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Hydroxyproline is a major component of the protein collagen, [3] comprising roughly 13.5% of mammalian collagen. Hydroxyproline and proline play key roles for collagen stability. [4] They permit the sharp twisting of the collagen helix. [5]
In protein, hydroxyproline is incorporated into protein by hydroxylation of proline. Pipecolic acid, a heavier analog of proline, is found in efrapeptin. Sarcosine is a N-methylized glycine so its methyl group is used in many biochemical reactions. Azetidine-2-carboxylic acid, which is a smaller homolog of proline in plants.
Proline or hydroxyproline constitute about 1/6 of the total sequence. With glycine accounting for the 1/3 of the sequence, this means approximately half of the collagen sequence is not glycine, proline or hydroxyproline, a fact often missed due to the distraction of the unusual GX 1 X 2 character of collagen alpha
Furthermore, proline is rarely found in α and β structures as it would reduce the stability of such structures, because its side chain α-nitrogen can only form one nitrogen bond. Additionally, proline is the only amino acid that does not form a red-purple colour when developed by spraying with ninhydrin for uses in chromatography. Proline ...
In 1954, Ramachandran & Kartha (13, 14) advanced a structure for the collagen triple helix on the basis of fiber diffraction data. It consists of a triple helix made of the repetitious amino acid sequence glycine-X-Y, where X and Y are frequently proline or hydroxyproline. [2] [3] Collagen folded into a triple helix is known as tropocollagen.
The most frequently hydroxylated amino acid residue in human proteins is proline. This is because collagen makes up about 25–35% of the protein in our bodies and contains a hydroxyproline at almost every 3rd residue in its amino acid sequence. Collagen consists of both 3‐hydroxyproline and 4‐hydroxyproline residues. [6]
When proline is incorporated into the Y position of the Gly-X-Y sequence, it is post-translationally modified to hydroxyproline. [10] The hydroxyproline can enter into favorable interactions with water, which stabilizes the triple helix because the Y residues are solvent-accessible in the triple helix structure.
In enzymology, a proline 3-hydroxylase (EC 1.14.11.28) is an enzyme that catalyzes the chemical reaction. L-proline + 2-oxoglutarate + O 2 cis-3-hydroxy-L-proline + succinate + CO 2. The 3 substrates of this enzyme are L-proline, 2-oxoglutarate, and O 2, whereas its 3 products are cis-3-hydroxy-L-proline, succinate, and CO 2.