Dp. Barondeau et al., STABILITY OF THE NI-C STATE AND OXIDATIVE TITRATIONS OF DESULFOVIBRIO-GIGAS HYDROGENASE MONITORED BY EPR AND ELECTRONIC ABSORPTION SPECTROSCOPIES, Journal of the American Chemical Society, 116(8), 1994, pp. 3442-3448
The Ni-C state of the NiFe hydrogenase from Desulfovibrio gigas was fo
und to be stable for over 40 h at pH 8.0 in the strict absence of H-2.
This demonstrates that Ni-C is unable to spontaneously reduce protons
to H-2 under these conditions, a result that is contrary to earlier r
eports. The form of the nickel that spontaneously reduces protons is p
robably Ni-R, the EPR-silent state that appears to be one electron mor
e reduced than Ni-C, The stability of Ni-C permitted, for the first ti
me, stoichiometric oxidative titrations of the H-2-free, reduced enzym
e. Four such titrations were performed, using the oxidant thionin and
monitoring the progress of the titrations by electronic absorption (at
410 nm) and EPR spectroscopies. Redox changes in the enzyme's two Fe4
S4 clusters were readily followed by changes at 410 nm, while those of
the Ni were observed by EPR. Redox changes of the Fe3S4 cluster were
monitored by both spectroscopic methods. At the start of the titration
s, the Ni center was in the Ni-C state and the Fe-S clusters were part
ially oxidized. Adding thionin caused disappearance of Ni-C, developme
nt of the EPR-silent intermediate state, and eventually the appearance
of Ni-B. The Fe-S clusters oxidized gradually throughout every stage
of the titrations. An average of 4.2 oxidizing equiv/mol of thionin wa
s consumed overall. After the number of equiv/mol consumed by the Fe-S
clusters and the EPR-active Ni ions were substracted, an average of 1
.2 oxidizing equiv/mol remained unassigned. The activity of the enzyme
appears to be correlated to the ability of the Ni site to undergo red
ox chemistry.