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

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1gl

1

(determined chromatographically); and heavy

metals As, Pb, Zn, and Cu, which must not exceed

concentrations of 0.5 p.p.m. for the first two and

10 p.p.m. for the last two.

0042 The concentration of reducing material must not

exceed 35 g l

1

. Finally, commercial gins must

not contain apreciable amounts of nitrogenated com-

pounds or furfural.

See also: Alcohol: Properties and Determination;

Chromatography: Gas Chromatography; Essential Oils:

Properties and Uses; Flavor (Flavour) Compounds:

Structures and Characteristics; Herbs: Herbs and Their

Uses; Sensory Evaluation: Aroma; Whisky, Whiskey,

and Bourbon: Composition and Analysis of Whisky

Further Reading

Barros C, Bahlsen C and Marin E (eds) (1998) Legislacion

Alimentaria, vol. XXX, Official Methods of Gin Analy-

sis, pp. 11–24. Madrid: Sid Alimentaria.

Clutton DW (1972) The history of gin. Flavour Industry 3:

454–456.

Clutton DW and Evans MB (1978) The flavour constituents

of gin. Journal of Chromatography 167: 409–419.

Guerra Herna

ndez E, Lopez Martı

nez MC and Garcı

Villanova R (1987) Analytical study in samples of com-

mercial gins. Anales de Bromatologı

´a

39: 219–227.

Guerra Herna

ndez E, Lopez Martı

nez MC and Garcı

a Vil-

lanova R (1987) Volatile components identified by gas

chromatography of digestion products of Juniperus in

ethanol. Anales de Bromatolı

a 39: 229–237.

Helrich K (ed.) (1990) AOAC. Official Methods of Analysis

of the Association of Official Analytical Chemists, 15th

edn, pp. 690–707. Virginia: Association of Official Ana-

lytical Chemists.

Hui YH (ed.) (1992) Encyclopaedia of Food Science and

Technology vol. 1, p. 594. New York: John Wiley.

Martı

nez-Alvarez PJ and Herranz A (1991) Application of

multivariate statistical methods to the differentiation of

gin brands. Journal of the Science of Food and Agricul-

ture 57: 263–272.

Simpson AC (1966) Gin manufacture. Process Biochemis-

try 10: 355–365.

GLUCOSE

Contents

Properties and Analysis

Function and Metabolism

Maintenance of Blood Glucose Level

Glucose Tolerance and the Glycemic (Glycaemic) Index

Properties and Analysis

C V K Sreeranjit and J J Lal, Centre for Development

of Science & Technology, Thrissur, Kerala, India

Background

0001 Glucose is a word derived from the Greek word

‘gleukos’ meaning sweet wine. The term glucose was

introduced by Andre

Dumas in 1838 to refer to the

sweet compound obtained from honey and grapes.

Later, Frederich August Kekule

von Stradonitz called

it dextrose because the naturally occurring glucose is

dextrorotatory in nature. When Fisher studied this

sugar, he called it glucose, and chemists have used

the same word ever since.

0002 Glucose is a widely distributed, sweet, simple sugar

(monosaccharide) synthesized in plants by photosyn-

thesis and usually stored as polysaccharides. In

animals dietary carbohydrates, after digestion, dif-

fuse into the blood in the form of glucose and are

assimilated by the liver. In the liver, glucose is used for

the synthesis of other carbohydrates, used as a major

fuel (for energy production) for the tissues of organ-

isms, or converted to other carbohydrates with highly

specific functions such as glycogen for storage, ribose

in nucleic acid, galactose in lactose of milk, in certain

complex lipids (glycolipids), and in combination with

proteins in glycoproteins and proteoglycans. The

storage form of glucose is glycogen (liver or muscle

glycogen) in animals, and starch or cellulose in plants.

Chemical Structure

0003The structure of glucose can be represented in differ-

ent ways as a straight chain, simple ring, or chair

form. The straight-chain form (aldo sugars are writ-

ten with the aldehyde group at the top and the pri-

mary alcohol at the bottom) can account for some of

2898 GLUCOSE/Properties and Analysis

the properties of glucose, but a cyclic structure is

favored on thermodynamic grounds. X-ray diffrac-

tion analysis has revealed that it has chair form

(Figures 1–3).

Isomerism

0004 Compounds that have the same structural formula

but differ in their spatial configuration are known as

stereoisomers. This phenomenon is mainly due to the

presence of an asymmetric carbon atom in the com-

pound. In the glucose molecule, there are four such

asymmetric carbon atoms, and so there are 16

isomers in nature.

