genic, cytotoxic, and carcinogenic processes; thus

untreated juice of certain cruciferous vegetables can

induce genetic mutations in both bacterial and mam-

malian systems and subsequent analysis implicated

glucosinolate breakdown products as the primary

source of mutagenesis. Differences in genotoxicity

amongst varieties of cruciferous vegetables have

been interpreted on the basis of the nature and

amount of individual glucosinolates. Cytotoxicity

and genotoxicity studies using, for example, human

colon adenocarcinoma HT29 cells, have demon-

strated the dose-dependent cytotoxicity of individual

glucosinolate hydrolysis products (diindolylmethane

(DIM) and sulforaphane) at < 1.0 mgml

1

; these

products were, moreover, able to inhibit quiescent

cells from reentering the cell cycle. Although very

effective on undifferentiated HT29 cells, DIM and

sulforaphane do not appear to affect the viability of

differentiated intestinal cells – a significant finding

given that cancer cells are undifferentiated.

Bioavailability

0027 The biological effects of glucosinolate hydrolysis

products depend greatly on their bioavailability,

defined not only by oral dose, but also by their ab-

sorption, distribution, metabolism, and excretion

(ADME) that, together, offer a measure of tissue

bioavailability. Thus, phenylhexylisothiocyanate (a

synthetic product) enhances esophageal cancer

because of its extreme bioavailability (the highly

lipophilic phenylhexenyl group increasing membrane

permeability and, hence, absorption) whereas lower

homologs decrease esophageal cancer (phenylpropyl-

isothiocyanate > *phenylethylisothiocyanate > phe-

nylbutylisothiocyanate > *benzylisothiocyanate: the

asterisk denotes that it is naturally occurring) since

reduced bioavailability leads to optimal tissue levels.

The broad opinion that even low concentrations of

nitriles, arising from glucosinolate breakdown, may

exert toxic effects needs to be reconsidered, since

their individual chemical structures, and subsequent

bioavailabilities, will be key parameters in determin-

ing their mode of action. Thus, 1-cyano-2-hydroxy-3-

butene may be the most active anticarcinogen in

Brussels sprouts, acting via induction of phase II

enzymes.

0028Although chewing will liberate intact glucosino-

lates, it also causes enzymatic hydrolysis; the extent

of these processes has not been investigated. The same

is true for the effect of gastric digestion, including the

binding of intact glucosinolates/hydrolysis products

to peptides, smaller glycoproteins, and other food

components. A substantial proportion of the intact

glucosinolates in the food may, therefore, not be

absorbed in the small intestine and will reach the

colon, to be hydrolyzed by endogenous microflora;

they might, therefore, have potential for delivering

bioactive hydrolysis products directly to the colon

and so be effective against colon tumor development.

In this context, the precise role of microbial thiogly-

cosidases is controversial and needs further investi-

gation.

0029Even if absorption is high, bioavailability may be

limited by rapid and extensive metabolism. The

major form of first-pass metabolism of isothiocya-

nates is conjugation with glutathione (catalyzed

by glutathione-S-transferases), but the site of this

conjugation is unclear. Little is known about the me-

tabolism of nitriles but structural features appear to

influence the formation of cyanide and thiocyanate

ions.

0030Lack of fecal recovery of intact glucosinolates

indicates that extensive metabolism is likely and nu-

merous animal studies have shown that, following

first-pass metabolism, extensive transformation of

glucosinolates and hydrolysis products occurs. The

site of metabolism and the nature of intermediates

remain unclear. Most studies have been unable to

study metabolism quantitatively and 25–65% of me-

tabolites are unidentified. Depending on the animal

species, the major urinary metabolites are N-acetyl

cysteine conjugates (mercapturic acids), thiocyanate

2928 GLUCOSINOLATES

ion, and hippuric acid conjugates. Attempts to

identify primary compounds or metabolites in blood

failed but the feeding of radiolabeled compounds

demonstrated a rapidly occurring peak of radio-

activity in the blood that was both species-specific

and of short half-life.

0031 There is only very limited knowledge on the me-

tabolism of nitriles; some, e.g., benzyl cyanide, re-

lease hydrogen cyanide (HCN) following the action

of a microsomal mixed-function oxidase, probably as

a result of a-methylene hydroxylation to form an

unstable cyanohydrin. The liberated HCN can be

converted to less toxic SCN



by sulfur transferase.

The individual structure of the organonitrile affects

SCN



excretion.

