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C-CAPPING AND HELIX STABILITY - THE PRO C-CAPPING MOTIF

机译：C-CAPPING和螺旋稳定性-PRO C-CAPPING MOTIF

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Here we have performed a statistical analysis of the protein database to find new putative local C-terminal motifs in alpha-helices. Our analysis shows that certain combinations of X-Pro pairs (Asn, Cys, His, Phe, Tyr, Trp, Ile, Val and Leu), in which residue X is the C-cap and the Pro is at position C', are more abundant than expected. In those pairs, except for the aliphatic residues; the presence of the Pro residue at C' tends to restrict the phi and psi dihedral angles of the residue at position C-cap, around -130 degrees, 70 degrees, respectively. For the aromatic residues as well as for His, the chi(1) angle is around -60 degrees and the edge of the His and aromatic rings are close to the carbonyl group of the residue i-4. Ln all the pairs having the above dihedral angles for residue C-cap, the main-chain amino group of Pro at C' is close to the last three main-chain carbonyls of the alpha-helix. The above structural arrangements suggests the existence of a stabilising electrostatic interaction of the residues at positions C-cap and C' with the helix macrodipole. We have denominated this putative local motif, the Pro-capping motif. To asses its importance in helix stability we have analysed by nuclear magnetic resonance (NMR) and far-UV circular dichroism (CD) a set of polyalanine-based peptides containing two of the above pairs: His-Pro and Phe-Pro, as well as the corresponding controls. In the case of the His-Pro pair we have found NMR evidence for the formation of the Pro-capping motif in aqueous solution. CD analysis shows that-the presence of a Pro residue alters the C-cap properties of the preceding amino acids in the case of His and Phe makes them more favourable. The Pro-capping motif with the appropriate sequence, determines the location of the C terminus of Ir-helices and stabilises the helical conformation having Pro as the C' residue. (C) 1997 Academic Press Limited. [References: 47]
机译：在这里，我们已经对蛋白质数据库进行了统计分析，以发现新的假定的局部α-螺旋C端基序。我们的分析表明，某些X-Pro对组合（Asn，Cys，His，Phe，Tyr，Trp，Ile，Val和Leu）的残基X为C-帽，Pro位于C'位置，比预期的要丰富。在那些对中，除了脂肪族残基; C'处Pro残基的存在往往会限制C-cap位置残基的phi和psi二面角，分别在-130度和70度左右。对于芳族残基以及His，chi（1）角约为-60度，His和芳环的边缘靠近残基i-4的羰基。在残基C-cap具有上述二面角的所有对中，Pro在C'的主链氨基基团接近α-螺旋的最后三个主链羰基。上述结构安排表明C-cap和C'位置的残基与螺旋大偶极子之间存在稳定的静电相互作用。我们将这种假定的局部主题命名为Pro-capping主题。为了评估其在螺旋稳定性中的重要性，我们通过核磁共振（NMR）和远紫外圆二色性（CD）分析了一组基于聚丙氨酸的肽，其中包含上述两个对：His-Pro和Phe-Pro作为相应的控件。在His-Pro对的情况下，我们发现了在水溶液中形成Pro-封端基序的NMR证据。 CD分析表明，在His和Phe的情况下，Pro残基的存在改变了前述氨基酸的C-cap性质，使其更有利。具有适当序列的Pro-封端基序确定Ir-螺旋的C末端的位置并稳定具有Pro作为C'残基的螺旋构象。 （C）1997 Academic Press Limited。 [参考：47]

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