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Hydroxyproline is a major component of the proteincollagen, comprising roughly 13.5% of mammalian collagen. Hydroxyproline and proline play key roles for collagen stability. They permit the sharp twisting of the collagen helix. In the canonical collagen Xaa-Yaa-Gly triad (where Xaa and Yaa are any amino acid), a proline occupying the Yaa position is hydroxylated to give a Xaa-Hyp-Gly sequence. This modification of the proline residue increases the stability of the collagen triple helix. It was initially proposed that the stabilization was due to water molecules forming a hydrogen bonding network linking the prolyl hydroxyl groups and the main-chain carbonyl groups. It was subsequently shown that the increase in stability is primarily through stereoelectronic effects and that hydration of the hydroxyproline residues provides little or no additional stability.
In addition to collagen, the mammalian proteins elastin and argonaute 2 have collagen-like domains in which hydroxyproline is formed. Some snail poisons, conotoxins, contain hydroxyproline, but lack collagen-like sequences.
Increased serum and urine levels of hydroxyproline have also been demonstrated in Paget's disease.
Other hydroxyprolines also exist in nature. The most notable ones are 2,3-cis-, 3,4-trans-, and 3,4-dihydroxyproline, which occurs in diatomcell walls and are postulated to have a role in silica deposition. Hydroxyproline is also found in the walls of oomycetes, fungus-like protists related to diatoms. (2S,4S)-cis-4-Hydroxyproline is found in the toxic cyclic peptides from Amanita mushrooms (e.g., phalloidin).
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^Nelson, D. L. and Cox, M. M. (2005) Lehninger's Principles of Biochemistry, 4th Edition, W. H. Freeman and Company, New York.
^Brinckmann, J., Notbohm, H. and Müller, P.K. (2005) Collagen, Topics in Current Chemistry 247, Springer, Berlin.
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