0032 The short-term biological activity of phytochem-

icals (including glucosinolate hydrolysis products) is

relatively weak; they have multiple targets and may

elicit both beneficial and adverse effects. This poses

particular problems in determining the net effect

because plant foods contain a complex mixture of

phytochemicals with a wide range of physiological

effects. One key question to address is: how much

does a particular food component contribute to a

specific health outcome compared with other dietary

constituents? Another is: to what extent does this

affect the disease outcome? The availability of a

robust and reliable surrogate biomarker should pro-

vide answers by facilitating measurement of exposure

and by indicating a positive/negative physiological

effect prior to the final outcome.

Future Research

0033 The need for future research has been identified in the

individual sections above. After a century of glucosi-

nolate study, research into their biological activity

and physiological significance remain in their infancy.

Determination of their ADME and human studies,

following on from animal and cell culture experi-

ments, should be a prime focus of future research.

As yet, there are no validated biomarkers of exposure,

effect, and susceptibility to dietary glucosinolates

that would enable correlation of dose and effect in

human intervention studies. Since presystemic

metabolism affects tissue bioavailability, knowledge

of how individual enzyme status and polymorphism

in human glucosinolate-metabolizing enzymes

affect ADME is vital. Current data are both insuffi-

cient and controversial. Insufficient attention has

been paid to the effect of the food matrix, since food

contains a range of bioactive compounds, nutrients

and non-nutrients, and detailed investigation of

their interactions, competition, and synergism with

one another and with proteins and carbohydrates

will be necessary if an understanding of overall

sensory and physiological characteristics is to be

achieved.

0034A second focus for research is elucidation of the

complex parameter affecting levels of glucosinolates

and hydrolysis products in food. If the biological

effects of glucosinolates are to be exploited for social

and economic benefit through the breeding or genetic

engineering of nutritionally enhanced crops, further

information on their biosynthesis, on the genes in-

volved, and on their function and possible linkages

is needed. Details of the myrosinase-glucosinolate

system is required and factors responsible for direct-

ing hydrolysis toward specific products understood;

recommendations for processing cruciferous vege-

tables to optimize glucosinolate hydrolysis products

may then follow.

0035Finally, understanding of the antimicrobial, insecti-

cidal, and fungicidal properties of glucosinolate hy-

drolysis products will assist targeting of future

breeding programs, facilitate integrated crop protec-

tion, and provide natural options for plant husbandry

in response to consumer concerns.

See also: Cancer: Diet in Cancer Prevention;

Carcinogens: Carcinogenic Substances in Food:

Mechanisms; Chromatography: High-performance

Liquid Chromatography; Functional Foods; Goitrogens

and Antithyroid Compounds

Further Reading

European Commission (1990) Official Journal of the Euro-

pean Community no. L170/33. Community regulation

(ECC no. 1864/90), pp. 27–34.

Fahey JW and Talalay P (1999) Antioxidant functions of

sulphoraphane: a potent inducer of phase II detoxication

enzymes. Food Chemistry and Toxicology 37: 973–979.

Fahey JW, Zalcmann AT and Talalay P (2000) The chemical

diversity and distribution of glucosinolates and isothio-

cyanates among plants. Phytochemistry 56: 5–51.

Fenwick GR, Heaney RK and Mawson R (1989) Glucosi-

nolates. In: Cheeke RR (ed.) Toxicants of Plant Origin.

Vol. II. Glycosides. pp. 1–40. Boca Raton, FL: CRC

Press.

Heaney RK and Fenwick GR (1993) Methods for glucosi-

nolate analysis. Methods in Plant Biochemistry 8:

531–550.

Hecht SS (1999) Chemoprevention of cancer by isothiocya-

nates, modifiers of carcinogen metabolism. Journal of

Nutrition 129: 768S–774S.

Kelloff GJ, Crowell JA, Steele VE et al. (2000) Progress in

cancer chemoprevention: development of diet-derived

chemopreventive agents. Journal of Nutrition 130:

467–471.

Mithen RF, Dekker M, Verkerk R, Rabot S and Johnson IT

(2000) The nutritional significance, biosynthesis and

GLUCOSINOLATES 2929

bioavailability of glucosinolates in human foods. Journal

of the Science of Food and Agriculture 80: 967–984.

Rosa EAS, Heaney RK, Fenwick GR and Portas CAM

(1997) Glucosinolates in crop plants. Horticultural

Reviews 19: 99–215.

Steinmetz KA and Potter JD (1996) Vegetables, fruit, and

cancer prevention: a review. Journal of the American

Dietary Association 96: 1027–1039.

Tawfiq N, Heaney RK, Plumb JA et al. (1995) Dietary

glucosinolates as blocking agents against carcinogenesis:

glucosinolate breakdown products assessed by induction

of quinone reductase activity in murine hepa1c1c7 cells.

Carcinogenesis 16: 1191–1194.