0005 D and L forms The orientation of the —H and —OH

groups around the carbon atom adjacent to the ter-

minal primary alcohol carbon determines whether it

belongs to the d or l series. In this spatial relationship,

if the —OH group on the fifth carbon is on the right, it

is d (dextrorotatory)-glucose, and if it is on the left, it

is l (levorotatory)-glucose (Figure 4), i.e., the presence

of asymmetric carbon atoms confers the optical activ-

ity of glucose. When a beam of plane-polarized light

passes through glucose solution, it is rotated either to

the right (dextrorotatory) (þ) or to the left (levorota-

tory) (), but if the mixture contains an equal quantity

of dextro- and levorotatory forms (a dl mixture),

it does not exhibit any optical activity, since the

activities of each isomer cancel each other out. This

mixture of isomers is also called a racemic mixture.

0006Pyranose and furanose forms In some cases, glucose

is found in a stable ring form similar to pyran (a-d-

glucopyranose) or furan (a-d-glucofuranose) as in

Figures 5 and 6. In solution, about 99% of glucose

is in the pyranose form, and the remainder is in the

furanose form.

0007a and b anomers A ring structure, called a hemi-

acetal, is formed by the combination of an aldehyde

fig0001 Figure 1 Straight chain.

HOCH

fig0003 Figure 3 Chair form.

CHOH

fig0004Figure 4 D-Glucose and L-glucose.

HOCH

fig0002 Figure 2 Ring form.

α-D-Glucopyranose

Pyran

HOCH

fig0005Figure 5 Pyranose form of glucose.

GLUCOSE/Properties and Analysis 2899

and an alcohol group in glucose. While dissolving in

water, the optical rotatory power of the solution of

monosaccharides gradually changes until it reaches

an equilibrium. The freshly prepared solution of glu-

cose has a specific rotation of þ112



, and when this

solution is allowed to stand, the rotation falls to

þ52.7



and remains constant. This change in specific

rotation is called mutarotation (Figure 7). In solution,

the ring structure (a-d-glucopyranose) undergoes

isomerism at position 1 or the anomeric carbon

atom to form b-d-glucopyranose.

0008 Epimers Epimers are isomers of glucose with differ-

ent configurations of —OH and —H on the 2, 3 and 4

carbon atoms. The biologically important epimers

of glucose are mannose and galactose formed by epi-

merization at the second and fourth carbon atoms

(Figure 8).

0009 Aldoses and ketoses Glucose and fructose have the

same molecular formula but differ in their structural

formula. In glucose, the first carbon atom has an

aldehyde group, while in fructose, there is a keto

group in the second carbon.

Configuration of C1 in glucose

0010 The configuration of C1 in glucose may be a (with the

—H atom to the left and the —OH group to the right)

or b (with the —H atom to the right and the —OH

group to the left). The empirical rule states that the

a anomer has a higher dextrarotation. Hence, the a-

d(þ) glucose has a specific rotation of þ111



,andb-

d(þ) glucose has a specific rotation of þ18.7



.In

negative rotation, the b anomer has a more negative

rotation than the a-anomer.

Properties

Oxidation

0011Sugars contain numerous oxidizable groups. Mild

oxidation converts only the aldehyde into a carbox-

ylic acid (aldonic acid), but more vigorous oxidation

results in the oxidation of both aldehyde and primary

alcohols to carboxylic acids (alaric acid). Even more

vigorous oxidation ruptures the six-carbon frame-

work due to the oxidation of the secondary hydroxyl

groups. Mild oxidation of glucose using the enzyme

glucose oxidase produces gluconic acid, vigorous oxi-

dation of glucose with strong oxidizing agents

produces glucaric acid, and even more vigorous

oxidation results in the formation of glucosaccharic

acid. Periodic acid treatment results in the complete

breakdown of the carbon chain.

Reduction

0013Similar to the process of oxidation, the product of

reduction depends on the nature of the reagent. Sodium

amalgam, sodium borohydride, high-pressure catalytic

hydrogenation, or electrolyte reduction in an acid solu-

tion converts aldoses into the corresponding alcohols,

and similarly, glucose is converted to sorbitol. Reduc-

tion of glucose with concentrated hydrochloric acid

and red phosphorus at 100



C produces 2-iodohexane,

and prolonged heating finally gives n-hexane.

Fermentation

0014Glucose is fermented by yeast to produce ethanol and

forms glucosates with various metallic hydroxides.

Treatment with calcium hydroxide produces calcium

glucosate (C

CaO).

Furan

α-

D-Glucofuranose

HOCH

HCOH

fig0006 Figure 6 Furanose form of glucose.

HOCH

β-

D-Glucopyranoseα-D-Glucopyranose

Acyclic aldehyde form

fig0007Figure 7 Mutarotation of glucose.

