Cytochrome c oxidase contains an extra charged amino acid cluster in a new type of respiratory chain in the amino-acid-producing Gram-positive bacterium Corynebacterium glutamicum.

The membranes from Corynebacterium glutamicum cells contain a hydrophobic di-haem C protein as the cytochrome c subunit of the new type of cytochrome bc complex (complex III in the respiratory chain) encoded by the qcrCAB operon [Sone, N., Nagata, K., Kojima, H., Tajima, J., Kodera, Y., Kanamaru, T., Noguchi, S. & Sakamoto, J. (2001). Biochim Biophys Acta 1503, 279-290]. To characterize complex IV, cytochrome c oxidase and its structural genes were isolated. The oxidase is of the cytochrome aa(3) type, but mass spectrometry indicated that the haem is haem As, which contains a geranylgeranyl side-chain instead of a farnesyl group. The enzyme is a SoxM-type haem-copper oxidase composed of three subunits. Edman degradation and mass spectrometry suggested that the N-terminal signal sequence of subunit II is cleaved and that the new N-terminal cysteine residue is diacylglycerated, while neither subunit I nor subunit III is significantly modified. The genes for subunits II (ctaC) and III (ctaE) are located upstream of the qcrCAB operon, while that for subunit I (ctaD) is located separately. The oxidase showed low enzyme activity with extrinsic substrates such as cytochromes c from horse heart or yeast, and has the Cu(A)-binding motif in its subunit II. A prominent structural feature is the insertion of an extra charged amino acid cluster between the beta2 and beta4 strands in the substrate-binding domain of subunit II. The beta2-beta4 loop of this oxidase is about 30 residues longer than that of major cytochrome c oxidases from mitochondria and proteobacteria, and is rich in both acidic and basic residues. These findings suggest that the extra charged cluster may play a role in the interaction of the oxidase with the cytochrome c subunit of the new type of bc complex.