Review and Hypothesis. New insights into the reaction mechanism of transhydrogenase: Swivelling the dIII component may gate the proton channel

Abstract

Edited by Peter BrzezinskiKeywords: Transhydrogenase Membrane-protein structure Nicotinamide nucleotide Proton-pump Proton-gating a b s t r a c tThe membrane protein transhydrogenase in animal mitochondria and bacteria couples reduction of NADP + by NADH to proton translocation. Recent X-ray data on Thermus thermophilus transhydrogenase indicate a significant difference in the orientations of the two dIII components of the enzyme dimer (Leung et al., 2015). The character of the orientation change, and a review of information on the kinetics and thermodynamics of transhydrogenase, indicate that dIII swivelling might assist in the control of proton gating by the redox state of bound NADP + /NADPH during enzyme turnover.

Critical Role of Water Molecules in Proton Translocation by the Membrane-Bound Transhydrogenase

“…Interestingly, our MD simulations indicate that transient water flows across this dry region only occur when His42 α2 is in the protonated state, which is more likely at pH 6.5 or near a physiologic pH than at basic pHs. The current data are thus consistent with a model previously postulated for dII-mediated proton translocation (Jackson et al, 2015; Jackson et al, 1999; Leung et al, 2015) that occurs through a dynamic protonation and deprotonation of a central histidine residue. The proton motive force favors protonation of His42 α2 from the periplasm and deprotonation to the cytoplasm and drives the chemical reaction in the forward direction, generating NADPH.…”

“…The third domain, dII, is comprised of between 12 and 14 transmembrane helices (depending on species) and contains a proton-conducting pathway between the bulk aqueous phases on either side of the membrane (Leung et al, 2015). Hydride transfer from NADH to NADP + is accompanied by the translocation of one proton from the “outside” (mitochondrial intermembrane space or bacterial periplasm) to the inside (mitochondrial matrix or bacterial cytoplasm) (Jackson et al, 2015). Under most physiological conditions the proton motive force down a concentration gradient drives protons across the membrane (out → in) which is coupled to the formation of NADPH.…”

Nicotinamide nucleotide transhydrogenase (NNT) deficiency dysregulates mitochondrial retrograde signaling and impedes proliferation

“…It couples the proton flow down electrochemical proton gradient to hydride transfer from NADH to NADP + [10]. Under pathophysiological conditions, NNT catalyzes the reverse reaction to generate NADH and maintain mitochondrial membrane potential (ΔΨm) via proton pumping.…”

Pyridine nucleotide transhydrogenase PntAB is essential for optimal growth and photosynthetic integrity under low‐light mixotrophic conditions in Synechocystis sp. PCC 6803

“…Although membrane-bound transhydrogenases from different organisms vary in their polypeptide composition and quaternary structure, the native enzyme always is composed of three distinct structural components: dI and dIII which bind NAD + :NADH and NADP + :NADPH, respectively, and dII which forms the integral membrane proton channel. Characteristically, these are arranged in two protomers (designated A and B) which form an asymmetric native dI-dII-dIII 'dimer' as summarized in detail by Jackson (2003) and Jackson et al (2015). The enzymes can be grouped into four different categories based on the overall topology: In mammals the enzyme is a (1) single polypeptide without clearly defined a and b regions, whereas the bacterial counterparts are characteristically heteromultimers composed of different arrangements of separate a and b polypeptides.…”

“…Thorough structural and functional characterization has revealed that transhydrogenasesdespite the different topologies employ a common binding-change mechanism, which couples the proton translocation to allosteric conformational changes upon substrate binding and release (see reviews of Jackson, 2012; Jackson et al, 2015). During the catalytic cycle the enzyme shifts between two alternative conformations: The open conformation allows substrate binding and product release in dI and dIII, whereas the occluded conformation brings the reacting species to appropriate orientation to allow the redox reaction to take place.…”