Verhoeven DTH, Verhagen H, Goldbohm RA, Vandenbrandt

PA and VanPoppel G (1997) A review of mechanisms

underlying anticarcinogenicity by brassica vegetables.

Chemico-Biological Interactions 103: 79–129.

Wallsgrove RM, Doughty K and Bennett RN (1999) Glu-

cosinolates. In: Singh BK (ed.) Plant Amino Acids,

pp. 523–561. New York: Marcel Dekker.

Gluten See Wheat: The Crop; Grain Structure of Wheat and Wheat-based Products; Celiac (Coeliac) Disease

GLYCOGEN

M H M Rocha Lea

o, Universidade Federal do Rio de

Janeiro, Rio de Janeiro, Brazil

Introduction

0001 Glycogen is a glucose polysaccharide occurring in

most mammalian and nonmammalian cells, in micro-

organisms, and even in some plants. It is an important

and quickly mobilized source of stored glucose. In

vertebrates it is stored mainly in the liver as a reserve

of glucose for other tissues. In hepatocyte cells it is

accumulated and mobilized according to blood glu-

cose availability and to extrahepatic cells. Glycogen is

also stored in muscles and fat cells. In the muscle it

seems to be mainly used for energy purposes as

metabolic fuel for glucolysis producing glucose 6-

phosphate. Thus, glycogen plays a crucial role as a

systemic and cellular energy source and also as an

energy store. A great number of enzymes and

hormones control the synthesis and degradation

of glycogen. Consequently, stores of human body

glycogen may vary dramatically due to diet, exercise,

and stress.

Structure, Isolation, and Location

0002 The glycogen molecule is formed by (1!4) linked

chains of alpha-d-glucose residues and branched at

intervals of 8–12 glucose residue units, with (1!6)-

alpha-d-linkages which produce a bush-like struc-

ture, called beta-particles. Dimers and trimers of

beta-particles are linked upon a single protein back-

bone known as glycogenin. The backbones of several

fundamental units are joined to form a macro-

molecule named an alpha-particle, of 100–150 nm

diameter. Due to its polymeric nature, glycogen is

osmoticaly inactive and can be stored in large quan-

tities within the cells. The branched glycogen has two

advantages: it increases its solubility and facilitates

the rapid turnover of sugar residues. The latter is due

to exhibition of a greater number of nonreducing

ends for simultaneous enzymatic reactions.

0003Studies on the cellular location of glycogen in

several cell types have led to some different inter-

pretations which are due to different procedures

employed for glycogen extraction (hot potassium hy-

droxide (KOH), cold or hot tricarboxylic acid (TCA),

hot sulfuric acid (H

), and cold or boiling water)

used in different laboratories. In yeast, it is accepted

that there are two pools of glycogen location. One is

water-insoluble or acid-extractable, occurring outside

the cellular membrane. This pool corresponds to the

major fraction of the yeast glycogen. The second is

water-soluble or alkali-extractable, occurring inside

the cellular membrane. In the past some authors

showed the relationship between the location and

their metabolic compartmentalization. However if

the two pools have distinct roles in yeast metabolism

this remains to be proven.

0004Recently, an original glycogen structure surround-

ing the finger-like plasma membrane invaginations

in a ring was discovered and visualized by electron

microscopy. In tissues, the glycogen is associated with

2–4 times its weight with proteins by adsorption

2930 GLYCOGEN

mechanism. These proteins appear to be mainly the

enzymes of its own metabolism. Because glycogen is

intimately associated with protein in the cell and the

protein–glycogen complex forms an independent

functional unit, it is considered as a dynamic

organelle called glycosome without a membranous

envelope and morphologically similar to ribosome.

0005 Both types of supramolecular structure have pro-

tein particles and both are attached to endoplasmic

reticulum membranes and cyto-skeleton components.

Glycosomes may occur free in cytosol (lyoglicosomes)

and are acid-labile, whereas the glycosomes associ-

ated with cellular structures (desmoglycosomes) are

acid-resistant. Although glycogen structure has been

studied after isolation using different mild extraction

(cold water, cold TCA, or enzymes to lysis cell walls),

or by direct observations by means of electron micro-

scopy, the results from the literature are sometimes

controversial and this matter is still debated. The

different methodologies used by different groups of

researchers have led to different results inducing

sometimes different interpretations. Nowadays it is

hypothesized that there are two forms of physio-

logical glycogen which differ in the proportion of

protein relative to carbohydrate and location in the

cells. One form corresponds to protein-rich, and it is

acid-insoluble, while another is protein-poor and is

acid-soluble. The former is called desmoglycosomes

and the latter is called lyoglycosomes. Progress for

establishing the relationship between glycosomes

(lyo- and desmo-) and glycogen metabolism is

expected.

