15.5 Proteins

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Fig. 15.5 A piece of RNA (from the Qbetaβ replicase MDV-1) showing the characteristic loops formed

by single-strand base-pairing

15.5

Proteins

Proteins are appropriately named after the mythological being Proteus, who could

assume many forms. The main functions of proteins are structural and catalytic. The

catalytic function is especially important, for almost all of the other macromolecules

of life, as well as small metabolites, are synthesized with the help of enzymes (cata-

lysts). A rough overview of the protein world reveals the existence of the following.

Small polypeptides typically with no definite structure, acting as hormones, toxins,

and so forth⁷ (examples: bradykinin, mellitin);

Globular proteins typically able to assume a small number of stable configurations.

This is the most numerous and varied class of proteins, comprising enzymes, trans-

porters, regulators, motors, and so forth (examples: glucose oxidase, haemoglobin,

kinesin, and tumour necrosis factoralphaα). Others in this class can polymerize to form

fairly rigid rods (examples: flagellin, tubulin);

Fibrous proteins, which may be very long. They often have modular structures

with many identical or at least very similar modules, which are folded up into

small globules (“globulets”) joined by short linker sections (“beads on a string”).⁸

Their rôle is mostly structural, both within and without the cell, but they actively

interact with objects in their environment (e.g., neurites growing on them; i.e.,

as extracellular basement membranes they show chemical specificity) (examples:

actin, collagen, and laminin);

7 See Zamyatnin et al. (2006).

8 Rocco et al. (1987) describe this for fibronectin.