2900 GLUCOSE/Properties and Analysis

Glycosides

0015 Generally, hemiacetals react with another molecule of

an alcohol to form acetals. Similarly, sugars (in their

ring form) react with a molecule of an alcohol to form

an acetal derivative generally called a glycoside. The

glycosides of glucose are called glucosides.

Analysis

0016 Monosaccharides contain free aldehyde (—CHO) or

ketone (

—

CO) groups, and so their chemical proper-

ties vary, depending on the number of hydroxyl groups

and the presence or absence of —CHO/=CO groups.

These variations are the basis of color reactions for

identifying various monosaccharides. The common

methods for conformational (structural), qualitative,

and quantitative analyses of glucose are as follows.

Conformational Analysis

0017 Various methods are used to study the conformation

of monosaccharides. The common method is to esti-

mate the instability rating of various conformations

by using various instability factors such as chair or

boat forms, number of axial hydroxyl groups (by

considering each axial —OH group result in one

instability unit), 1,3 interactions involving the axial

—OH group (0.5 instability unit) an axial CH

group (result in two instability units) and if the —OH

group on C2 is axial, and the oxygen atom on C1 is

equatorial (resulting in 2.5 instability units).

0018 In addition to this instability rating for conforma-

tional analysis, various physical methods are used for

conformational analysis and to identify and elucidate

the structures of various monosaccharides.

0019 X-ray analysis This method is limited to studies on

crystals (solid components), and the results may not

apply to conformations when the compound in solu-

tion. The X-ray analysis of d-glucose has shown that

it is in pyranose form.

0020 Infrared spectroscopy This method is used to iden-

tify, and determine the structure, configuration, and

conformation of, monosaccharides.

0021Nuclear magnetic resonance spectroscopy This

technique is used for the purposes of identification

and assignment of configuration and conformation.

Deuterium oxide is used as the solvent to examine all

C—H protons, and dimethylsulfoxide is used as the

solvent to examine —OH protons.

0022Mass spectrometry This is the most advanced tech-

nique used to determine the size of a ring of a mono-

saccharide (pyranose or furanose), the number and

position of methyl, ether, and acetyl groups in methyl-

ated sugar, the position of linkage in a disaccharide,

etc.

0023Chromatography This is used to separate, estimate,

and identify monosaccharides, using various tech-

niques such as paper, column, gas–liquid (GLC), and

thin-layer chromatography (TLC). Column chroma-

tography is used almost exclusively for preparative

purposes, and GLC and TLC are particularly useful in

analytical work. Definite identification is carried out

by isolating the sugar then determining the physical

characteristic and precipitating the crystalline deriva-

tive. The presence of a reducing sugar on a paper

chromatogram is detected using various reagents,

e.g., spraying with ammoniacal silver nitrate (black

spot formation) or a solution of salt of an aromatic

amine, e.g., N,N- dimethylaniline hydrochloride, to

examine the appearance of colors characteristic of the

type of reducing sugar.

Qualitative Analysis

0025The occurrence of glucose in a carbohydrate solution

can be detected by various methods. The commonest

tests are as follows.

0026Molisch’s test The principle of this test is that

monosaccharides, derived from carbohydrates by

hydrolyzing the glycosidic bonds (using concentrated

sulfuric acid), become dehydrated to furfural and

its derivatives. These products then combine with

sulfonated a-naphthol to give a purple complex.

0027Two drops of a-naphthol solution are added to

2 ml of the test solution in a test tube, and then

α-D-Galactose

HOCH

HOH

α-D-Glucose

HOCH

α-D-Mannose

HOCH

fig0008 Figure 8 Epimerization of glucose.

GLUCOSE/Properties and Analysis 2901

about 1 ml of concentrated sulfuric acid is added

down the side of the tube so as to form two layers.

A color change observed at the interface of the two

liquids confirms the presence of carbohydrates in the

test solution.

0028 Anthrone reaction The principle of this test is that

the aforementioned furfural and derivatives react

with anthrone (10-keto, 9,10-dihydroanthracene) to

give a blue–green complex.

0029 Five drops of the test solution are added to about

2 ml of anthrone reagent (a mixture of 2 g of anthrone

per liter of sulfuric acid) and the solution mixed

thoroughly. The appearance of a blue–green color

confirms the presence of carbohydrates in the test

solution.

0030 Fehling’s test This test uses two reagents: Fehling’s

reagent A (34.65 g of copper sulfate dissolved in dis-

tilled water and made up to 500 ml) and Fehling’s

reagent B (125 g of potassium hydroxide and 173 g

of potassium sodium tartrate dissolved in distilled

water and made up to 500 ml). The blue alkaline

cupric hydroxide present in the solution when heated

in the presence of reducing sugar is then reduced to

yellow or red cuprous oxide and precipitated.