Function

0006 Glycogen metabolism is strongly related to glucose

metabolism despite the cell type or the organism

where it is found. On the other hand, fats are the

major source of energy in the human body but they

are only mobilized during prolonged periods of star-

vation from adipose tissue by lipolysis. The fatty acids

produced are degraded to acetyl Coenzyme A (CoA)

in the liver via the beta-oxidation pathway. Under

these conditions the amounts of acetyl CoA are very

high and exceed the degradative capacity of the citric

acid cycle. Thus, acetyl CoA is used as a substrate to

form ketone bodies which are utilized by all body

organs as an energy source under starvation. So, in

the human liver, fatty acids are not transformed into

blood glucose. If starvation is very prolonged,

proteins are also used as an energy source.

0007 Glycogen as an energy reserve stored in the liver is

low when compared to fat. The body has more fat

tissue and fats yield more energy than carbohydrates.

When the energy supply is low the liver responds,

mobilizing stored, glycogen and after 6–12 h it is

exhausted. When glycogen is depleted there is an

increase in the glyconeogenesis rate for glucose syn-

thesis. Humans can store up to 150 g (a third of all

glycogen in the body) glycogen in the liver which will

serve to maintain the blood glucose level in the

postresorptive phase. Therefore, in this organ it falls

drastically and can decrease to almost zero during

starvation lasting more one day. The buffering effect

of liver glycogen to maintain blood glucose level is

caused by glucose 6-phosphatase catalysis. Since this

enzyme only exists in liver, glycogenolysis in muscles

channel glucose 6-phosphate into glycolysis without

any adenosine triphosphate (ATP) waste.

0008Muscle glycogen serves as an energy source during

mechanical work. During light exercise glucose, fatty

acids, and ketone bodies are completely degraded to

and H

O. ATP and creatine phosphate present

in resting muscles sustain the necessary energy to

muscle contraction for a few seconds.

0009Glycogen is the most important energetic reserve

in muscle tissue. It can form 2% of muscle mass.

Glucose 1-phosphate, the first metabolite produced

when glycogen is degraded, is processed to glucose 6-

phosphate via isomerization. This metabolite yields

ATP either via oxidative phosphorylation, when O

present, or through conversion to lactate via anaer-

obic glucose. There are two types of muscle in

animals: the white muscle of rapid contraction and

the red muscle of slow contraction. The former works

for short periods of time during heavy exercise with

high ATP requirements via anaerobic glycolysis. The

latter works for long periods of time during pro-

longed exercise and in continuous contraction, as in

cardiac muscle. Red muscle requires low ATP per

minute because it is rich in mitochondrial organelles

and therefore converts glucose 6-phosphate more

slowly to CO

and H

O when compared with white

muscle.

0010Knowledge of microbial nutrition has become

more relevant in recent years with the increased

use of microorganisms and their products as food

additives and supplements. Saccharomyces cerevisiae

(baker’s yeast) is an important microorganism used in

food technology which mobilizes glycogen after glu-

cose is exhausted from the medium and in the bud-

ding process as carbon and energy sources. Recently,

it was found that glycogen plays another important

role when S. cerevisiae is submitted to stress condi-

tions in the medium of glucose. The effect of stress is

to stimulate the recycling of glycogen instead of pro-

moting either its accumulation or its degradation. It is

important to note that, under stress, cell growth is

arrested, thus there are ATP imbalances in the upper

and lower part of the glycolytic pathway. Like a

GLYCOGEN 2931

‘turbo’ design model, glucose oxidation by glycolysis

leads to overproduction of ATP, glucose 6-phsphate,

phosphate depletion, and cell death. The solution to

this cellular problem is possible because stress induces

transcriptional activation and the relative enzymes

for glycogen metabolism are under this control.

Moreover, as a result, both synthesis and degradation

enzymes are expressed to the same extent. Thus, it has

been suggested that glycogen turnover may function

as a glycolytic safety valve to avoid substrate-acceler-

ated death and also that glycogen in these cells con-

fers survival advantages for cell proliferation and

stress resistance. In this manner, the glycogen level

or glycogen used as a futile cycle contributes to the

fitness of the yeast in different environments and

under industrial processes. In brewery fermentation

yeast is added at the beginning of each batch for eight

or more cycles. Before each cycle it is replaced in

proliferation conditions and must break down its

glycogen for sterol and fatty acids biosynthesis.

Glycogen is the sole carbon source used in membrane

component biosynthesis. Naturally, the glycogen con-

tent at the beginning of the brewery fermentation has

a direct impact on yeast vitality (metabolic activity)

and a significant improvement in brewer quality.

