Cytochrome oxidase, also from Pseudomonas

In addition to the ubiquinol oxidase mentioned above, cytochrome c oxidase is known, which contains heme O, as already described. Cytochrome aco (or cao3) has one molecule each of hemes A, B and O in the molecule, and shows cytochrome c oxidase activity (Qureshi et al., 1990). cytochromes co purified from Pseudomonas aeruginosa (Matsushita et al., 1982), Methylophilus methylotrophus (Fraud and Anthony, 1984) and Pseudomonas stutzeri (Heiss et al., 1989) also act as cytochrome c oxidase. 

The biological significance of these reactions is considered further in Chapters 18 and 24. The 132-kDa dimeric N20 reductase from Pseudomonas stiltzeri contains four copper atoms per subunit.546 One of its copper centers resembles the CuA centers of cytochrome c oxidase. A second copper center consists of four copper ions, held by seven histidine side chains in a roughly tetrahedral array around one sulfide (S2 ) ion. Rasmussen et al. speculate that this copper-sulfide cluster may be an acceptor of the oxygen atoms of N20 in the formation of N2.546a There is also a cytochrome cdj type of nitrite reductase.1433... 

Yamanaka and co-workers (364-366) have crystallized a cytochrome oxidase from P. aeruginosa which oxidizes Pseudomonas ferrocytochrome c-551. It is also capable of nitrite reduction with a turnover number of 4000 moles nitrite reduced under anaerobic conditions to nitric oxide per minute at 37°. It is an adaptive enzyme, nitrate being essential for its biosynthesis. The enzyme has a molecular weight of 120,000, with two subunits of equivalent molecular weight, 2 heme c and 2 heme d groups per mole (Fig. 38) (366a). Nitrite reductase activity is 94% inhibited by 8 X 10 M KCN, but only by CO. The lack of CO inhibition appears to be related to the fact that the enzyme has a greater affinity for nitrite than for carbon monoxide. 

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