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Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid (used in the biosynthesis of proteins), although it does not contain the amino group but is rather a secondary amine. The secondary amine nitrogen is in the protonated form (NH2+) under biological conditions, while the carboxyl group is in the deprotonated −COO− form. The "side chain" from the α carbon connects to the nitrogen forming a pyrrolidine loop, classifying it as a aliphatic amino acid. It is non-essential in humans, meaning the body can synthesize it from the non-essential amino acid L-glutamate. It is Genetic code by all the starting with CC (CCU, CCC, CCA, and CCG).
Proline is the only proteinogenic amino acid which is a secondary amine, as the nitrogen atom is attached both to the α-carbon and to a chain of three carbons that together form a five-membered ring.
The name proline comes from pyrrolidine, one of its constituents.
A diet rich in proline was linked to an increased risk of depression in humans in a study from 2022 that was tested on a limited pre-clinical trial on humans and primarily in other organisms. Results were significant in the other organisms.
Peptide bond formation with incoming Pro-tRNAPro in the ribosome is considerably slower than with any other tRNAs, which is a general feature of N-alkylamino acids.. 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.
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 thermophile organisms. Protein secondary structure can be described in terms of the φ, ψ and ω of the protein backbone. The cyclic structure of proline's side chain locks the angle φ at approximately −65°.
Proline acts as a structural disruptor in the middle of regular secondary structure elements such as alpha helix and ; however, proline is commonly found as the first residue of an alpha helix and also in the edge strands of . Proline is also commonly found in turns (another kind of secondary structure), and aids in the formation of beta turns. This may account for the curious fact that proline is usually solvent-exposed, despite having a completely aliphatic side chain.
Multiple prolines and/or in a row can create a polyproline helix, the predominant secondary structure in collagen. The hydroxylation of proline by prolyl hydroxylase (or other additions of electron-withdrawing substituents such as fluorine) increases the conformational stability of collagen significantly. Hence, the hydroxylation of proline is a critical biochemical process for maintaining the connective tissue of higher organisms. Severe diseases such as scurvy can result from defects in this hydroxylation, e.g., mutations in the enzyme prolyl hydroxylase or lack of the necessary vitamin C cofactor.
From a kinetic standpoint, cis– trans proline isomerization is a very slow process that can impede the progress of protein folding by trapping one or more proline residues crucial for folding in the non-native isomer, especially when the native protein requires the cis isomer. This is because proline residues are exclusively synthesized in the ribosome as the trans isomer form. All organisms possess prolyl isomerase to catalyze this isomerization, and some bacteria have specialized prolyl isomerases associated with the ribosome. However, not all prolines are essential for folding, and protein folding may proceed at a normal rate despite having non-native conformers of many X–Pro peptide bonds.
In brewing, proteins rich in proline combine with polyphenols to produce haze (turbidity). Accessed July 12, 2010.
L-Proline is an osmoprotectant and therefore is used in many pharmaceutical and biotechnological applications.
The growth medium used in plant tissue culture may be supplemented with proline. This can increase growth, perhaps because it helps the plant tolerate the stresses of tissue culture. For proline's role in the stress response of plants, see .
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, instead, produces an orange-yellow colour.
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