Protein

Proteins are major constituents of all living organisms, from the single-celled bacterium and the submicroscopic virus to complex animals like ourselves. There are thousands of different proteins known to exist, and an advanced animal or plant may be capable of producing as many as 10,000 distinct protein types during its lifetime. Biochemically, proteins are large molecules that are formed by the biological polymerization of amino acids. Each protein produced has a specific structure and a particular function, and proteins by their action enable the living cell to grow and perform all the activities necessary for its survival.

Structure

A protein consists of one or more chains of amino acids linked together by means of peptide bonds, and each protein has a unique structure that is determined at the molecular level by the particular amino acids in the chain and their sequence. There are only 20 common amino acids, and all proteins are assembled from these basic building blocks. Plants are able to form all of the amino acids they need, but animals must obtain some of them from food. A peptide bond is formed as a result of a condensation reaction between the amino group, –NH₂, of one amino acid and the carboxylic acid group, –COOH, of another amino acid:

Chains of amino acids linked together by means of peptide bonds are sometimes called polypeptides. The peptide bond is the only type of covalent bond found between amino acid units in proteins, apart from the disulfide bridge, –S–S–, which sometimes occurs between two units of the sulfur-containing amino acid cysteine, HSCH₂CH(NH₂)–COOH, to link together different parts of a protein chain. Weaker ionic and hydrogen bonds may also occur between atoms in the chain. It is the order, or sequence, in which the amino acids are arranged in the chain that confers three-dimensional structure, and therefore a particular function, on a given protein. Theoretically, the number of ways a polypeptide chain could fold up is enormous. However, X-ray crystallography has shown that, for a given protein, only one particular folded shape, or conformation, is preferred. Certain sequences seem to specify coils, or a-helices, or so-called b-pleated sheets, while others commonly produce sharp bends in the polypeptide chain. Most of the protein’s structure is, however, unique to that particular molecule, and the mechanisms that stabilize these complex conformations are incompletely understood. One thing is certain, though, and that is that certain amino acids (the hydrophobic, or water-repelling, amino acids) prefer to be near the center of the protein molecule, and some amino acids (the hydrophilic, or water-attracting, amino acids) prefer the outside of the molecule. In all known protein structures, the polypeptide chain folds so that hydrophilic amino acids tend to be on the outside, while the center of the molecule forms a hydrophobic core. Two main types of protein—fibrous and globular—result from the chemical bonds linking the protein chains. Fibrous proteins form long chains that may run parallel to one another and are connected by cross-links. These types of protein are very stable and often physically strong. Globular proteins, as their name suggests, are folded in such a way that they form globules. These proteins have irregular amino acid sequences and are relatively unstable.