Gene—protein relationships

Abstract

Proteins play a great diversity of essential roles in all living cells; in the comparatively simple bacterium E. coli there are about 2000 different types of protein, while in humans the number is estimated at 100 000. Many proteins play largely structural roles; thus, collagen, a fibrous protein, is an important component of bones, tendons and ligaments, myosin is the main muscle protein, and the protein coat of a virus encapsulates and protects the viral nucleic acid. Other proteins, such as the myoglobins and haemoglobins, are involved in oxygen transport and storage, while hormones regulate various types of chemical activity in the cell. Many proteins are enzymes, such as DNA polymerase and RNA polymerase, biological catalysts which promote or speed up specific biochemical reactions in the cell without becoming changed themselves, so that a single molecule of an enzyme can catalyse the synthesis or breakdown of many hundreds of substrate molecules. Each enzyme has an active site which binds the specific substrate molecules, forming an enzyme–substrate complex; this brings closer together the reactive groups of the substrate molecules and promotes the formation of covalent bonds between them. After the covalent bonds have formed, the products are released and the enzyme molecule recycles. Each enzyme is highly specific, and usually, catalyses one specific biochemical reaction.