Prostate gland. A gland of male mammals that surrounds and opens into the urethra where it leaves the bladder. During ejaculation it secretes a fluid into semen that activates the sperms and prevents them from sticking together.

Prosthetic group. A tightly bound nonpeptide inorganic or organic component of a protein. Prosthetic groups may be lipids, carbohydrates, metal ions, phosphate groups, etc. Some coenzymes are more correctly regarded as prosthetic groups.

Protamine. Any of a group of proteins of relatively low molecular weight found in association with the chromosomal DNA of vertebrate male germ cells. They contain a single polypeptide chain comprising about 67% arginine. Protamines are thought to protect and support the chromosomes.

Protease. Any enzyme that splits proteins such as Pepsin, Trypsin, Erepsin or Rennin.

Proteolysis. The splitting of protein molecular by the hydrolysis of their Peptide Bonds.

Proteolytic enzyme. Any enzyme taking part in breaking down of proteins. A system of several such peptidases is necessary to break proteins down to constituent amino acids, and these frequently occur, e.g. in plant and animal cells and in plant seeds and animal digestive juices.

Prothallus. Independent, gametophyte plant of Pteridophyta (ferns and related plants). Small, green parenchymatous thallus bearing antheridia and archegonia; shows little differentiation. Usually prostrate on soil surface attached by rhizoids; may be subterranean, mycotrophic.