0031 To 1 ml of the Fehling’s solution A and 1 ml solu-

tion B few drops of test solution are added and the

mixture boiled for a few minutes. The appearance

of a yellow or brownish precipitate confirms the

presence of glucose in the solution.

0032 Benedict’s test Benedict’s reagent is made by dis-

solving 173 g of sodium citrate and 100 g of sodium

carbonate in 800 ml of warm water, and the solution

filtered and made up to 850 ml with water. Then,

17.3 g of copper sulfate is dissolved in 100 ml of

water, the solution made up to 150 ml, and the copper

sulfate solution slowly added to the first solution

while stirring. The alkaline cupric hydroxide present

in the solution, when heated in the presence of reduc-

ing sugar, is reduced to yellow or red cuprous oxide

and is precipitated.

0033 Five drops of the test solution are added to 2 ml of

Benedict’s reagent the mixture placed in a boiling

water bath for 5 min and the color (green–yellow–

orange–red) development against a standard glucose

solution. The color of the final solution depends on

the concentration of glucose present.

0034 Barfoed’s test Barfoed’s reagent is a weakly acidic

solution and is reduced by monosaccharides. The

precipitate of cuprous oxide is less dense than with

the previous two tests, and it is best to leave the tube

to stand to allow the precipitate to settle. The color of

the cuprous oxide is also different, giving a brick-red

color in the presence of carbohydrates.

0035One milliliter of the test solution is added to 2 ml

of Barfoed’s reagent (made by dissolving 13.3 g of

copper acetate in about 200 ml of water and adding

1.8 ml of glacial acetic acid), and the mixture is then

boiled for 1 min and allowed to stand and cool. The

development of a brick-red color confirms the

presence of carbohydrate in the test solution.

0036Preparation of osazones Compounds containing the

—CO—CHOH— group form crystalline osazones

with phenyl hydrazine with characteristic shapes

and melting points. This can be used as a better

method by which to identify a reducing sugar. Phenyl

hydrazine reacts with the carbonyl group of the sugar,

resulting in the formation of phenylhydrazone and

the osazone (Figure 9).

0037Five milliliters of the sugar solution are acidified

with 10 drops of glacial acetic acid, a pinch of

phenylhydrazine powder, and 0.1 g of solid sodium

acetate. The mixture is then warmed in a boiling

water bath for 10 min with occasional shaking and

filtered. The filtrate is warmed in the water bath for a

further 20 min and cooled very slowly. The osozone

crystals can then be examined under a microscope

and the shapes compared. Glucose produces needle-

shaped yellow osazone crystals.

Quantitative Analysis

0038Anthrone method This is a very rapid and conveni-

ent method for determining hexoses either in free

form or in polysaccharides. The resulting blue–green

coloration shows a maximum absorption at 620 nm.

0039Four milliliters of anthrone reagent are added to

1 ml of a protein-free (the presence of protein results

in the development of false colors) carbohydrate

solution, mixed rapidly, and the tubes with lids (to

prevent loss of water by evaporation) placed in a

boiling water bath for 10 min. After cooling, the

fig0009Figure 9 Crystalline appearance of glucosazone (needles or

feathery).

2902 GLUCOSE/Properties and Analysis

optical density is measured at 620 nm against a re-

agent blank. A standard curve should be prepared

using known glucose and glycogen solutions.

0040 Nelson–Somogyi method In this method, the sugar

is heated with an alkaline solution of copper tartrate,

and cuprous oxide is produced, which reacts with

arsenomolybdate to give molybdenum blue; the

intense blue color is then measured in a colorimeter.

Sodium sulfate is included in the reaction mixture to

minimize the entry of atmospheric oxygen into the

solution, which would reoxidate the cuprous oxide.

0041 One milliliter of the test solution (containing ap-

proximately 50–150 mg of sugar) is mixed with 1.5 ml

of water in a test tube and mixed thoroughly. Next,

0.2 ml of barium hydroxide solution (0.3 mol l

1

)is

added to the mixture followed by 0.2 ml of aqueous

zinc sulfate solution (50 g l

1

), then shaken thor-

oughly and centrifuged. The supernatant is used for

analysis.

