Xaa-Pro dipeptidase, also known as prolidase, hydrolizes dipeptides having proline as the second residue. Enzymes of this type have been found in mammals, bacteria, and archeon Pyrococcus horikoshii. In humans, prolidase is involved in the final stage of the degradation of endogenous and dietary proteins, and is particularly important in collagen catabolism. Mutations in the gene encoding for human prolidase cause an autosomal recessive disorder characterized by skin lesions, mental retardation, and recurrent infections. The physiological role of the enzyme in bacteria is not established. In general, proline-specific peptidases participate with other peptidases in the terminal degradation of intracellular proteins, and may function in the recycling of proline.