Protein in bodybuilding: Protein metabolism (II)

Proteins are important molecules comprised of amino acids — compounds containing an amino group (–NH 2), a carboxylic acid group (–COOH), and a radical group (different for each of the amino acids). Structural proteins include cytoskeletal proteins such as dystrophin, vimentin, and desmin, and connective tissue proteins such as collagen; regulatory proteins include enzymes such as lactate dehydrogenase, citrate synthase, or cytochrome c oxidase. There are 20 amino acids that are found as constituents of proteins or present as free amino acids.

Nine amino acids are onsidered essential or indispensable (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine), and arginine is sometimes considered to be conditionally essential. The essential amino acids must come from the diet or from endogenous protein breakdown. Since proteins serve such critical roles in the survival of the organism, it is not surprising that their metabolism is complex, tightly regulated, and in a constant state of flux with simultaneous synthesis and degradation.