Abstract
Cytochrome cd 1 nitrite reductase ( cd 1 NIR) is a haem-containing enzyme responsible for the reduction of nitrite into nitric oxide (NO), a key step in the bacterial anaerobic respiratory process of denitrification. The active site of cytochrome cd 1 contains the unique d 1 -haem cofactor, from which NO must be released. In general, reduced haems bind NO tightly relative to oxidised haems. The mechanism of NO release employed by cd 1 NiRs has been recently outlined, highlighting the essential role of the peculiar d 1 -haem, whose presence in these enzymes was largely unexplained. Briefly, we have shown that cd 1 NiRs can release quite rapidly the NO bound to the reduced d 1 haem. This finding challenged the classical idea that NO is often an “irreversible” inhibitor of reduced hemoproteins and prompted new studies on the reactivity of ferrous d 1 haem with NO, indicating that the d 1 haem has evolved to have low affinity for NO, as compared to other ferrous haems.
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