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Activation of the Cytochrome cd1 Nitrite Reductase from Paracoccus pantotrophus

Jessica H. van Wonderen,C. Knight,V. Oganesyan,S. George,W. Zumft,M. Cheesman

Published 2007 in Journal of Biological Chemistry

ABSTRACT

Cytochromes cd1 are dimeric bacterial nitrite reductases, which contain two hemes per monomer. On reduction of both hemes, the distal ligand of heme d1 dissociates, creating a vacant coordination site accessible to substrate. Heme c, which transfers electrons from donor proteins into the active site, has histidine/methionine ligands except in the oxidized enzyme from Paracoccus pantotrophus where both ligands are histidine. During reduction of this enzyme, Tyr25 dissociates from the distal side of heme d1, and one heme c ligand is replaced by methionine. Activity is associated with histidine/methionine coordination at heme c, and it is believed that P. pantotrophus cytochrome cd1 is unreactive toward substrate without reductive activation. However, we report here that the oxidized enzyme will react with nitrite to yield a novel species in which heme d1 is EPR-silent. Magnetic circular dichroism studies indicate that heme d1 is low-spin FeIII but EPR-silent as a result of spin coupling to a radical species formed during the reaction with nitrite. This reaction drives the switch to histidine/methionine ligation at FeIII heme c. Thus the enzyme is activated by exposure to its physiological substrate without the necessity of passing through the reduced state. This reactivity toward nitrite is also observed for oxidized cytochrome cd1 from Pseudomonas stutzeri suggesting a more general involvement of the EPR-silent FeIII heme d1 species in nitrite reduction.

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