Abstract
cd(1) 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 cd(1) nitrite reductase of Pseudomonas aeruginosa have 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. stutzeri enzyme is the absence of a flexible N-terminal extension in the Ps. stutzeri enzyme. In Ps. aeruginosa cd(1) nitrite reductase the N-terminal extension wraps around the second subunit of the homodimer and with Tyr(10) stabilizing a water molecule co-ordinated to the d(1)-haem. Given the intimate association of the N-terminal extension with the d(1)-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. stutzeri cd(1) 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. stutzeri cd(1) 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. stutzeri cd(1) 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. aeruginosa in the kinetics of these processes. Our data indicate that global structural reorganization following CO photodissociation is an intrinsic property of the haem domains in cd(1) nitrite reductases. The absence of an N-terminal extension, as in the Ps. stutzeri cd(1) nitrite reductase, does not lead to loss of global structural reorganization following CO photodissociation.