Fig. 1.

Model of electron flow in the oxidative stress defense system of P. furiosus. The oxidation of maltose results in the reduction of ferredoxin, which is used to produce H₂ by the MBH. Two different routes are proposed to connect the reduced ferredoxin (Fd) pool to the NADPH pool, which is the proposed electron source for the oxidative stress defense enzymes FdpA, SOR, and Rbr. Route A: An unknown enzyme (X) directly transfers electrons from reduced Fd to NADP. Route B: The H₂ that is produced by MBH is recycled by oxidation by the soluble hydrogenases SHI and SHII forming NADPH. NROR and Rd are proposed to transfer electrons from NADPH to SOR, Rbr, and FdpA.