In bacteria, repressor proteins are important control elements that silence specific segments of the DNA of a cell. In higher organisms, growth and differentiation are controlled by growth factor proteins. For example, nerve growth factor guides the formation of neural networks. The activities of different cells in multicellular organisms are coordinated by hormones. The simplest amino acid is glycine which has just a hydrogen atom as its side chain.
Alanine comes next, with a methyl group as it side chain.
Larger hydrocarbon side chains are found in valine leucine, and isoleucine . The different sizes and shapes of these hydrocarbon side chains enable them to pack together to form compact structures with few holes. Proline also has an aliphatic side chain but it differs form other members of the set of twenty in that its side chain is bonded to both nitrogen and carbon atom. The resulting cyclic structure markedly influences protein architecture. Proline, often found in the bends of folded protein chains, is not averse to being exposed to water.