protein metabolism
all the biochemical reactions involved in the manufacture and breakdown of proteins. The body makes a large variety of complex proteins from amino acids, including enzymes, antibodies, certain hormones (e.g., insulin), and structural proteins, such as keratin, collagen, and the actin and myosin molecules of muscle. Proteins are broken down into their constituent amino acids during the basic turnover of cell proteins as well as for the elimination of toxic peptides and abnormal proteins. In case of severe energy starvation, with depletion of the body’s fat reserves, protein can be broken down to provide carbon to make glucose, but this impairs normal body functions.