0042 One milliliter of alkaline copper reagent (made by

dissolving 4 g of CuSO

 5H

O, 24 g of Na

16 g of Na–K tartrate, and 180 g of anhydrous

in water and diluting to 1 l) is added and

the solution mixed well. The solution is then heated

on a boiling water bath for 15 min (in a test tube with

a lid), the tubes cooled in cold water, 1 ml of the

arsenomolybdate reagent (commercially available)

added, and the solution left to stand for a minute

until the effervescence ceases. The blue color is di-

luted with water to a final volume of 10 ml, and the

extinction is read at 510 or 660 nm. A standard

should be prepared, and the reading should be taken

against a reagent blank.

0043 Dinitrosalicylic acid method In this method, the

sugar is heated with dinitrosalicylic acid reagent

followed by a 40% solution of potassium sodium

tartrate, and the dark red color developed is measured

in a colorimeter at 510 nm.

0044 The sample is mixed with dinitrosalicylic acid re-

agent (prepared by dissolving 1 g of dinitrosalicylic

acid, 200 mg of crystalline phenol, and 50 mg of

sodium sulfate in 100 ml of 1% NaOH and storing

the solution at 4



C) and heated for 5 min in a boiling

water bath. Then, 1 ml of 40% potassium sodium

tartrate is added and the solution allowed to cool.

The intensity of the dark red color developed is read

at 510 nm, and the concentration of glucose in the

sample is calculated by plotting the optical density

against a standard curve prepared using various con-

centrations of known glucose solutions.

0045 Glucose oxidase method As glucose is a simple

sugar, one of the most specific methods for the

quantitative estimation of glucose is the use of glucose

oxidase (EC 1.1.3.4), which catalyzes the oxidation

of a-d-glucose to d-glucono-1,5 lactone (gluconic

acid) with the formation of hydrogen peroxide. The

oxygen liberated from hydrogen peroxide by perox-

idase reacts with the O-dianisidine and oxidases it to

a red chromophore product.

0046One milliliter of glucose oxidase peroxidase re-

agent (made by dissolving 25 mg of O-dianisidine in

1 ml of methanol, adding 49 ml of 0.1 M phosphate

buffer (pH 6.5), then adding 5 mg of peroxidase and

5 mg of glucose oxidase to the mixture) is added to

the sample. The tubes are then incubated at 35



C for

40 min and the reaction terminated by the addition of

2 ml of 6 N HCl. The intensity of the color developed

at 540 nm is read against a reagent blank, and the

concentration of the sample is then calculated by

plotting a standard curve using different concentra-

tions of known glucose solutions.

0047Phenol sulfuric acid method In a hot acidic medium,

glucose is dehydrated to hydroxymethyl furfural.

This forms a green product with phenol and has an

absorbed maximum at 490 nm.

0048To 0.2 ml of the sample solution, 1 ml of 5%

phenol solution and 5 ml of 95% sulfuric acid are

added and the solution mixed well. After 10 min,

the test tubes are shaken and placed in a water bath

at 25–30



C for 20 min. The green color is then read

at 490 nm against a reagent blank, and the amount of

glucose in the sample is calculated by plotting a stand-

ard curve.

See also: Chromatography: Principles; Thin-layer

Chromatography; High-performance Liquid

Chromatography; Gas Chromatography; Combined

Chromatography and Mass Spectrometry; Glucose:

Properties and Analysis; Spectroscopy: Infrared and

Raman; Atomic Emission and Absorption; Nuclear

Magnetic Resonance

Further Reading

Guthrie RD (1974) Introduction to Carbohydrate

Chemistry, 4th edn. Oxford: Clarendon Press.

Pigman W and Horten D (1972) The Carbohydrates, 2nd

edn, vol. 1A. New York: Academic Press.

Sadasivam S and Manikam A (1992) Biochemical Methods

for Agricultural Sciences, pp. 1–21. New Delhi, India:

Wiley Eastern.

Whelam WJ (1975) Biochemistry of Carbohydrates.

London: Butterworth and University Park, University

Park Press.

Whistler RL (1962–1971) Methods in Carbohydrate

Chemistry, vols. 1–6. New York: Academic Press.

GLUCOSE/Properties and Analysis 2903

Function and Metabolism

D A Bender, University College London, London, UK

Background

0001 Glucose, arising from dietary carbohydrates, is the

major metabolic fuel, providing 50–75% of energy

in most diets. In the fed state, glucose in excess of

immediate requirements is used for the synthesis of

the reserve carbohydrate glycogen in liver and muscle.

Between meals these glycogen reserves are used to

maintain a supply of glucose for the red blood cells

and central nervous system – other tissues use mainly

fatty acids in the fasting state, to spare glucose. In the

fed state, when glucose is plentifully available, it be-

comes the major fuel of all tissues, and the respiratory

quotient (RQ) rises from about 0.8 to near 1, indicat-

ing that glucose is the substrate for most energy-

yielding metabolism.

