Abstract
Nicotinamide nucleotide transhydrogenase (TH) is an enzyme complex in animal mitochondria and bacteria that utilizes the electrochemical proton gradient across membranes to drive the production of NADPH. The enzyme plays an important role in maintaining the redox balance of cells with implications in aging and a number of human diseases. TH exists as a homodimer with each protomer containing a proton-translocating transmembrane domain and two soluble nucleotide binding domains that mediate hydride transfer between NAD(H) and NADP(H). The three-domain architecture of TH is conserved across species but polypeptide composition differs substantially. The complex domain coupling mechanism of TH is not fully understood despite extensive biochemical and structural characterizations. Herein the progress is reviewed, focusing mainly on structural findings from 3D crystallization of isolated soluble domains and more recently of the transmembrane domain and the holo-enzyme from Thermus thermophilus. A structural perspective and impeding challenges in further elucidating the mechanism of TH are discussed.
Highlights
Nicotinamide nucleotide transhydrogenase (TH) is a key enzyme residing in mitochondrial inner membrane and bacterial cytoplasmic membrane that helps to maintain cellular redox balance through the following reaction: H+out + NADP+ + NADH ↔ H+in + NADPH + NAD+
In E. coli TH, domain I and the first four transmembrane helices (TM 1–4) of domain II are encoded by one gene α, whereas in T. thermophilus, two separate genes (α1 and α2) encode domain I and the first three TM helices (TM 2–4) of domain II; the remaining TM helices (TM 6–14) of domain II together with domain III in both species are encoded by gene β
The use of stably expressed and purified TH from T. thermophilus has resulted in the recent structural determinations of the membrane-intercalated domain II and holo-TH
Summary
Nicotinamide nucleotide transhydrogenase (TH) is a key enzyme residing in mitochondrial inner membrane and bacterial cytoplasmic membrane that helps to maintain cellular redox balance through the following reaction: H+out + NADP+ + NADH ↔ H+in + NADPH + NAD+ In this reaction, proton motive force across the membrane is utilized to drive hydride transfer from NADH to NADP+, resulting in the generation of NADPH under most physiological conditions (Jackson, 2003). A novel mechanism of domain III swiveling has been proposed for its communication with each of the other two domains alternately within the biological dimer (Jackson et al, 2015) This manuscript will review progress in determining TH structure, new mechanistic insights that have resulted, and some questions that remain in order to fully elucidate the conformational dynamics and mechanism by which proton translocation is coupled to hydride transfer events that occur remotely. Local conformational changes in the NAD(H) binding site as well as relative movement between the two subdomains within domain I appear to cause a “distal-toproximal” motion of bound NAD(H) (Mather et al, 2004)
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