Berg J.M., Tymoczko J.L., Stryer L. Biochemistry

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of galactose joined to glucose by a β -1,4 linkage. Maltose (from starch) consists of two glucoses joined by an α -1,4

linkage. Starch is a polymeric form of glucose in plants, and glycogen serves a similar role in animals. Most of the

glucose units in starch and glycogen are in α -1,4 linkage. Glycogen has more branch points formed by α -1,6 linkages

than does starch, which makes glycogen more soluble. Cellulose, the major structural polymer of plant cell walls,

consists of glucose units joined by β -1,4 linkages. These β linkages give rise to long straight chains that form fi-brils

with high tensile strength. In contrast, the α linkages in starch and glycogen lead to open helices, in keeping with their

roles as mobilizable energy stores. Cell surfaces and extracellular matrices of animals contain polymers of repeating

disaccharides called glycosaminoglycans. One of the units in each repeat is a derivative of glucosamine or

galactosamine. These highly anionic carbohydrates have a high density of carboxylate or sulfate groups. Proteins bearing

covalently linked glycosaminoglycans are termed proteoglycans.

Carbohydrates Can Attach to Proteins to Form Glycoproteins

Specific enzymes link the oligosaccharide units on proteins either to the side-chain oxygen atom of a serine or threonine

residue or to the side-chain amide nitrogen atom of an asparagine residue. Protein glycosylation takes place in the lumen

of the endoplasmic reticulum. The N-linked oligosaccharides are synthesized on dolichol phosphate and subsequently

transferred to the protein acceptor. Additional sugars are attached in the Golgi complex to form diverse patterns.

Lectins Are Specific Carbohydrate-Binding Proteins

Carbohydrates are recognized by proteins called lectins, which are found in animals, plants, and microorganisms. In

animals, the interplay of lectins and their sugar targets guides cell-cell contact. The viral protein hemagglutinin on the

surface of the influenza virus recognizes sialic acid residues on the surfaces of the cells invaded by the virus. A small

number of carbohydrate residues can be joined in many different ways to form highly diverse patterns that can be

distinguished by the lectin domains of protein receptors.

Key Terms

monosaccharide

triose

ketose

aldose

enantiomer

tetrose

pentose

hexose

heptose

diastereoisomer

epimer

hemiacetal

pyranose

hemiketal

furanose

anomer

glycosidic bond

reducing sugar

nonreducing sugar

oligosaccharide

disaccharide

polysaccharide

glycogen

starch

cellulose

glycosaminoglycan

proteoglycan

glycosyltransferase

glycoprotein

endoplasmic reticulum

Golgi complex

dolichol phosphate

lectin

selectin

I. The Molecular Design of Life 11. Carbohydrates

Problems

Word origin. Account for the origin of the term

carbohydrate.

See answer

Diversity. How many different oligosaccharides can be made by linking one glucose, one mannose, and one

galactose? Assume that each sugar is in its pyranose form. Compare this number with the number of tripeptides that

can be made from three different amino acids.

See answer

Couples. Indicate whether each of the following pairs of sugars consists of anomers, epimers, or an aldose-ketose

pair:

See answer

(a)

d-glyceraldehyde and dihydroxyacetone

(b)

d-glucose and d-mannose

(c)

d-glucose and d-fructose

(d) α-

d-glucose and β-d-glucose

(e)

d-ribose and d-ribulose

(f)

d-galactose and d-glucose

Tollen's test. Glucose and other aldoses are oxidized by an aqueous solution of a silver-ammonia complex. What are

the reaction products?

See answer

Mutarotation. The specific rotations of the α and β anomers of d-glucose are +112 degrees and +18.7 degrees,

respectively. Specific rotation, [ α ]

, is defined as the observed rotation of light of wavelength 589 nm (the d line

of a sodium lamp) passing through 10 cm of a 1 g ml

-1

solution of a sample. When a crystalline sample of α-d-

glucopyranose is dissolved in water, the specific rotation decreases from 112 degrees to an equilibrium value of 52.7

degrees. On the basis of this result, what are the proportions of the α and β anomers at equilibrium? Assume that the

concentration of the open-chain form is negligible.

See answer

Telltale adduct. Glucose reacts slowly with hemoglobin and other proteins to form covalent compounds. Why is

glucose reactive? What is the nature of the adduct formed?

See answer

Periodate cleavage. Compounds containing hydroxyl groups on adjacent carbon atoms undergo carbon-carbon bond

cleavage when treated with periodate ion (IO

). How can this reaction be used to distinguish between pyranosides

and furanosides?