Biosynthesis and Degradation

0011 Glycogen phosphorylase is a dimmer catalyzing

the first and controlled step in glycogen degradation

generating glucose 1-phosphate. The cyclic adenosine

monophosphate (cAMP) cascade action yields the

active form of glycogen phosphorylase a, which

has a phosphoryl group linked to Ser14 in each

subunit. The less active form, which is phosphorylase

b, is dephosphorylated. Both forms are finely regu-

lated. They contain the vitamin B

derivative pyri-

doxal-5-phosphate, covalently linked, which is

required for their activities. The glucose 1-phosphate

is converted by phosphoglucomutase to a glucose 1,6-

bisphosphate intermediary which is an important

enzyme regulator occurring in vivo at low concen-

tration to maintain phosphoglucomutase fully active.

To debranch the glycogen, two catalytic sites in deb-

ranching enzyme for both transferase and alpha

(1!6)-glucosidase are required, increasing enzyme

efficiency. Although the reaction catalyzed by glyco-

gen phosphorylase is reversible, under physiological

conditions glycogen breakdown is a thermodyn-

amically favorable process. Consequently, glycogen

biosynthesis and degradation must occur by two sep-

arate pathways. This strategy has two advantages:

biosynthesis and degradation may occur at the

same time and each pathway may be independently

regulated. Since under physiological conditions the

glycogen biosynthesis from glucose 1-phosphate to

glycogen is an unfavorable process, the glycogen bio-

synthesis needs an exergonic step. So, glucose 1-phos-

phate reacts with uridine triphosphate (UTP) to

form uridine diphosphate glucose (UDPG) via

UDPG pyrophosphorylase catalysis. This reaction

produces pyrophosphate (PPi) which is hydrolyzed

in a thermodynamically highly favorable reaction

catalyzed by pyrophosphatase. Both free energy of

PPi and free energy of UTP are utilized to drive the

glycogen biosynthesis pathway entropically. Glyco-

gen synthase can only act on preexisting alpha

(1!4)-linked glucose residues chain. So, the first

step in glycogen biosynthesis is the attachment of

glucose residue from UDPG to the Tyr group of an

autocatalytic enzyme named glycogenin. Sequen-

tially, the autocatalytical glycogenin extends the glu-

can chain by six or seven alpha-1,4-linked glucose

residues, again derived from UDPG. Note that glyco-

genin is a glucosyl transferase enzyme and serves as

an initiator for glycogen biosynthesis. The latter

shows that glycogenin has a structural role in glyco-

gen molecule and its biogenesis. Glycogen synthase

associates with branching enzyme, elongating the glu-

can chain and building the glycogen molecule on to

the protein backbone. The branching enzyme amylo-

(1,4!1,6) transglycosilase breaks segments of at

least 11 glucose residues of the linear chain (alpha-

amylose), transferring to the C

-OH groups to the

same or another chain.

Allosteric Regulation

0012Glycogen phosohorylase and glycogen synthase

are under finely tuned and reciprocal regulation.

The activity of glycogen phosphorylase is allosteri-

cally controlled by metabolic effects produced in cel-

lular metabolism. In muscle this enzyme is activated

by adenosine monophosphate (AMP) and inhibited

by ATP and glucose 6-phosphate. The liver isoform,

unlike the muscle isoform, is more tightly controlled

by phosphorylation than by allosteric regulation. In

addition, in the liver, free glucose readly inhibits

glycogen phosphorylase a. So, when cellular energy

is high the enzyme is inhibited, and when cellular

energy is low it is activated. Naturally, when ATP

and glucose 6-phosphate concentrations are high,

glycogen biosynthesis is favored. In terms of the sym-

metry model of allosterism, glycogen phosphorylase

may assume the enzymatically inactive T (tense)

conformation or the catalitically active R (relaxed)

conformation which are differently responsive to

allosteric effects. When in R-state, the enzyme has

a high-affinity substrate phosphate-binding site,

whereas in the T-state this catalytic enzyme site is

2932 GLYCOGEN

inaccessible. Moreover, the R-state is favored by

dephosphorylation and allosteric activators, while

the T-state is favored by dephosphorylation and by

inhibitors. Besides allosteric controls there are enzym-

atic interconversions through a bicyclic cascade in-

volving three enzymes: phosphorylase kinase, cAMP

protein kinase, and phosphoprotein phosphatase,

which are themselves under allosteric control

(Figure 1). The cascade functions in order to amplify

the sensitivity of the system to an allosteric effect. So,

a small alteration in cAMP concentration produces

large modifications in the cascade action.

