A molecule composed of long chains of amino acids linked by peptide bonds.
Proteins make up about half of the dry weight of the bodies of living organisms.
The amino acid sequence determines the final three-dimensional shape of the protein molecule. Globular proteins include enzymes, antibodies, and many other types. Some proteins, such as haemoglobin, show 'quaternary' structure, in which complete, 'tertiary' level molecules are fitted together to make a larger protein assembly. In the human diet, proteins are needed to repair or replace existing molecules, or for growth, but may also be exploited as an energy source. The Bloomsbury Good Word Guide 2.