See answer

Oxygen source. Does the oxygen atom attached to C-1 in methyl α-d-glucopyranoside come from glucose or

methanol?

See answer

Sugar lineup. Identify the following four sugars.

See answer

10.

Cellular glue. A trisaccharide unit of a cell-surface glycoprotein is postulated to play a critical role in mediating

cell-cell adhesion in a particular tissue. Design a simple experiment to test this hypothesis.

See answer

11.

Mapping the molecule. Each of the hydroxyl groups of glucose can be methylated with reagents such as

dimethylsulfate under basic conditions. Explain how exhaustive methylation followed by compete digestion of a

known amount of glycogen would enable you to determine the number of branch points and reducing ends.

See answer

12.

Component parts. Raffinose is a trisaccharide and a minor constituent in sugar beets.

See answer

(a) Is raffinose a reducing sugar? Explain.

(b) What are the monosaccharides that compose raffinose?

of β-galactosidase treatment of raffinose?

13.

Anomeric differences. α -d-Mannose is a sweet-tasting sugar. β-d-Mannose, on the other hand, tastes bitter. A pure

solution of α-d-mannose loses its sweet taste with time as it is converted into the β anomer. Draw the β anomer and

explain how it is formed from the α anomer.

See answer

14.

A taste of honey. Fructose in its β-d-pyranose form accounts for the powerful sweetness of honey. The β-d-furanose

form, although sweet, is not as sweet as the pyranose form. The furanose form is the more stable form. Draw the

two forms and explain why it may not always be wise to cook with honey.

See answer

15.

Making ends meet. (a) Compare the number of reducing ends to nonreducing ends in a molecule of glycogen. (b)

As we will see in Chapter 21, glycogen is an important fuel storage form that is rapidly mobilized. At which

end the reducing or nonreducing would you expect most metabolism to take place?

See answer

16.

Carbohydrates and proteomics. Suppose that a protein contains six potential N-linked glycosylation sites. How

many possible proteins can be generated, depending on which of these sites is actually glycosylated? Do not

include the effects of diversity within the carbohydrate added.

See answer

Chapter Integration Problem

17.

Stereospecificity. Sucrose, a major product of photosynthesis in green leaves, is synthesized by a battery of

enzymes. The substrates for sucrose synthesis, d-glucose and d-fructose, are a mixture of α and β anomers as well

as acyclic compounds in solution. Nonetheless, sucrose consists of α-d-glucose linked by its carbon-1 atom to the

carbon-2 atom of β-d-fructose. How can the specificity of sucrose be explained in light of the potential substrates?

See answer

I. The Molecular Design of Life 11. Carbohydrates

Selected Readings

Where to start

N. Sharon and H. Lis. 1993. Carbohydrates in cell recognition Sci. Am. 268: (1) 82-89. (PubMed)

L.A. Lasky. 1992. Selectins: Interpreters of cell-specific carbohydrate information during inflammation Science 258:

964-969. (PubMed)

P. Weiss and G. Ashwell. 1989. The asialoglycoprotein receptor: Properties and modulation by ligand Prog. Clin. Biol.

Res. 300: 169-184. (PubMed)

N. Sharon. 1980. Carbohydrates Sci. Am. 245: (5) 90-116. (PubMed)

J.C. Paulson. 1989. Glycoproteins: What are the sugar side chains for? Trends Biochem. Sci. 14: 272-276. (PubMed)

R.J. Woods. 1995. Three-dimensional structures of oligosaccharides Curr. Opin. Struct. Biol. 5: 591-598. (PubMed)

Books

Varki, A., Cummings, R., Esko, J., Freeze, H., Hart, G., and Marth, J., 1999. Essentials of Glycobiology. Cold Spring

Harbor Laboratory Press.

Fukuda, M., and Hindsgaul, O., 2000. Molecular Glycobiology. IRL Press at Oxford University Press.

El Khadem, H. S., 1988. Carbohydrate Chemistry. Academic Press.

Ginsburg, V., and Robbins, P. W. (Eds.), 1981. Biology of Carbohydrates (vols. 1

3). Wiley.

Fukuda, M. (Ed.), 1992. Cell Surface Carbohydrates and Cell Development. CRC Press.

Preiss, J. (Ed.), 1988. The Biochemistry of Plants: A Comprehensive Treatise: Carbohydrates. Academic Press.

