Proton translocating nicotinamide nucleotide transhydrogenase from E-coli.Mechanism of action deduced from its structural and catalytic properties

Transhydrogenase couples the stereospecific and reversible transfer of hydr ide equivalents from NADH to NADP(+) to the translocation of proton across the inner membrane in mitochondria and the cytoplasmic membrane in bacteria . Like all transhydrogenases, the Escherichia coli enzyme is composed of th ree domains. Domains I and III protrude from the membrane and contain the b inding site for NAD(H) and NADP(H), respectively. Domain II spans the membr ane and constitutes at least partly the proton translocating pathway. Three -dimensional models of the hydrophilic domains I and III deduced from cryst allographic and NMR data and a new topology of domain II are presented. The new information obtained from the structures and the numerous mutation stu dies strengthen the proposition of a binding change mechanism, as a way to couple the reduction of NADP(+) by NADH to proton translocation and occurri ng mainly at the level of the NADP(H) binding site. (C) 2000 Elsevier Scien ce B.V. All rights reserved.