DETECTION AND CHARACTERIZATION OF EXCHANGEABLE PROTONS BOUND TO THE HYDROGEN-ACTIVATION NICKEL SITE OF DESULFOVIBRIO-GIGAS HYDROGENASE - A H-1 AND H-2 Q-BAND ENDOR STUDY

This paper represents a Q-band ENDOR study of the nickel site of the as-isolated (Ni-A), H2-reduced (Ni-C), and reoxidized (Ni-A/Ni-B) states of Desulfovibrio gigas hydrogenase. Through proton and deuteron ENDOR measurements we detect and characterize the possible products of heterolytic cleavage of H2, namely two distinct types of exchangeable protons, bound to the Ni-C site. One proton, H(1), has a hyperfine coupling A(H)(1) = 16.8 MHz and appears to interact directly with Ni-C. The other proton, H(2), has A(H)(2) almost-equal-to 4.4 MHz and could be associated with H2O or OH-bound to nickel. We discuss possible binding modes for H(1) and H(2). One type of exchangeable deuteron(s), D(2), associated with the Ni-C center remains associated with the Ni-B center after oxidation of the Ni-C. In addition we confirm that the Ni-A site is inaccessible to solvent protons.