Solution structure of the NADP(H)-binding component (dIII) of proton-translocating transhydrogenase from Rhodospirillum rubrum

Transhydrogenase is a proton pump found in the membranes of bacteria and an imal mitochondria. The solution structure of the expressed, 21.5 kDa, NADP( H)-binding component (dIII) of transhydrogenase from Rhodospirillum rubrum has been solved by NMR methods. This is the first description of the struct ure of dIII from a bacterial source. The protein adopts a Rossmann fold: an open, twisted, parallel beta-sheet, flanked by helices. However, the bindi ng of NADP(+) to dIII is profoundly different to that seen in other Rossman n structures, in that its orientation is reversed the adenosine moiety inte racts with the first beta alpha beta alpha beta motif, and the nicotinamide with the second. Features in the structure that might be responsible for c hanges in nucleotide-binding affinity during catalysis, and for interaction with other components of the enzyme, are identified. The results are compa red with the recently determined, high-resolution crystal structures of hum an and bovine dIII which also show the reversed nucleotide orientation. (C) 2000 Elsevier Science B.V. All rights reserved.