0013 Glycogen synthase, a homotetrameric enzyme,

contains 1–9 Ser residues to be phosphorylated. Thus,

it can exist in several intermediate phosphorylated

forms that have depressed ratios but have enhanced

sensitivities to glucose 6-phosphate concentration. In

muscles a less active form of glycogen synthase b is

strongly inhibited by physiological concentrations of

ATP, adenosine diphosphate (ADP), and Pi, its allos-

teric inhibitors, and due to low physiological concen-

tration of glucose 6-phosphate, its allosteric activator,

the enzyme b-form is almost totally inactive. In con-

trast, the more active enzyme a-form is independent

of these effects. In this way, the glycogen synthase

activity in vivo varies with the fraction of the enzyme

in dephosphorylated form and the degree of the

enzyme phosphorylation. The fraction of a-form is

controlled by the same bicyclic cascade involved

in glycogen phosphorylase through phosphorylase

kinase and phosphoprotein phosphatase 1 enzymes.

Moreover, the involvement of multiple protein

kinases and protein phosphatases in glycogen

synthase activity has been suggested. cAMP protein

kinase, protein kinase C, calmodulin-dependent pro-

tein kinase, and glycogen synthase kinase-3 are

present in this process. The phosphoprotein phos-

phatase 1 in muscle is only active when it is bound

to glycogen via its G subunit. It is regulated by phos-

phorylation by insulin-stimulated protein kinase

and by cAMP-protein kinase. Phosphatase 1 is also

inhibited by its binding to the phosphoprotein

phosphatase inhibitor 1, which also is modified by

phosphorylation/dephosphorylation. Due to this fine

control glycogen synthase has been considered to

control the glycogen synthesis rate. In muscle it was

demonstrated through theoretical quantitative model

that glucose transport/hexokinase controls the glyco-

gen synthesis rate and that the role of covalent phos-

phorylation of glycogen synthase is to adapt the

activity of the enzyme to the flux and to control the

metabolite level, not the flux. Phosphoprotein phos-

phatase 1 in the liver can be bound to R and T states

of glycogen phosphorylase but only in the T-state

enzyme is it accessible for hydrolysis by phosphopro-

tein phosphatase 1, which converts glycogen phos-

phorylase a to glycogen phosphorylase b. Thus,

phosphoprotein phosphatase 1 results in glycogen

phosphorylase inactivation and glycogen synthase

activation.

0014In yeast, two distinct genes GSY1 and GSY2

encode two distinct glycogen synthases. The former

OHP-O OHP-O

ADP ATP ADP ATP

Glycogen

phosphorylase b

(Inactive)

Phosphorylase kinase

Phosphoprotein

Phosphatase I

Glycogen

phosphorylase a

(Active)

cAMP

inhibits

cAMP

activates

Phosphoprotein

Phosphatase I

Glycogen

synthase

(Active)

Glycogen

synthase

(Inactive)

Glycogen synthase kinase (GSK3)

Insulin

inhibits

Insulin

activates

fig0001 Figure 1 Controls of glycogen phosphorylase and glycogen synthase via enzyme phosphorylase.

GLYCOGEN 2933

is expressed constitutively and the latter is expressed

under catabolite depression, heat shock, and nitrogen

starvation. The gene product (Gsy2) seems to be the

predominant glycogen synthase. Gsy1 and Gsy2

isoforms occur in phosphorylated (less active) and in

dephosphorylated (active) forms and have activities

also subjected to allosteric control. Glucose 6-

phosphate is a direct activator and also controls the

phosphorylation state of this enzyme by inhibiting a

glycogen synthase kinase. The cAMP-protein kinase

and a (Ser/Thr) protein kinase named Snf1 also

appear to play a central role in the control of glycogen

metabolism in yeast. Using depressed mutant or less

repressed sugars it was demonstrated that glycogen

accumulation occurs associated to cell growth

under both oxidative metabolism and low specific

growth rate. Lipase–glycogen interactions have been

observed in Yarrowia lipolytica lipase in vitro as well

as during lipase production process. Y. lipolytica

under stress condition shows a relationship between

the lipase liberation rate and glycogen recycling

followed decline.

Hormonal Control

0015 In the past, research on glycogen metabolism led to

general important discoveries such as the concept of

second messenger. The great number of these mol-

ecules synthesized after hormone–receptor complex

binding in cellular membranes amplifies the hormone

signal in target cells. Glycogen metabolism regulation

in the body is adjusted with meals at periodic inter-

vals which can vary from short to longer periods.

