1.Substitution of proline32 by α-methylproline preorganizes β2-microglobulin for oligomerization but not for aggregation into amyloids.
Torbeev V1, Ebert MO, Dolenc J, Hilvert D. J Am Chem Soc. 2015 Feb 25;137(7):2524-35. doi: 10.1021/ja510109p. Epub 2015 Feb 16.
Conversion of soluble folded proteins into insoluble amyloids generally proceeds in three distinct mechanistic stages: (1) initial protein misfolding into aggregation-competent conformers, (2) subsequent formation of oligomeric species and, finally, (3) self-assembly into extended amyloid fibrils. In the work reported herein, we interrogated the amyloidogenesis mechanism of human β2-microglobulin (β2m), which is thought to be triggered by a pivotal cis-trans isomerization of a proline residue at position 32 in the polypeptide, with nonstandard amino acids. Using chemical protein synthesis we prepared a β2m analogue in which Pro32 was replaced by the conformationally constrained amino acid α-methylproline (MePro). The strong propensity of MePro to adopt a trans prolyl bond led to enhanced population of a non-native [trans-MePro32]β2m protein conformer, which readily formed oligomers at neutral pH. In the presence of the antibiotic rifamycin SV, which inhibits amyloid growth of wild-type β2m, [MePro32]β2m was nearly quantitatively converted into different spherical oligomeric species.
2.Beta-turns induced in bradykinin by (S)-alpha-methylproline.
Welsh JH1, Zerbe O, von Philipsborn W, Robinson JA. FEBS Lett. 1992 Feb 10;297(3):216-20.
The ability of (S)-alpha-methylproline (alpha-MePro) to stabilise reverse-turn conformations in the peptide hormone bradykinin (BK = Arg1-Pro2-Pro3-Gly4-Phe5-Ser6-Pro7-Phe8-Arg9) has been investigated. Two BK analogues containing alpha-MePro at position 3 or position 7 were synthesised and their conformations in aqueous solution investigated by NMR spectroscopy. Whereas BK is largely disordered on the NMR time scale both analogues showed ROE connectivities in 2D-ROESY spectra indicative of reverse-turn conformations at both Pro2-Phe5 and Ser6-Arg9, whose formation appears to be cooperative. Some potential applications of alpha-MePro as a reverse-turn mimetic in the construction of synthetic peptide libraries is discussed.
3.Conformational analysis of homochiral and heterochiral diprolines as beta-turn-forming peptidomimetics: unsubstituted and substituted models.
Baures PW1, Ojala WH, Gleason WB, Johnson RL. J Pept Res. 1997 Jul;50(1):1-13.
The effect of replacing one of the proline residues in either unsubstituted homochiral or heterochiral diproline segments with either a 2- or a 3-substituted prolyl residue on the allowed conformational of the diproline template has been examined. In heterochiral (L-D) diprolines, placement of a 2-methyl-D-proline residue in the i + 2 position and placement of either a cis- or trans-3-methyl-L-proline residue in the i + 1 position results in substituted diproline peptides that adopt the same type II beta-turn conformation as that defined experimentally for the unsubstituted diproline peptides. In contrast, placement of a cis-3-methyl-D-proline residue in the i + 1 position of a homochiral (D-D) diproline peptide seems to promote a different conformation than that seen in the unsubstituted case, whereas the trans-3-methyl-D-proline residue seems to provide a stabilizing influence for the predicted type VI' beta-turn. The demonstrated ability of certain substituted diproline templates to adopt predictable conformations coupled with the development of asymmetric synthetic routes to both 2- and 3-substituted prolyl residues, capable of mimicking a variety of side chains should make these templates useful tools in designing specific turn mimics of biologically active molecules.
4.Effects of ring contraction on the conformational preferences of α-substituted proline analogs.
Revilla-López G1, Warren JG, Torras J, Jiménez AI, Cativiela C, Alemán C. Biopolymers. 2012;98(2):98-110. doi: 10.1002/bip.21716. Epub 2011 Sep 6.
The structural consequences derived from the incorporation of either a methyl or a phenyl group at the α carbon of proline were recently investigated by quantum mechanical calculations (J Org Chem 2008, 73, 3418). In this work, the effect produced by contraction of the pyrrolidine ring on such α-substituted proline analogs has been explored using the same computational methods. Specifically, the intrinsic conformational preferences of the N-acetyl-N'-methylamide derivatives of the lower proline homolog L-azetidine-2-carboxylic acid (Aze), characterized by a four- instead of a five-membered ring, and its α-methyl (αMeAze) and α-phenyl (αPhAze) derivatives have been determined using quantum mechanical calculations and compared to those observed before for the proline counterparts. Replacement of the pyrrolidine ring by an azetidine cycle leads to a reduction of the conformational flexibility, especially for the Aze and αMeAze derivatives, which should be attributed to the quasi-planar geometry of the four-membered ring.