0002 Most glucose is metabolized by glycolysis, leading

to the formation of 2 mol of pyruvate per mol of

glucose, which then undergoes oxidative decarboxyl-

ation to acetyl coenzyme A (CoA), and oxidation in

the citric acid cycle. In red blood cells and tissues that

are active in fatty acid synthesis (liver, adipose tissue,

and lactating mammary gland) the pentose phosphate

pathway provides an alternative to part of the path-

way of glycolysis.

Glycolysis: The (Anaerobic) Metabolism of

Glucose

0003 Overall, the pathway of glycolysis is cleavage of the

6-carbon glucose molecule into two 3-carbon units.

The key features of the pathway are:

0004 Two phosphorylation reactions to form successively

glucose-6-phosphate and fructose bisphosphate.

0005 Cleavage of fructose bisphosphate to yield two

molecules of triose (3-carbon sugar) phosphate.

0006 Two steps in which phosphate is transferred from a

3-carbon substrate on to adenosine diphosphate

(ADP), forming adenosine triphosphate (ATP).

0007 One step in which NAD

is reduced to NADH.

The substrate for glycolysis is glucose-6-phosphate;

this may arise from two sources:

0008 Phosphorylation of glucose, catalyzed by hexo-

kinase;

0009 Phosphorolysis of glycogen in liver and muscle to

yield glucose-1-phosphate, catalyzed by glycogen

phosphorylase. Glucose-1-phosphate is readily

isomerized to glucose-6-phosphate.

The pathway of glycolysis is shown in Figure 1. Al-

though the aim of glucose metabolism is ultimately to

phosphorylate ADP !ATP, there is a moderate initial

ATP cost; there are two steps in which ATP is used,

one to form glucose-6-phosphate when glucose is the

substrate, and the other to form fructose bisphos-

phate.

0010The formation of fructose bisphosphate, catalyzed

by phosphofructokinase, is an important step for the

regulation of glucose metabolism. Phosphofructo-

kinase is inhibited by normal intracellular concentra-

tions of ATP. As the ratio of ATP to ADP falls, 5

AMP

is formed; it relieves the inhibition of phosphofructo-

kinase by ATP, leading to an increased rate of glycoly-

sis and increased formation of ATP.

0011Fructose bisphosphate is cleaved to dihydroxy-

acetone phosphate and glyceraldehyde-3-phosphate,

which are interconvertible. The onward metabolism

of these 3-carbon sugars is linked to both the reduc-

tion of NAD

to NADH, and direct (substrate-level)

phosphorylation of ADP ! ATP.

0012There is a net yield of 8 ADP þ phosphate !ATP

from the oxidation of 1 mol of glucose to 2 mol of

pyruvate. Initially, 2 mol of ATP are required to form

fructose bisphosphate, but the pathway yields

4 ATP by direct phosphorylation of ADP, and

2 NADH (formed from NAD

), which is equiva-

lent to a further 6 ATP when oxidized in the

electron transport chain.

0013The glycolytic pathway also provides a route

for the metabolism of fructose, galactose (which

undergoes phosphorylation to galactose-1-phosphate

and isomerization to glucose-1-phosphate), and gly-

cerol. Some fructose is phosphorylated directly to

fructose-6-phosphate by hexokinase, but most is

phosphorylated to fructose-1-phosphate by a specific

fructokinase. Fructose-1-phosphate is then cleaved to

yield dihydroxyacetone phosphate and glyceralde-

hyde; the glyceraldehyde can be phosphorylated to

glyceraldehyde-3-phosphate by triose kinase.

0014Glycerol, arising from the hydrolysis of triacylgly-

cerols, can be phosphorylated and oxidized to dihy-

droxyacetone phosphate. In triacylglycerol synthesis

most glycerol phosphate is formed from dihydroxy-

acetone phosphate.

The Reduction of Pyruvate to Lactate:

Anaerobic Glycolysis

0015In red blood cells, which lack mitochondria, reoxida-

tion of NADH formed in glycolysis cannot be by way

of the electron transport chain, as occurs in other

tissues. Similarly, under conditions of maximum

exertion, for example, in sprinting, the rate at which

oxygen can be taken up into the muscle is inadequate

2904 GLUCOSE/Function and Metabolism

HC O

HO CH

HC O

HO CH

ATP

ADP

ATP

ADP

HO CH

NAD

NADH

COOH

Pyruvate Phosphoenolpyruvate 2-Phosphoglycerate 3-Phosphoglycerate Bisphosphoglycerate

