Proteins


A protein is a macromolecule with amino acids as its monomeric subunits. A polypeptide chain is a single chain of amino acids, covalently linked by peptide bonds. A protein may consist of either a single, or multiple polypeptide chains. Proteins are the 'chief workers' in the cell, having numerous roles in both cellular structure and in the catalysis of virtually all biochemical reactions (as enzymes). The information stored in DNA is used to dictate the chemical structure, and thus function, of all cellular proteins via the series of processes that constitute protein biosynthesis.

The particular sequence of amino acids on a polypeptide chain is said to be its primary structure; the way this sequence folds due to bonding interactions between amino acid residues is referred to as its secondary structure; and its final 3D conformation is its tertiary structure. Proteins may consist of one polypeptide chain, or several polypeptides that are non-covalently linked (proteins consisting of several polypeptides are said to have, in addition to the above, a quaternary structure).

Amino acids have the following generalized structure:


external image aminoacid.gif
where COOH is the carboxylic acid group and H2N is the amino group. The R-group (sometimes called the 'side chain') is variable between amino acids: it may be hydrophilic or hydrophobic; acidic, basic or non-polar. Because the R-group is variable, it is this group which gives each amino acid its unique chemical properties. Amino acids join chemically by way of a double covalent bond called a peptide bond. This involves a condensation reaction between the carboxylic acid group of one amino acid and the amino group of its adjacent partner. At one end of a full polypeptide chain, there is an exposed carboxyl group - the C-terminus - and an exposed amino group - the N-terminus. Polypeptide chains are synthesised from the N terminus to the C terminus, and each amino acid is coded for by a triplet codon in DNA.

As well as being the monomeric constituents of proteins, amino acids have a number of biochemical roles as precursors in biosynthetic pathways. Significantly, amino acids are often the starter molecules in the metabolic pathways that generate nucleobases; the building blocks of DNA and RNA .

Proteins may have structural or functional roles in the cell (e.g. intrinsic proteins are found in the plasma membrane and regulate substance transport, while enzymes are proteins which act as biological catalysts). DNA regulates cellular activity by commanding the exact primary structure (and thereby, the subsequent structures) of cellular proteins. By regulating structure, DNA effectively determines the precise function of a protein.