Structure of carbohydrate-binding proteins

U. Ünligil and J.M. Rini. 2000. Glycosyltransferase structure and mechanism Curr. Opin. Struct. Biol. 10: 510-517.

(PubMed)

J. Bouckaert, T. Hamelryck, L. Wyns, and R. Loris. 1999. Novel structures of plant lectins and their complexes with

carbohydrates Curr. Opin. Struct. Biol. 9: 572-577. (PubMed)

W.I. Weis and K. Drickamer. 1996. Structural basis of lectincarbohydrate recognition Annu. Rev. Biochem. 65: 441-473.

(PubMed)

N.K. Vyas. 1991. Atomic features of protein-carbohydrate interactions Curr. Opin. Struct. Biol. 1: 732-740.

W.I. Weis, K. Drickamer, and W.A. Hendrickson. 1992. Structure of a C-type mannose-binding protein complexed with

an oligosaccharide Nature 360: 127-134. (PubMed)

C.S. Wright. 1992. Crystal structure of a wheat germ agglutinin/ glycophorin-sialoglycopeptide receptor complex:

Structural basis for cooperative lectin-cell binding J. Biol. Chem. 267: 14345-14352. (PubMed)

B. Shaanan, H. Lis, and N. Sharon. 1991. Structure of a legume lectin with an ordered N-linked carbohydrate in complex

with lactose Science 254: 862-866. (PubMed)

Glycoproteins

R.G. Spiro. 2000. Glucose residues as key determinants in the biosynthesis and quality control of glycoproteins with N-

linked oligosaccharides J. Biol. Chem. 275: 35657-35660. (PubMed)

M. Bernfield, M. Götte, P.W. Park, O. Reizes, M.L. Fitzgerald, J. Lincecum, and M. Zako. 1999. Functions of cell

surface heparan sulfate proteoglycans Annu. Rev. Biochem. 68: 729-777. (PubMed)

R.V. Iozzo. 1998. Matrix proteoglycans: From molecular design to cellular function Annu. Rev. Biochem. 67: 609-652.

(PubMed)

E.S. Trombetta and A. Helenius. 1998. Lectins as chaperones in glycoprotein folding Curr. Opin. Struct. Biol. 8: 587-

592. (PubMed)

M. Yanagishita and V.C. Hascall. 1992. Cell surface heparan sulfate proteoglycans J. Biol. Chem. 267: 9451-9454.

(PubMed)

R.V. Iozzo. 1999. The biology of small leucine-rich proteoglycans: Functional network of interactive proteins J. Biol.

Chem. 274: 18843-18846. (PubMed)

Carbohydrates in recognition processes

W.I. Weis. 1997. Cell-surface carbohydrate recognition by animal and viral lectins Curr. Opin. Struct. Biol. 7: 624-630.

(PubMed)

N. Sharon and H. Lis. 1989. Lectins as cell recognition molecules Science 246: 227-234. (PubMed)

M.L. Turner. 1992. Cell adhesion molecules: A unifying approach to topographic biology Biol. Rev. Camb. Philos. Soc.

67: 359-377. (PubMed)

T. Feizi. 1992. Blood group-related oligosaccharides are ligands in cell-adhesion events Biochem. Soc. Trans. 20: 274-

278. (PubMed)

T.M. Jessell, M.A. Hynes, and J. Dodd. 1990. Carbohydrates and carbohydrate-binding proteins in the nervous system

Annu. Rev. Neurosci. 13: 227-255. (PubMed)

C. Clothia and E.V. Jones. 1997. The molecular structure of cell adhesion molecules Annu. Rev. Biochem. 66: 823-862.

(PubMed)

Carbohydrate sequencing

G. Venkataraman, Z. Shriver, R. Raman, and R. Sasisekharan. 1999. Sequencing complex polysaccharides Science 286:

537-542. (PubMed)

Y. Zhao, S.B.H. Kent, and B.T. Chait. 1997. Rapid, sensitive structure analysis of oligosaccharides Proc. Natl. Acad.

Sci. U.S.A. 94: 1629-1633. (PubMed) (Full Text in PMC)

P.M. Rudd, G.R. Guile, B. Küster, D.J. Harvey, G. Opdenakker, and R.A. Dwek. 1997. Oligosaccharide sequencing

technology Nature 388: 205-207. (PubMed)

I. The Molecular Design of Life

12. Lipids and Cell Membranes

The boundaries of cells are formed by biological membranes, the barriers that define the inside and the outside of a cell

(Figure 12.1). These barriers prevent molecules generated inside the cell from leaking out and unwanted molecules from

diffusing in; yet they also contain transport systems that allow specific molecules to be taken up and unwanted

compounds to be removed from the cell. Such transport systems confer on membranes the important property of

selective permeability.

