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Corynebacterium glutamicum is an aerobic bacterium that requires oxygen as exogenous electron acceptor for respiration. Recent molecular and biochemical analyses together with information obtained from the genome sequence showed that C. glutamicum possesses a branched electron transport chain to oxygen with some remarkable features. Reducing equivalents obtained by the oxidation of various substrates are transferred to menaquinone via at least eight different dehydrogenases, i.e. NADH dehydrogenase, succinate dehydrogenase, malate:quinone oxidoreductase, pyruvate:quinone oxidoreductase, d-lactate dehydrogenase, l-lactate dehydrogenase, glycerol-3-phosphate dehydrogenase and l-proline dehydrogenase. All these enzymes contain a flavin cofactor and, except succinate dehydrogenase, are single subunit peripheral membrane proteins located inside the cell. From menaquinol, the electrons are passed either via the cytochrome bc1 complex to the aa3-type cytochrome c oxidase with low oxygen affinity, or to the cytochrome bd-type menaquinol oxidase with high oxygen affinity. The former branch is exceptional, in that it does not involve a separate cytochrome c for electron transfer from cytochrome c1 to the CuA center in subunit II of cytochrome aa3. Rather, cytochrome c1 contains two covalently bound heme groups, one of which presumably takes over the function of a separate cytochrome c. The bc1 complex and cytochrome aa3 oxidase form a supercomplex in C. glutamicum. The phenotype of defined mutants revealed that the bc1–aa3 branch, but not the bd branch, is of major importance for aerobic growth in minimal medium. Changes of the efficiency of oxidative phosphorylation caused by qualitative changes of the respiratory chain or by a defective F1F0-ATP synthase were found to have strong effects on metabolism and amino acid production. Therefore, the system of oxidative phosphorylation represents an attractive target for improving amino acid productivity of C. glutamicum by metabolic engineering. [Copyright &y& Elsevier]

In this work, the genes for cytochrome aa3 oxidase and the cytochrome bc1 complex in the gram-positive soil bacterium Corynebacterium glutamicum were identified. The monocistronic ctaD gene encoded a 65-kDa protein with all features typical for subunit I of cytochrome aa3 oxidases. A ctaD deletion mutant lacked the characteristic 600 nm peak in redox difference spectra, and growth in glucose minimal medium was strongly impaired. The genes encoding subunit III of cytochrome aa3 (ctaE) and the three characteristic subunits of the cytochrome bc1 complex (qcrABC) were clustered in the order ctaE-qcrCAB. Analysis of the deduced primary structures revealed a number of unusual features: (1) cytochrome c1 (QcrC, 30 kDa) contained two Cys-X-X-Cys-His motifs for covalent heme attachment, indicating that it is a di-heme c-type cytochrome; (2) the 'Rieske' iron-sulphur protein (QcrA, 45 kDa) contained three putative transmembrane helices in the N-terminal region rather than only one; and (3) cytochrome b (QcrB, 60 kDa) contained, in addition to the conserved part with eight transmembrane helices, a C-terminal extension of about 120 amino acids, which presumably is located in the cytoplasm. Staining of C. glutamicum proteins for covalently bound heme indicated the presence of a single, membrane-bound c-type cytochrome with an apparent molecular mass of about 31 kDa. Since this protein was missing in a qcrCAB deletion mutant, it most likely corresponds to cytochrome c1. Similar to the ΔctaD mutant, the ΔqcrCAB mutant showed strongly impaired growth in glucose minimal medium, which indicates that the bc1-aa3 pathway is the main route of respiration under these conditions. [ABSTRACT FROM AUTHOR]