a b s t r a c tAnaerobic ammonium oxidation (anammox) and denitrification are two distinct microbial reactions relevant to the global nitrogen cycle. The proposed initial step of the anammox reactions, reduction of nitrite to nitric oxide, has been postulated to be identical to that in denitrification catalyzed by the dissimilatory nitrite reductase of the cytochrome cd 1 -type. Here, we characterized the copper-containing nitrite reductase homolog encoded by nirK detected in the genome of an anammox bacterium strain KSU-1. We hypothesize that this NirK-type nitrite reductase, rather than a nitrite reductase of the cytochrome cd 1 -type (NirS), is likely to catalyze nitrite reduction in anammox organism KSU-1.
The O 2 and CO reactions with the heme, ␣-hydroxyheme, and verdoheme complexes of heme oxygenase have been studied. and protein residues in the heme pocket. The CO affinities estimated for both isoforms are only 1-6-fold higher than the corresponding O 2 affinities. Thus, heme oxygenase discriminates much more strongly against CO binding than either myoglobin or hemoglobin. The CO binding reactions with the ferrous ␣-hydroxyheme complex are similar to those of the protoheme complex, and hydroxylation at the ␣-meso position does not appear to affect the reactivity of the iron atom. In contrast, the CO affinities of the verdoheme complexes are >10,000 times weaker than those of the heme complexes because of a 100-fold slower association rate constant (k CO Ϸ 0.004 M ؊1 s
؊1) and a 300-fold greater dissociation rate constant (k CO Ϸ 3 s ؊1 ) compared with the corresponding rate constants of the protoheme and ␣-hydroxyheme complexes. The positive charge on the verdoporphyrin ring causes a large decrease in reactivity of the iron.
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