Pro·te·in n.  Physiol. Chem. any polymer of an amino acid joined by peptide (amide) bonds.  Most natural proteins have alpha-amino acids as the monomeric constituents.  All classical enzymes are composed of protein, and control most of the biochemical transformations carrie dout in living cells.  They may be soluble, as casein, albumins, and other globular proteins, or insoluble (e. g. "structural proteins"), as collagen or keratin.  "albumin", an older term for protein, is now used primarily to refer to certain specific soluble globular proteins found in eggs or blood serum, e.g.  bovine serum albumin, the main soluble protein in teh serum of cattle, used as an enzymatically inert protein in biochemical research.
 Note: In the 1913 dictionary, protein was defined as: “A body now known as alkali albumin, but originally considered to be the basis of all albuminous substances, whence its name.”
 Protein crystal. Bot. See Crystalloid, n., 2.

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 Crys·tal·loid, n.
 1. Chem. A body which, in solution, diffuses readily through animal membranes, and generally is capable of being crystallized; -- opposed to colloid.
 2. Bot. One of the microscopic particles resembling crystals, consisting of protein matter, which occur in certain plant cells; -- called also protein crystal.

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