Proteins Vary in Their Biological Roles

Functionally, proteins may be divided into four main categories: structural proteins, enzymes, regulatory proteins and transport proteins.

1. Structural proteins make up many sub-cellular structures. The flagella with which bacteria swim around, the microtubules used to control traffic flow inside cells of higher organisms, the fibers involved in contractions of a muscle cell, and the outer coats of viruses are a few examples of structures constructed using proteins.

2. Enzymes are proteins that facilitate chemical reactions. An enzyme first binds another molecule, known as its substrate, and then performs some chemical operations with it. Some enzymes bind only a single substrate molecule; others may bind two or more, and react them together to make the final product. In any case, the enzyme needs an active site, a pocket or cleft in the protein, where the substrate binds and the reaction occurs. The active site of the protein is produced by the folding up of its polypeptide chain correctly so that amino acid residues that were spread out at great distances in the linear chain now come together and will cooperate in binding the substrate to facilitate the enzyme reaction (Fig. 3.27).

3. Although regulatory proteins are not enzymes, they do bind other molecules and so they also need active sites to accommodate these. Regulatory proteins vary enormously. Many of them can bind both small signal molecules and DNA. The presence or absence of the signal molecule determines whether or not the gene is switched on (Fig. 3.28).

active site Special site or pocket on a protein where the substrate binds and the enzyme reaction occurs regulatory protein A protein that regulates the expression of a gene or the activity of another protein structural protein A protein that forms part of a cellular structure substrate The molecule altered by the action of an enzyme transport protein A protein that carries other molecules across membranes or around the body

Signal molecule b

Regulatory protein changes shape...

Regulatory protein changes shape...

FIGURE 3.28 Protein

Regulatory

Regulatory proteins usually exist in two conformations. Receiving a signals promotes a change in shape. The regulatory protein may then bind to DNA and alter the expression of a gene.

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