Fetrow J S, Cardillo T S, Sherman F
Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142.
Proteins. 1989;6(4):372-81. doi: 10.1002/prot.340060404.
omega (omega)-loops are protein secondary structural elements having small distances between segment termini. It should be possible to delete or replace certain of these omega-loops without greatly distorting the overall structure of the remaining portion of the molecule. Functional requirements of regions of iso-1-cytochrome c from the yeast Saccharomyces cerevisiae were investigated by determining the biosynthesis and activity in vivo of mutant forms in which four different omega-loops were individually deleted, or in which one omega-loop was replaced with five different segments. Deletions encompassing amino acid positions 27-33 and 79-83 either prevented synthesis of the holoprotein, or produced highly labile iso-1-cytochromes c, whereas deletions encompassing positions 42-45 and 48-55 allowed partial synthesis and activity. These two latter regions, therefore, are not absolutely required for any biosynthetic process such as heme attachment, mitochondrial import, or for enzymatic interactions. All replacements in Loop A (residue positions 24-33) with same size (10 amino acid residues), longer (13 and 15 amino acid residues), or shorter segments (6 amino acid residues), resulted in strains having at least partial levels of iso-1-cytochrome c; however, the relative activities ranged from zero to almost the normal level. Thus, Loop A does not appear to be essential for such biosynthetic steps as heme attachment and mitochondrial import. In contrast, the full range of relative activities suggest that this region interacts with physiological partners to carry out efficient electron transport.
ω（欧米伽）环是蛋白质二级结构元件，其片段末端之间距离较短。删除或替换其中某些ω环而不会严重扭曲分子其余部分的整体结构应该是可行的。通过测定四种不同ω环被单独删除或其中一个ω环被五个不同片段替换的突变形式在体内的生物合成和活性，研究了酿酒酵母中异 - 1 - 细胞色素c区域的功能需求。包含氨基酸位置27 - 33和79 - 83的缺失要么阻止了全蛋白的合成，要么产生了高度不稳定的异 - 1 - 细胞色素c，而包含位置42 - 45和48 - 55的缺失允许部分合成和活性。因此，对于诸如血红素附着、线粒体导入或酶促相互作用等任何生物合成过程，这两个后一个区域并非绝对必需。用相同大小（10个氨基酸残基）、更长（13和15个氨基酸残基）或更短片段（6个氨基酸残基）对环A（残基位置24 - 33）进行的所有替换，都产生了至少具有部分异 - 1 - 细胞色素c水平的菌株；然而，相对活性范围从零到几乎正常水平。因此，环A对于血红素附着和线粒体导入等生物合成步骤似乎并非必不可少。相比之下，相对活性的完整范围表明该区域与生理伙伴相互作用以进行有效的电子传递。
