Surface accessibility of different types of the same elements of secondary structure has been studied in 10 non-redundant sets of proteins (total number of three-dimensional structures is 1730) with a help of DSSP (Dictionary of Secondary Structure of Proteins). Random coils (C), beta strands (B), and alpha helices (H) have been classified according to their flanking elements of secondary structure in a polypeptide chain. Thanks to this kind of classification, for the first time it has been shown that random coils situated between a beta strand and an alpha helix (BCH) contain significantly lower fraction of exposed residues compared to other types of random coils; the least accessible beta strands are situated between two alpha helices (HBH), and the least accessible alpha helices are situated between a beta strand and an alpha helix (BHH). Discovered trends are explained as consequences of the natural selection that had been stabilizing the secondary structure of proteins on early steps of their evolution. Acquired differences in amino acid content of different types of random coils, alpha helices, and beta strands led to the formation of partially buried but hydrophilic BCH random coils because of their enrichment by Ser, Thr, and Asp residues. As a result, BCH random coils became prone to bind cations because of their lower hydration and decreased usage of positively charged amino acid residues. The mechanism described above led to the formation of active centers in ancient metalloenzymes. Nowadays one can observe decreased surface accessibility of amino acid residues in BCH random coils, in HBH beta strands, and in BHH alpha helices in proteins possessing hydrophobic cores.

中文翻译：

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位于Beta链和Alpha螺旋之间的随机排列的蛋白质表明溶剂可及性降低。

借助DSSP（蛋白质二级结构词典），在10组非冗余蛋白质（三维结构总数为1730）中研究了不同类型的二级结构相同元素的表面可及性。根据多肽链中二级结构的侧翼元素，对随机线圈（C），β链（B）和α螺旋（H）进行了分类。由于这种分类，首次证明，与其他类型的随机线圈相比，位于β链和α螺旋（BCH）之间的随机线圈包含的残留残渣比例要低得多；最不易接近的β链位于两个α螺旋（HBH）之间，最不易接近的α螺旋位于β链与α螺旋（BHH）之间。发现的趋势被解释为自然选择的结果，这种选择在蛋白质进化的早期步骤中已经稳定了蛋白质的二级结构。由于不同类型的无规卷曲，α螺旋和β链氨基酸含量的获得差异，由于其富含Ser，Thr和Asp残基，导致部分掩埋但亲水的BCH无规卷曲形成。结果，BCH无规线圈因其较低的水合作用和带正电荷的氨基酸残基的使用减少而易于结合阳离子。上述机制导致了古代金属酶中活性中心的形成。如今，人们可以观察到具有疏水核心的蛋白质中，BCH随机螺旋，HBHβ链和BHHα螺旋中氨基酸残基的表面可及性降低。