Semiquinone radical anion of 1-(p-tolylsulfinyl)-2,5-benzoquinone (ToISQ(center dot-)) forms a strong hydrogen bond with protonated histidine (ToISQ(center dot-)/His center dot 2H(+)), which was successfully detected by electron spin resonance. Strong hydrogen bonding between ToISQ(center dot-) and His center dot 2H(+) results in acceleration of electron transfer (ET) from ferrocenes [R(2)Fc, R = C5H5, C5H4(n-Bu), C5H4Me] to ToISQ, when the one-electron reduction potential of ToISQ is largely shifted to the positive direction in the presence of His center dot 2H(+). The rates of His center dot 2H(+)-promoted ET from R2Fc: to ToISQ exhibit deuterium kinetic isotope effects due to partial dissociation of the N-H bond in His center dot 2H(+) at the transition state, when His center dot 2H(+) is replaced by the deuterated compound (His center dot 2D(+)-d(6)). The observed deuterium kinetic isotope effect (k(H)/k(D)) decreases continuously with increasing the driving force of ET to approach k(H)/k(D) = 1.0. On the other hand, His center dot 2H+ also promotes a hydride reduction of ToISQ by an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH(2)). The hydride reduction proceeds via the one-step hydride-transfer pathway. In such a case, a large deuterium kinetic isotope effect is observed in the rate of the hydride transfer, when AcrH2 is replaced by the dideuterated compound (AcrD(2)). In sharp contrast to this, no deuterium kinetic isotope effect is observed, when His-2H(+) is replaced by His center dot 2D(+)-d(6). On the other hand, direct protonation of ToISQ and 9,10-phenanthrenequinone (PQ) also results in efficient reductions of ToISQH(+) and PQH(+) by AcrH2, respectively. In this case, however, the hydride-transfer reactions occur via the ET pathway, that is, ET from AcrH2 to ToISQH+ and PQH+ occurs in preference to direct hydride transfer from AcrH2 to ToISQH+ and PQH+, respectively. The AcrH(2)center dot(+) produced by the ET oxidation of AcrH2 by ToISQH+ and PQH+ was directly detected by using a stopped-flow technique.