After a meal, blood glucose may increase from 90 to

140 mg dl

1

in an hour but reverts to normal in a

nondiabetic person in 2 h due to the buffering effect

of the liver. Although the general mechanism of glyco-

gen biosynthesis and breakdown is the same in all

tissues, the rates of these important pathway are dif-

ferently regulated by chemical signaling to hormone

release into the blood by gland cells. Then, from the

blood to target tissues, insulin, epinephrine (adrena-

line), and glucagon start the signal transduction in

response to available blood glucose and other stimu-

lus such as fight or flight. Under epinephrine and

glucagon (Figure 2) a fine cellular control of the signal

amplification is mediated by G-protein followed by

adenylate cyclase activation, which catalyzes the

synthesis of cAMP from ATP. Then, the signal is

transduced via phosphorylation/dephosphorylation

cascades adjusting glycogen biosynthesis and

degradation to several physiological states. The

rapid changes in the cAMP concentration are also

under phosphodiesterase control, promoting its hy-

drolysis to AMP. On the other hand, insulin collects a

protein family called Ser/Thr kinases in its target cells

to advance its anabolic and anticatabolic programs.

In the presence of insulin glycogen synthase kinase 3

is inactivated via protein kinase B, thus preventing the

latter inhibiting glycogen synthase. Simultaneously,

insulin activates protein phosphatase 1, thus activat-

ing glycogen synthase. Liver uptake of glucose across

hepatocyte plasma membrane by transporter systems

is outside the control of insulin. An increase in glyco-

gen synthesis caused by insulin induction of the glyco-

gen synthase in response to blood glucose level also

activates the hexose monophosphate shunt, produ-

cing NADPH in fatty acid synthesis. The insulin/

glucagon rate increase accelerates glycolysis by induc-

tion and activation of the glycolytic key enzymes.

Besides blood glucose level, the protein and lipids

supplied in food producing amino acids and fatty

acids in the digestive system place the liver under

anabolic states during the absorptive state. In con-

trast, in the postresorptive state the insulin/glucagon

rate decreases causing a direct effect on gluconeogen-

esis activation as well as on glycogen degradation and

on glycolysis deceleration, conducing the liver to

enter in to a catabolic state. Glucagon induces the

gluconeogenesis enzymes glucose 6-phosphatase,

fructose 6-phosphase, fructose 1,6 bisphosphatase,

and pyruvate carboxylase and represses the glycolytic

enzyme pyruvate kinase, whereas insulin induces

the glycolytic enzymes, phosphofructokinase 1, pyru-

vate kinase, and hexokinase. Glucagon also acts on

the glycogen phosphorylation/dephosphorylation

enzyme cascades via cAMP control. A similar gluca-

gon effect is found in pyruvate kinase deactivation.

Glucose 6-phosphate, ATP, AMP, acetyl CoA and

fructose 2,6-bisphosphatase regulate both glycogen

biosynthesis and glycogen degradation. The latter is

synthesized from fructose 6-phosphate by nonpho-

sphorylated phosphofructokinase 2 under cAMP

protein kinase control. In its phosphorylated form

it removes phosphate from fructose 2,6 bispho-

sphate, producing fructose 6-phosphate. Fructose

2,6-bisphosphate is the major glycolysis and gluco-

neogenesis regulator. It occurs at low cellular

concentrations, activating phosphofructokinase 1

and inhibiting fructose 2,6-bisphosphatase. In this

way it regulates biosynthesis and the degradation

glycogen pathway. In muscle and fat tissues glucose

uptake is regulated by insulin and similarly in the liver

this hormone activates both glycogen biosynthesis

and glycolysis by activation/inhibition, as well as in-

duction/repression of the key enzymes of carbohy-

drate metabolism. On the other hand, in muscle

epinephrine but not glucagon released in fight-or-

flight situations activates glycogen degradation and

inhibits glycogen biosynthesis at the same time. The

2934 GLYCOGEN

activation of glycogen phosphorylase by epinephrine

is preceded by cyclic cascade via cAMP protein kinase

action, after signaling epinephrine–receptor complex.

2þ

ions are released from the sarcoplasmic reticu-

lum to cytoplasm, after nervous signal, during muscle

contraction. Under emergency degradation of glyco-

gen (fight-or-flight), Ca

2þ

ions sustain glycogen

phosphorylase in full activity via subunit-calmodulin

activation. This process is cAMP-independent. But,

under normal muscle contraction, Ca

2þ

ions partly

activate phosphorylase kinase which gives partial

activation of glycogen phosphorylase.

Glycogenolysis and Physical Activity

0016 The glycogen content stored in the human muscle

before exercising is related to the state of preservation.

