Pyrococcus furiosus glyceraldehyde 3-phosphate oxidoreductase has comparable W(6+/5+) and W(5+/4+) reduction potentials and unusual [4Fe-4S] EPR properties

Pyrococcus furiosus glyceraldehyde 3-phosphate oxidoreductase has been characterized using EPR-monitored redox titrations. Two different W signals were found. W₁/⁵⁺ is an intermediate species in the catalytic cycle, with the midpoint potentials E(m)(W(6+/5+)) = -507 mV and E(m)(W(5+/4+)) = -491 mV. W₂/⁵⁺ represents an inactivated species with E(m)(W(6+/5+)) = -329 mV. The cubane cluster exhibits both S = 3/2 and S = 1/2 signals with the same midpoint potential: E(m)([4Fe-4S](2+/1+)) = -335 mV. The S = 1/2 EPR signal is unusual with all g values below 2.0. The titration results combined with catalytic voltammetry data are consistent with electron transfer from glyceraldehyde 3-phosphate first to the tungsten center, then to the cubane cluster and finally to the ferredoxin.