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The thioreduction component CcmG confers efficiency and the heme ligation component CcmH ensures stereo-specificity during cytochrome c maturation.


ABSTRACT: In many Gram-negative bacteria, including Rhodobacter capsulatus, cytochrome c maturation (Ccm) is carried out by a membrane-integral machinery composed of nine proteins (CcmA to I). During this process, the periplasmic thiol-disulfide oxidoreductase DsbA is thought to catalyze the formation of a disulfide bond between the Cys residues at the apocytochrome c heme-binding site (CXXCH). Subsequently, a Ccm-specific thioreductive pathway involving CcmG and CcmH reduces this disulfide bond to allow covalent heme ligation. Currently, the sequence of thioredox reactions occurring between these components and apocytochrome c and the identity of their active Cys residues are unknown. In this work, we first investigated protein-protein interactions among the apocytochrome c, CcmG, and the heme-ligation components CcmF, CcmH, and CcmI. We found that they all interact with each other, forming a CcmFGHI-apocytochrome c complex. Using purified wild-type CcmG, CcmH, and apocytochrome c, as well as their respective Cys mutant variants, we determined the rates of thiol-disulfide exchange reactions between selected pairs of Cys residues from these proteins. We established that CcmG can efficiently reduce the disulfide bond of apocytochrome c and also resolve a mixed disulfide bond formed between apocytochrome c and CcmH. We further show that Cys-45 of CcmH and Cys-34 of apocytochrome c are most likely to form this mixed disulfide bond, which is consistent with the stereo-specificity of the heme-apocytochrome c ligation reaction. We conclude that CcmG confers efficiency, and CcmH ensures stereo-specificity during Ccm and present a comprehensive model for thioreduction reactions that lead to heme-apocytochrome c ligation.

SUBMITTER: Verissimo AF 

PROVIDER: S-EPMC5555179 | biostudies-literature | 2017 Aug

REPOSITORIES: biostudies-literature

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The thioreduction component CcmG confers efficiency and the heme ligation component CcmH ensures stereo-specificity during cytochrome <i>c</i> maturation.

Verissimo Andreia F AF   Khalfaoui-Hassani Bahia B   Hwang Josephine J   Steimle Stefan S   Selamoglu Nur N   Sanders Carsten C   Khatchikian Camilo E CE   Daldal Fevzi F  

The Journal of biological chemistry 20170620 32


In many Gram-negative bacteria, including <i>Rhodobacter capsulatus,</i> cytochrome <i>c</i> maturation (Ccm) is carried out by a membrane-integral machinery composed of nine proteins (CcmA to I). During this process, the periplasmic thiol-disulfide oxidoreductase DsbA is thought to catalyze the formation of a disulfide bond between the Cys residues at the apocytochrome <i>c</i> heme-binding site (C<i>XX</i>CH). Subsequently, a Ccm-specific thioreductive pathway involving CcmG and CcmH reduces t  ...[more]

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