Membranes are dynamic structures in which proteins float in a sea of lipids. The lipid components of the membrane form

the permeability barrier, and protein components act as a transport system of pumps and channels that endow the

membrane with selective permeability.

In addition to an external cell membrane (called the plasma membrane), eukaryotic cells also contain internal membranes

that form the boundaries of organelles such as mitochondria, chloroplasts, peroxisomes, and lysosomes. Functional

specialization in the course of evolution has been closely linked to the formation of such compartments. Specific systems

have evolved to allow targeting of selected proteins into or through particular internal membranes and, hence, into

specific organelles. External and internal membranes have essential features in common, and these essential features are

the subject of this chapter.

Biological membranes serve several additional important functions indispensable for life, such as energy storage and

information transduction, that are dictated by the proteins associated with them. In this chapter, we will examine the

general properties of membrane proteins

how they can exist in the hydrophobic environment of the membrane while

connecting two hydrophilic environments and delay a discussion of the functions of these proteins to the next and later

chapters.

I. The Molecular Design of Life 12. Lipids and Cell Membranes

Figure 12.1. Red-Blood-Cell Plasma Membrane. An electron micrograph of a preparation of plasma membranes from

red blood cells showing the membranes as seen "on edge," in cross section. [Courtesy of Dr. Vincent Marchesi.]

I. The Molecular Design of Life 12. Lipids and Cell Membranes

The surface of a soap bubble is a bilayer formed by detergent molecules. The polar heads (red) pack together leaving

the hydrophobic groups (green) in contact with air on the inside and outside of the bubble. Other bilayer structures define

the boundary of a cell. [(Left) Photonica.]

I. The Molecular Design of Life 12. Lipids and Cell Membranes

12.1. Many Common Features Underlie the Diversity of Biological Membranes

Membranes are as diverse in structure as they are in function. However, they do have in common a number of important

attributes:

1. Membranes are sheetlike structures, only two molecules thick, that form closed boundaries between different

compartments. The thickness of most membranes is between 60 Å (6 nm) and 100 Å (10 nm).

2. Membranes consist mainly of lipids and proteins. Their mass ratio ranges from 1:4 to 4:1. Membranes also contain

carbohydrates that are linked to lipids and proteins.

3. Membrane lipids are relatively small molecules that have both hydrophilic and hydrophobic moieties. These lipids

spontaneously form closed bimolecular sheets in aqueous media. These lipid bilayers are barriers to the flow of polar

molecules.

4. Specific proteins mediate distinctive functions of membranes. Proteins serve as pumps, channels, receptors, energy

transducers, and enzymes. Membrane proteins are embedded in lipid bilayers, which create suitable environments for

their action.

5. Membranes are noncovalent assemblies. The constituent protein and lipid molecules are held together by many

noncovalent interactions, which are cooperative.

6. Membranes are asymmetric. The two faces of biological membranes always differ from each other.

7. Membranes are fluid structures. Lipid molecules diffuse rapidly in the plane of the membrane, as do proteins, unless

they are anchored by specific interactions. In contrast, lipid molecules and proteins do not readily rotate across the

membrane. Membranes can be regarded as two-dimensional solutions of oriented proteins and lipids.

8. Most cell membranes are electrically polarized, such that the inside is negative [typically - 60 millivolts (mV)].

Membrane potential plays a key role in transport, energy conversion, and excitability (Chapter 13).

I. The Molecular Design of Life 12. Lipids and Cell Membranes

12.2. Fatty Acids Are Key Constituents of Lipids

Among the most biologically significant properties of lipids are their hydrophobic properties. These properties are

mainly due to a particular component of lipids: fatty acids, or simply fats. Fatty acids also play important roles in signal-

transduction pathways (Sections 15.2 and 22.6.2).

12.2.1. The Naming of Fatty Acids

Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with carboxylic acid

groups. The systematic name for a fatty acid is derived from the name of its parent hydrocarbon by the substitution of oic

for the final e. For example, the C

saturated fatty acid is called octadecanoic acid because the parent hydrocarbon is

octadecane. A C

fatty acid with one double bond is called octadecenoic acid; with two double bonds, octadecadienoic

acid; and with three double bonds, octadecatrienoic acid. The notation 18:0 denotes a C

fatty acid with no double

bonds, whereas 18:2 signifies that there are two double bonds. The structures of the ionized forms of two common fatty

acids

palmitic acid (C

, saturated) and oleic acid (C

, monounsaturated) are shown in Figure 12.2.