ATP

ADP

COOH

HC O

COOH

HC OH

COOH

ATP

ADP

HC OH

COO

Glucose-

6-phosphatase

Glucose-6-phosphate Fructose-1,6-phosphate Fructose-1,6-bisphosphatase

Dihydroxyacetone

phosphate

Fructose-1,6-

bisphosphatase

Aldolase

Triose

phosphate

isomerase

Glyceraldehyde-

3-phosphate

Glucose

Hexokinase

Pyruvate kinase Enolase Phosphoglyceromutase

Glyceraldehyde-3-phosphate

dehydrogenase

Phosphoglycerate

kinase

Phosphohexose

isomerase

Phosphofructokinase

3 carbon sugar molecules per glucose

Figure 1 The pathway of glycolysis. Hexokinase and glucokinase EC 2.7.1.1, glucose-6-phosphatase EC 3.1.3.9, phosphoglucomutase EC 5.4.2.2, phosphofructokinase EC 2.7.1.11, fructose-

1,6-bisphosphatase EC 3.1.3.11, aldolase EC 4.1.2.13, triose phosphate isomerase EC 5.3.1.1, glyceraldehyde 3-phosphate dehydrogenase EC 1.2.1.12, phosphoglycerate kinase EC 2.7.2.3,

phosphoglyceromutase EC 5.4.2.1, phosphopyruvate hydratase EC 4.2.1.11, pyruvate kinase EC 2.7.1.40, ATP, adenosine triphosphate; ADP, adenosine diphosphate.

fig0001

to permit reoxidation of all the NADH which is

formed in glycolysis. In order to maintain the oxida-

tion of glucose, and the net yield of 2 ATP per mol

of glucose oxidized (or 3 mol of ATP if the source is

muscle glycogen), NADH is oxidized to NAD

by the

reduction of pyruvate to lactate, catalyzed by lactate

dehydrogenase (Figure 2).

0016 Lactate is exported from muscle and red blood

cells, and taken up by the liver, where it is used for

the resynthesis of glucose – the Cori cycle, shown in

Figure 2. Synthesis of glucose from lactate is an

ATP (and guanosine triphosphate (GTP))-requiring

process. The oxygen debt after strenuous physical

activity is due to an increased rate of energy-yielding

metabolism to provide the ATP and GTP that are

required for gluconeogenesis from lactate. While

most of the lactate will be used for gluconeogenesis,

a proportion will undergo oxidation to CO

in order

to provide the ATP and GTP required for gluco-

neogenesis.

0017 The conversion of glucose to lactate is known as

anaerobic glycolysis, since it does not require oxygen.

However, it is not true to say that human metabolism

(apart from red blood cells) is ever wholly anaerobic.

The formation of lactate is the fate of much of the

pyruvate formed from glucose under conditions of

maximum muscle exertion when oxygen is limiting,

but as much as possible will continue to undergo

complete oxidation.

0018Many tumors have a low capacity for oxidative

metabolism, so that much of the energy-yielding

metabolism in the tumor is anaerobic. Lactate pro-

duced by anaerobic glycolysis in tumors is

exported to the liver for gluconeogenesis; this in-

creased cycling of glucose between anaerobic

glycolysis in the tumor and gluconeogenesis in the

liver may account for much of the hypermetabolism

and consequent weight loss seen in patients with

cancer cachexia.

0019Truly anaerobic glycolysis does occur in micro-

organisms which are capable of living in the absence

of oxygen. Here there are two possible fates for the

pyruvate formed from glucose, both of which involve

the oxidation of NADH to NAD

Anaerobic glycolysis in muscle Gluconeogenesis in liver

HO CH

COOH

2 NAD

NAD

2 NADH

2 NAD

2 NADH

NADH

CHOH

COOH

Glucose

4 ADP

4 ATP

2 ATP

2 ADP

2 pyruvate

per glucose

COOH

Pyruvate

Lactate

CHOH

COOH

Lactate

dehydrogenase

Lactate

dehydrogenase

Glucose

HC O

HO CH

4 ATP

4 ADP

2 GTP

2 GDP

NAD

NADH

fig0002 Figure 2 The Cori cycle – anaerobic glycolysis in muscle and gluconeogenesis in the liver. Lactate dehydrogenase EC 1.1.1.28. ATP,

adenosine triphosphate; ADP, adenosine diphosphate.

2906 GLUCOSE/Function and Metabolism

.0020 Reduction to lactate, as occurs in human muscle.

This is the pathway in lactic acid bacteria, which

are responsible for the fermentation of lactose in

milk to form yogurt and cheese;

0021 Decarboxylation and reduction to ethanol. This is

the pathway of fermentation in yeast, which is

exploited to produce alcoholic beverages.

