Rankin S E, Watts A, Roder H, Pinheiro T J
Department of Biological Sciences, University of Warwick, Coventry, United Kingdom.
Protein Sci. 1999 Feb;8(2):381-93. doi: 10.1110/ps.8.2.381.
Unfolded apocytochrome c acquires an alpha-helical conformation upon interaction with lipid. Folding kinetic results below and above the lipid's CMC, together with energy transfer measurements of lipid bound states, and salt-induced compact states in solution, show that the folding transition of apocytochrome c from the unfolded state in solution to a lipid-inserted helical conformation proceeds via a collapsed intermediate state (I(C)). This initial compact state is driven by a hydrophobic collapse of the polypeptide chain in the absence of the heme group and may represent a heme-free analogue of an early compact intermediate detected on the folding pathway of cytochrome c in solution. Insertion into the lipid phase occurs via an unfolding step of I(C) through a more extended state associated with the membrane surface (I(S)). While I(C) appears to be as compact as salt-induced compact states in solution with substantial alpha-helix content, the final lipid-inserted state (Hmic) is as compact as the unfolded state in solution at pH 5 and has an alpha-helix content which resembles that of native cytochrome c.
未折叠的脱辅基细胞色素c与脂质相互作用时会获得α-螺旋构象。在脂质临界胶束浓度（CMC）上下的折叠动力学结果，以及脂质结合态的能量转移测量结果，还有溶液中盐诱导的紧密态结果表明，脱辅基细胞色素c从溶液中的未折叠态转变为脂质插入的螺旋构象是通过一个塌缩的中间态（I(C)）进行的。这种初始紧密态是在没有血红素基团的情况下由多肽链的疏水塌缩驱动的，可能代表了在溶液中细胞色素c折叠途径上检测到的早期紧密中间体的无血红素类似物。通过与膜表面相关的更伸展状态（I(S)），I(C)的一个去折叠步骤导致其插入脂质相。虽然I(C)似乎与溶液中盐诱导的紧密态一样紧密，且具有大量的α-螺旋含量，但最终的脂质插入态（Hmic）在pH 5时与溶液中的未折叠态一样紧密，并且其α-螺旋含量类似于天然细胞色素c的α-螺旋含量。
