Proline (symbolPro orP)[4] is an organic acid classed as aproteinogenic amino acid (used in thebiosynthesis of proteins), although it does not contain theamino group-NH 2 but is rather asecondary amine. The secondary amine nitrogen is in the protonated form (NH2+) under biological conditions, while thecarboxyl group is in thedeprotonated −COO− form. The "side chain" from theα carbon connects to the nitrogen forming apyrrolidine loop, classifying it as aaliphaticamino acid. It is non-essential in humans, meaning the body can synthesize it from the non-essential amino acidL-glutamate. It isencoded by all thecodons starting with CC (CCU, CCC, CCA, and CCG).
Proline is the only proteinogenicamino 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.
Proline was first isolated in 1900 byRichard Willstätter who obtained the amino acid while studyingN-methylproline, and synthesized proline by the reaction of sodium salt ofdiethyl malonate with1,3-dibromopropane. The next year,Emil Fischer isolated proline fromcasein and the decomposition products of γ-phthalimido-propylmalonic ester,[5] and published the synthesis of proline from phthalimide propylmalonic ester.[6]
The name proline comes frompyrrolidine, one of its constituents.[7]
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.[16]
The distinctive cyclic structure of proline's side chain gives proline an exceptional conformational rigidity compared to other amino acids. It also affects the rate of peptide bond formation between proline and other amino 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 ahydrogen bond donor, but can be a hydrogen bond acceptor.
Peptide bond formation with incoming Pro-tRNAPro in the ribosome is considerably slower than with any other tRNAs, which is a general feature ofN-alkylamino acids.[17] 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.[18]
The exceptional conformational rigidity of proline affects thesecondary structure of proteins near a proline residue and may account for proline's higher prevalence in the proteins ofthermophilic organisms.Protein secondary structure can be described in terms of thedihedral anglesφ, ψ and ω[broken anchor] of the protein backbone. 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 regularsecondary structure elements such asalpha helices andbeta sheets; however, proline is commonly found as the first residue of analpha helix and also in the edge strands ofbeta sheets. Proline is also commonly found inturns (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 completelyaliphatic side chain.
Multiple prolines and/orhydroxyprolines in a row can create apolyproline helix, the predominantsecondary structure incollagen. Thehydroxylation of proline byprolyl hydroxylase (or other additions of electron-withdrawing substituents such asfluorine) increases the conformational stability ofcollagen significantly.[20] Hence, the hydroxylation of proline is a critical biochemical process for maintaining theconnective tissue of higher organisms. Severe diseases such asscurvy can result from defects in this hydroxylation, e.g., mutations in the enzyme prolyl hydroxylase or lack of the necessaryascorbate (vitamin C) cofactor.
Peptide bonds to proline, and to otherN-substituted amino acids (such assarcosine), are able to populate both thecis andtrans isomers. Most peptide bonds overwhelmingly adopt thetrans isomer (typically 99.9% under unstrained conditions), chiefly because the amide hydrogen (trans isomer) offers less steric repulsion to the preceding Cα atom than does the following Cα atom (cis isomer). By contrast, thecis andtrans isomers of the X-Pro peptide bond (where X represents any amino acid) both experience steric clashes with the neighboring substitution and have a much lower energy difference. Hence, the fraction of X-Pro peptide bonds in thecis isomer under unstrained conditions is significantly elevated, withcis fractions typically in the range of 3-10%.[21] However, these values depend on the preceding amino acid, with Gly[22] and aromatic[23] residues yielding increased fractions of thecis isomer.Cis fractions up to 40% have been identified for aromatic–proline peptide bonds.[24]
From a kinetic standpoint,cis–trans prolineisomerization is a very slow process that can impede the progress ofprotein folding by trapping one or more proline residues crucial for folding in the non-native isomer, especially when the native protein requires thecis isomer. This is because proline residues are exclusively synthesized in theribosome as thetrans isomer form. All organisms possessprolyl isomeraseenzymes to catalyze this isomerization, and somebacteria 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.
Proline is one of the two amino acids that do not follow along with the typicalRamachandran plot, along withglycine. Due to the ring formation connected to the beta carbon, theψ andφ angles about the peptide bond have fewer allowable degrees of rotation. As a result, it is often found in "turns" of proteins as its free entropy (ΔS) is not as comparatively large to other amino acids and thus in a folded form vs. unfolded form, the change in entropy is smaller. 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 withninhydrin for uses inchromatography. Proline, instead, produces an orange-yellow colour.
^Plimmer RH (1912) [1908], Plimmer RH, Hopkins FG (eds.),The chemical composition of the proteins, Monographs on biochemistry, vol. Part I. Analysis (2nd ed.), London: Longmans, Green and Co., p. 130, retrievedSeptember 20, 2010
^Shrestha A, Cudjoe DK, Kamruzzaman M, Siddique S, Fiorani F, Léon J, Naz AA (June 2021). "Abscisic acid-responsive element binding transcription factors contribute to proline synthesis and stress adaptation in Arabidopsis".Journal of Plant Physiology.261 153414.Bibcode:2021JPPhy.26153414S.doi:10.1016/j.jplph.2021.153414.PMID33895677.S2CID233397785.
^Thomas KM, Naduthambi D, Zondlo NJ (February 2006). "Electronic control of amidecis–trans isomerism via the aromatic-prolyl interaction".Journal of the American Chemical Society.128 (7):2216–2217.doi:10.1021/ja057901y.PMID16478167.
^Siebert KJ."Haze and Foam".Cornell AgriTech.Archived from the original on 2010-07-11. Retrieved2010-07-13. Accessed July 12, 2010.
^Pazuki A, Asghari J, Sohani MM, Pessarakli M, Aflaki F (2015). "Effects of Some Organic Nitrogen Sources and Antibiotics on Callus Growth of Indica Rice Cultivars".Journal of Plant Nutrition.38 (8):1231–1240.Bibcode:2015JPlaN..38.1231P.doi:10.1080/01904167.2014.983118.S2CID84495391.
Balbach J, Schmid FX (2000). "Proline isomerization and its catalysis in protein folding". In Pain RH (ed.).Mechanisms of Protein Folding (2nd ed.). Oxford University Press. pp. 212–249.ISBN978-0-19-963788-1..
