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Hydroxyproline is found in few proteins other than collagen. For this reason, hydroxyproline content has been used as an indicator to determine collagen and/or gelatin amount. However, the mammalian proteins elastin and argonaute 2 have collagen-like domains in which hydroxyproline is formed.
In 2001, biologically active hydroxyproline-rich glycopeptides were isolated from tobacco which activated the production of protease inhibitors in a similar way to systemin in tomatoes. [1] Although they are structurally unrelated to systemins, their similar function resulted in them being named hydroxyproline-rich systemins (HypSys).
Two tyrosines separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink. [11] This can be crosslinked further by the enzyme extensin peroxidase [12] [13] [14] to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.
Hydroxylation occurs at the γ-C atom, forming hydroxyproline (Hyp), which stabilizes the secondary structure of collagen due to the strong electronegative effects of oxygen. [9] Proline hydroxylation is also a vital component of hypoxia response via hypoxia inducible factors .
Procollagen-proline dioxygenase catalyzes the following reaction: L-proline + alpha-ketoglutaric acid + O 2 → (2S, 4R)-4-hydroxyproline + succinate + CO 2. The mechanism for the reaction is similar to that of other dioxygenases, and occurs in two distinct stages: [3] In the first, a highly reactive Fe(IV)=O species is produced.
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 cyclic structure of proline's side chain locks the angle φ at approximately −65°. [19] Proline acts as a structural disruptor in the middle of regular secondary structure elements such as alpha helices and beta sheets; however, proline is commonly found as the first residue of an alpha helix and also in the edge strands of beta sheets.
Adams E; Goldstone A (1960). "Hydroxyproline metabolism. III. Enzymatic synthesis of hydroxyproline from Delta1-pyrroline-3-hydroxy-5-carboxylate".
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