Proline is a nonessential amino acid, which means that it
is manufactured from other amino acids in the liver; it does not have to be
obtained directly through the diet.
Proline is the precursor to hydroxyproline, which is a
major amino acid found in the connective tissue of the body - collagen. Proline
is different from other amino acids in that it has a secondary amino group and
contains a pyrrole ring, such as found in hemoglobin and the cytochromes. This
structure causes proline in a protein to impart a rigid protein structure.
Proline has no known therapeutic use. Its effect in human
nutrition, other than as a source of nitrogen, is not well established.
Supplementation with proline has not been found to increase collagen formation.
Proline is an unusual, cyclic alpha-amino acid. As a
consequence of its structure, it imparts a unique set of physical
characteristics to proteins in which it is incorporated. Collagen, which is the
most abundant protein in the body, makes up much of the organic mass of skin,
tendon, blood vessels, bone, cornea, vitreous humor of the eye, and basement
membranes of all organs. Collagen is composed of the metabolic by-product of
proline called hydroxyproline. A closely related protein, elastin, is found in
the elastic, yellow, connective tissue fibers, which are abundant in ligaments
and walls of blood vessels. Both collagen and elastin are rich in
hydroxyproline, which comes from proline by way of a hydroxylation reaction
requiring adequate levels of vitamin C (ascorbic acid).
Proline is incorporated within a growing protein chain at
the ribosome and is then hydroxylated into hydroxyproline as the growing
peptide chains are still attached to the ribosome along with rough, endoplasmic
reticulum. There a number of genetic defects that have been identified as they
related to the synthesis of collagen which produce connective tissue
difficulties. These include Ehlers-Danlos syndrome, which, in some instances,
is accompanied with recurrent joint dislocations and curvature of the spine.
Osteoarthritis may also be related to abnormalities in collagen structure as
well as the bone disease, osteogenesis imperfecta.