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15

The Molecules of Life

hands. This gives a fair impression of typical protein structure at very low resolution.

alphaα-helices and betaβ-sheets are called secondary structures (the primary structure is the

sequence of amino acids). The arrangement of secondary structure elements is called

the tertiary structure. Quaternary structure denotes arrangements of individual folded

peptide chains (e.g., subunits) to form supramolecular complexes. Quinary structure

is the network of other proteins with which a protein interacts.

The number of basic shapes in which proteins fold (i.e., the variety of tertiary

structures) seems to be far smaller (tilde 10 Superscript 4∼10⁴) than the number of possible sequences.

Individual examples of sequences with less than 10% homology folding into essen-

tially the same structure are known. Some folds are very common, whereas others

are rare.

15.6

Polysaccharides

Monosaccharides (sugars) are carbohydrates whose chemical composition is given

by the empirical formula (CHSubscript 22O)Subscript nn, with typicallyn equals 3 comma 4 comma 5 comman = 3, 4, 5, and 6. They are linked

together via one of their oxygen atoms in an ether-like linkage to form oligomers and

polymers. Saccharide monomers have many –OH groups, and there is much variety

in their choice for linking. Some oligosaccharides are metabolic intermediates; they

are very often used to modify proteins and lipids, with profound influence on their

structure and reactivity.¹⁶ For example, if one sugar is missing from transferring,

an iron-transporting protein in the blood with several glycosylated amino acids, the

bearer has an abnormal skin colour, liver problems, and so forth. Oligosaccharides

are extensively used to confer specificity of binding (e.g., in the immune system).

Longer polysaccharides are used to store energy and as structural components. Their

assembly is not templated but is accomplished by enzymes. There is considerable

variety in the sequence of nominally identical heteroöligosaccharides.

Cellulose is a long unbranched chain of glucose monomers linked from head to

tail. As the major constituent of plant cell walls, cellulose is probably more abundant

on Earth than any other organic material. The chains are packed side by side to form

microfibrils, which are typically a mixture of two crystalline forms, ISubscript alphaα and ISubscript betaβ, and

whose diameter ranges from about 3 nm in most plants to about 20 nm in sea squirts.

The chains are held together by H-bonds.¹⁷

Glycomics encompasses both the numerous glycans (polysaccharides) that exist

independently and those that are conjugated—polysaccharides linked to proteins and

lipids. Among the latter, the mucins are especially important.¹⁸ By such conjugation

the variety of proteins and lipids can be enormously increased.

16 See Dwek and Butters (2002) for an overview.

17 See also “Symbols for specifying the confirmation of polysaccharide chains”, Eur. J. Biochem.

131 (1983) 5–7, or Pure Appl. Chem. 55 (1983) 1269–1272.

18 Perez-Vilar and Hill (1999).