The Pentose Phosphate Pathway – An

Alternative to Glycolysis

0022 There is an alternative pathway for the conversion

of glucose-6-phosphate to fructose-6-phosphate, the

pentose phosphate pathway (sometimes known as the

hexose monophosphate shunt), shown in Figure 3.

0023 Overall, the pentose phosphate pathway produces

2 mol of fructose-6-phosphate, 1 mol of glyceralde-

hyde-3-phosphate, and 3 mol of carbon dioxide

from 3 mol of glucose-6-phosphate, linked to the

reduction of 6 mol of NADP

to NADPH. The

sequence of reactions is as follows:

0024 Three mol of glucose are oxidized to yield 3 mol of

the 5-carbon sugar ribulose-5-phosphate þ 3 mol

of carbon dioxide.

0025 Two mol of ribulose-5-phosphate are isomerized to

yield 2 mol of xylulose-5-phosphate.

0026 One mol of ribulose-5-phosphate is isomerized to

ribose-5-phosphate.

0027 One mol of xylulose-5-phosphate reacts with the

ribose-5-phosphate, yielding (ultimately) fructose-

6-phosphate and erythrose-4-phosphate.

0028 The other mol of xylulose-5-phosphate reacts

with the erythrose-4-phosphate, yielding fructose-

6-phosphate and glyceraldehyde-3-phosphate.

0029 This is the pathway for the synthesis of ribose for

nucleotide synthesis; more importantly, it is the

source of half the NADPH required for fatty acid

synthesis; tissues that are active in lipogenesis have a

high activity of the pentose phosphate pathway.

0030 The pentose phosphate pathway is also important

in red blood cells, where the NADPH is required to

maintain an adequate pool of reduced glutathione

(GSH), the reducing agent for glutathione peroxidase,

which reduces H

! H

O and O

. Oxidized glu-

tathione (GSSG) is reduced back to active GSH by

glutathione reductase, which uses NADPH as the

reducing agent.

0031 Partial or total lack of glucose-6-phosphate dehy-

drogenase (and hence impaired activity of the pentose

phosphate pathway) is the cause of favism, an acute

hemolytic anemia with fever and hemoglobinuria,

precipitated in genetically susceptible people by the

consumption of broad beans (fava beans) and a

variety of drugs, all of which, like the toxins in fava

beans, undergo redox cycling, producing hydrogen

peroxide. Infection can also precipitate an attack,

because of the increased production of oxygen

radicals as part of the macrophage respiratory burst.

0032Because of the low activity of the pentose phos-

phate pathway, there is a lack of NADPH in red

blood cells, and hence an impaired ability to remove

hydrogen peroxide, which causes oxidative damage

to the cell membrane lipids, leading to hemolysis.

The Oxidation of Pyruvate to Acetyl CoA

0033The first step in the complete oxidation of pyruvate is

catalyzed by the pyruvate dehydrogenase multi-

enzyme complex; an oxidative decarboxylation that

results in the formation of acetyl CoA. The oxidation

involves the reduction of NAD

to NADH. Since

2 mol of pyruvate are formed from each mol of

glucose, this step represents the formation of 2 mol

of NADH, equivalent to 6 ATP for each mol of

glucose metabolized. The acetate is released from

the enzyme esterified to coenzyme A as acetyl CoA

(Figure 4), which undergoes oxidation in the citric

acid cycle.

0034The decarboxylation and oxidation of pyruvate to

form acetyl CoA requires the coenzyme thiamin di-

phosphate, which is formed from vitamin B

. In thia-

min deficiency this reaction is impaired, and deficient

subjects are unable to metabolize glucose normally.

Especially after a test dose of glucose or moderate

exercise they develop high blood concentration of

pyruvate and lactate. In some cases this may be severe

enough to result in life-threatening acidosis.

Glycogen Synthesis and Utilization

0035In the fed state, glycogen is synthesized from glucose

in both liver and muscle. The reaction is a stepwise

addition of glucose units on to the glycogen that is

already present. In muscle insulin stimulates both the

uptake of glucose from the blood stream by active

transport and also the activity of glycogen synthetase.

In the liver, glycogen synthetase is also stimulated

in response to insulin, but glucose uptake is insulin-

independent and occurs by a carrier-mediated

passive process followed by metabolic trapping as

glucose-6-phosphate. There are two isoenzymes of

hexokinase in the liver:

0036An isoenzyme with a low Michaelis constant (K

)

which is saturated, and therefore acting at its max-

imum rate, at concentrations of glucose very much

lower than occur in tissues. This isoenzyme there-

fore acts at a more or less constant rate regardless

of the concentration of glucose available, and

GLUCOSE/Function and Metabolism 2907