Hence, the hydroxylation of proline is a critical biochemical process for maintaining the connective tissue of higher organisms. The hydroxylation of proline by prolyl hydroxylase (or other additions of electron-withdrawing substituents such as fluorine) increases the conformational stability of collagen significantly.
Multiple prolines and/or hydroxyprolines in a row can create a polyproline helix, the predominant secondary structure in collagen. This may account for the curious fact that proline is usually solvent-exposed, despite having a completely aliphatic side chain. Proline is also commonly found in turns (another kind of secondary structure), and aids in the formation of beta turns. 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. The cyclic structure of proline's side chain locks the angle φ at approximately −65°. Protein secondary structure can be described in terms of the dihedral angles φ, ψ and ω of the protein backbone.
The exceptional conformational rigidity of proline affects the secondary structure of proteins near a proline residue and may account for proline's higher prevalence in the proteins of thermophilic organisms.
Peptide bond formation is also slow between an incoming tRNA and a chain ending in proline with the creation of proline-proline bonds slowest of all. Peptide bond formation with incoming Pro-tRNA Pro is considerably slower than with any other tRNAs, which is a general feature of N-alkylamino acids. When proline is bound as an amide in a peptide bond, its nitrogen is not bound to any hydrogen, meaning it cannot act as a hydrogen bond donor, but can be a hydrogen bond acceptor. It also affects the rate of peptide bond formation between proline and other amino acids. The distinctive cyclic structure of proline's side chain gives proline an exceptional conformational rigidity compared to other amino acids. In plants, proline accumulation is a common physiological response to various stresses but is also part of the developmental program in generative tissues (e.g. It has been proposed to be a potential endogenous excitotoxin. L-Proline has been found to act as a weak agonist of the glycine receptor and of both NMDA and non-NMDA ( AMPA/ kainate) ionotropic glutamate receptors. Zwitterionic structure of both proline enantiomers: ( S)-proline (left) and ( R)-proline Biological activity This can then either spontaneously cyclize to form 1-pyrroline-5-carboxylic acid, which is reduced to proline by pyrroline-5-carboxylate reductase (using NADH or NADPH), or turned into ornithine by ornithine aminotransferase, followed by cyclisation by ornithine cyclodeaminase to form proline. Glutamate-5-semialdehyde is first formed by glutamate 5-kinase (ATP-dependent) and glutamate-5-semialdehyde dehydrogenase (which requires NADH or NADPH). Proline is biosynthetically derived from the amino acid L- glutamate. The name proline comes from pyrrolidine, one of its constituents. The next year, Emil Fischer isolated proline from casein and the decomposition products of γ-phthalimido-propylmalonic ester, and published the synthesis of proline from phthalimide propylmalonic ester. Proline was first isolated in 1900 by Richard Willstätter who obtained the amino acid while studying N-methylproline, and synthesized proline by the reaction of sodium salt of diethyl malonate with 1,3-dibromopropane.
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