Valente F M, Saraiva L M, LeGall J, Xavier A V, Teixeira M, Pereira I A
Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Apt. 127, 2780-156 Oeiras, Portugal.
Chembiochem. 2001 Dec 3;2(12):895-905. doi: 10.1002/1439-7633(20011203)2:12<895::AID-CBIC895>3.0.CO;2-V.
A new tetraheme cytochrome c3 was isolated from the membranes of Desulfovibrio vulgaris Hildenborough (DvH). This cytochrome has a molecular mass of 13.4 kDa and a pI of 5.5 and contains four heme c groups with apparent reduction potentials of -170 mV, -235 mV, -260 mV and -325 mV at pH 7.6. The complete sequence of the new cytochrome, retrieved from the preliminary data of the DvH genome, shows that this cytochrome is homologous to the "acidic" cytochrome c3 from Desulfovibrio africanus (Da). A model for the structure of the DvH cytochrome was built based on the structure of the Da cytochrome. Both cytochromes share structural features that distinguish them from other cytochrome c3 proteins, such as a solvent-exposed heme 1 surrounded by an acidic surface area, and a heme 4 which lacks most of the surface lysine patch proposed to be the site of hydrogenase interaction in other cytochrome c3 proteins. Furthermore, in contrast to previously discovered cytochrome c3 proteins, the genes coding for these two cytochromes are adjacent to genes coding for two membrane-associated FeS proteins, which indicates that they may be part of membrane-bound oxidoreductase complexes. Altogether these observations suggest that the DvH and Da cytochromes are a new type of cytochrome c3 proteins (Type II: TpII-c3) with different redox partners and physiological function than the other cytochrome c3 proteins (Type I: TpI-c3). The DvH TpII-c3 is reduced at considerable rates by the two membrane-bound [NiFe] and [NiFeSe] hydrogenases, but catalytic amounts of TpI-c3 increase these rates two- and fourfold, respectively. With the periplasmic [Fe] hydrogenase TpII-c3 is reduced much slower than TpI-c3, and no catalytic effect of TpI-c3 is observed.
从希登伯勒脱硫弧菌（DvH）的膜中分离出一种新的四血红素细胞色素c3。这种细胞色素的分子量为13.4 kDa，pI为5.5，含有四个血红素c基团，在pH 7.6时其表观还原电位分别为-170 mV、-235 mV、-260 mV和-325 mV。从DvH基因组的初步数据中获取的这种新细胞色素的完整序列表明，该细胞色素与来自非洲脱硫弧菌（Da）的“酸性”细胞色素c3同源。基于Da细胞色素的结构构建了DvH细胞色素的结构模型。这两种细胞色素具有共同的结构特征，使其与其他细胞色素c3蛋白不同，例如一个暴露于溶剂中的血红素1被一个酸性表面区域包围，以及一个血红素4，它缺少其他细胞色素c3蛋白中被认为是氢化酶相互作用位点的大部分表面赖氨酸补丁。此外，与先前发现的细胞色素c3蛋白不同，编码这两种细胞色素的基因与编码两种膜相关铁硫蛋白的基因相邻，这表明它们可能是膜结合氧化还原酶复合物的一部分。总之，这些观察结果表明，DvH和Da细胞色素是一种新型的细胞色素c3蛋白（II型：TpII-c3），与其他细胞色素c3蛋白（I型：TpI-c3）相比，具有不同的氧化还原伙伴和生理功能。DvH TpII-c3被两种膜结合的[NiFe]和[NiFeSe]氢化酶以相当高的速率还原，但催化量的TpI-c3分别使这些速率提高了两倍和四倍。对于周质[Fe]氢化酶，TpII-c3的还原速度比TpI-c3慢得多，并且未观察到TpI-c3的催化作用。
