cd1 nitrite reductase from Pseudomonas stutzeri is a di-haem- containing enzyme, comprising a c-type haem and a d-type haem. Studies with the highly related cd1 nitrite reductase of Pseudomonas aeruginosahave established that this enzyme undergoes fast (microsecond) and global structural relaxation upon CO photodissociation from the reduced enzyme. A key difference between the Ps. aeruginosa and Ps. stutzerienzyme is the absence of a flexible N-terminal extension in the Ps. stutzeri enzyme. In Ps. aeruginosacd1 nitrite reductase the N-terminal extension wraps around the second subunit of the homodimer and with Tyr10 stabilizing a water molecule co-ordinated to the d1-haem. Given the intimate association of the N-terminal extension with the d1-haem, we hypothesized that the presence of the N-terminal extension likely contributes to the fast structural reorganization seen during photodissociation of CO from the reduced enzyme. In the present study we have investigated the kinetics of CO association and CO photodissociation of Ps. stutzericd1 nitrite reductase (which lacks the N-terminal arm seen in the Ps. aeruginosa enzyme) to probe the role and influence of the N-terminal arm in the fast global structural reorganization seen with Ps. aeruginosa. Surprisingly, we find that Ps. stutzericd1 nitrite reductase also undergoes fast structural reorganization during CO photodissociation. We also show, in stopped-flow experiments, that the kinetics of CO binding and dissociation with reduced Ps. stutzericd1 nitrite reductase are similar to those observed with Ps. aeruginosa enzyme, thus ruling out a major role for the N-terminal flexible arm found in Ps. aeruginosain the kinetics of these processes. Our data indicate that global structural reorganization following CO photodissociation is an intrinsic property of the haem domains in cd1 nitrite reductases. The absence of an N-terminal extension, as in the Ps. stutzericd1 nitrite reductase, does not lead to loss of global structural reorganization following CO photodissociation.

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