On a high-carbohydrate diet, muscle glycogen may

rise to 25 g kg

1

of muscle. If carbohydrate intake

occurs after glycogen is depleted by drastic exercise

it may rise to 30 g kg

1

and in dietary and exercise

programs it may reach 40 g kg

1

. This phenomenon is

called supercompensation and it is used in baker’s

yeast production in order to increase the glycogen

content of the commercial product. So, baker’s yeast

is submitted to periods of high-carbohydrate con-

sumption followed by fasting periods.

0017In athletes either short- or long-duration exercise

determines if the glycogen-lactic acid system can pro-

vide energy or if an aerobic system is necessary to

generate energy. This mechanism may be due both

to the lactic acid disappearing rate in the liver for

glucose conversion via gluconeogenesis, and the

oxygen-replenishing rate in the muscle. Obviously,

Activates a protein kinase allosterically

Glycogen degradation

Glucose-1-phosphate

Glucose-6-phosphate

Liver and blood

glucose

Muscle

glycolysis

ATP

ADP

ATP

ADP

ATP cAMP (second messenger)

Activate adenylate cyclase

Hormone−receptor complex

Inactive protein kinase A

Inactive glycogen

phosphorylase b kinase

Inactive glycogen phosphorylase b

(without AMP)

Active protein kinase A

Active glycogen

phosphorylase b kinase

Active glycogen phosphorylase a

(without AMP)

Hormone signals (epinephrine (adrenaline) in muscle and in liver, glucagon in liver)

fig0002 Figure 2 Glycogen degradation via hormone signaling.

GLYCOGEN 2935

in long-duration exercise aerobic glycogen degrad-

ation and oxidative phosphorylation are the most

important pathways, although fatty acids may also

sustain the working muscle. When glycogen is low

during exercise an expressive amount of energy

comes from fat degradation. Moreover, the muscle

macroglycogen (MG) pool, which is a protein-poor

glycogen (acid-soluble), as well as the proglycogen

(PG) pool, which is a protein-rich glycogen (acid-

insoluble) are differently affected by metabolic

changes, by diet manipulation, and by glycogen-

depleting exercises, respectively. The PG pool

accumulates more easily after drastic exercise and

the supercompensation phenomenon appears to be

related to an increase in MG. The PG pool is less

resistant to mobilization and its catabolism is in-

hibited less rapidly than the MG pool during intensive

exercise. Glycogen content and glycogenolysis are

positively related, thus glycogen amounts in the

muscle before exercise is a regulator of glycogenolysis

rate. All these facts suggest that the metabolic regula-

tion of the two pools must be different and that the

MG pool is a more stable form than the PG pool. On

the other hand, the greater fraction of human muscle

MG than PG is utilized in a marathon and accumula-

tion of MG is delayed after this strenuous physical

exercise, particularly in type 1 fibers, indicating local

control for the glycogen biosynthesis pattern.

Glycogen Storage Disease

0018 Due to the high number of enzymes and isoenzymes

involved in glycogen metabolism and because gene

expression, enzyme activities, and enzyme-regulatory

mechanisms vary between tissues, a large number

of diseases are linked to storage and/or structure of

the glycogen appearing in a specific tissue but not

in another. All of these abnormalities are named

glycogenoses or glycogen storage diseases. They are

divided into liver and muscle diseases respectively and

are classified numerically from I to VIII, including

subclassifications. Liver defects manifest hepato-

megaly, hypoglycemia and other metabolic disorders,

whereas muscle defects manifest cramps, pain, inabil-

ity to increase the blood lactate level, and exercise

intolerance. We must keep in mind that liver glycogen

metabolism but not muscle glycogen metabolism is

dramatically linked to glucose homeostasis of the

human body. Hence, glycogen-liver diseases play a

crucial role in human health and survival. Besides

glycogen storage diseases, mutations in genes

involved in insulin biosynthesis and action affect

glycogen-liver metabolism. Diabetes is a chronic

metabolic disorder afflicting millions of persons.

Type 2 diabetes or noninsulin-dependent diabetes is

a less severe form and is a polygenic disease charac-

terized by multiple defects in insulin actions. Patients

with type 2 diabetes have excessive hepatic glucose

production which contributes to diabetic hypergly-

cemia in postabsortive state and in postprandial

states. Compounds with action antiglycogenolytic

that decrease blood glucose in vivo such as CP-

91149, an allosteric inhibitor of the glycogen phos-

phorylase which is 200-fold more potent in direct

comparison with caffeine, have shown that liver

phosphorylase is a pharmacological candidate target

for controlling hyperglycemia in diabetes.

See also: Diabetes Mellitus: Etiology; Chemical

Pathology; Exercise: Metabolic Requirements; Liver:

Nutritional Management of Liver and Biliary Disorders