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In biochemistry, NAD(P) transhydrogenase (Si-specific) (EC 1.6.1.1) is an enzyme that catalyzes the chemical reaction

NADPH + NAD ${\displaystyle \rightleftharpoons }$ NADP + NADH

Thus, the two substrates of this enzyme are NADPH and NAD, whereas its two products are NADP and NADH. This enzyme participates in nicotinate and nicotinamide metabolism. It employs one cofactor, FAD.

Physiological function

Si-specific transhydrogenase is a soluble protein found in some Gammaproteobacteria and gram-positive bacteria. Enterobacteriaceae are known to possess both a soluble and a membrane-bound transhydrogenase. In living cells this enzyme primarily operates in the direction consuming NADPH and producing NADH, as the physiological ratio of NADPH/NADP is much higher than the ratio of NADH/NAD. Its chief function in vivo is the reoxidization of excess NADPH.

Nomenclature

This enzyme belongs to the family of oxidoreductases, specifically those acting on NADH or NADPH with NAD+ or NADP+ as acceptor. The systematic name of this enzyme is NADPH:NAD+ oxidoreductase (Si-specific). Other names in common use include non-energy-linked transhydrogenase, NAD(P)+ transhydrogenase (B-specific), and soluble transhydrogenase.

Older literature often uses ambiguous names such as pyridine nucleotide transhydrogenase, transhydrogenase, NAD(P)+ transhydrogenase, nicotinamide nucleotide transhydrogenase, NADPH-NAD+ transhydrogenase, pyridine nucleotide transferase, or NADPH-NAD+ oxidoreductase, which can equally apply to the more common NAD(P) transhydrogenase (Re/Si-specific).

References

1. ^ Cao Z, Song P, Xu Q, Su R, Zhu G (Jul 2011). "Overexpression and biochemical characterization of soluble pyridine nucleotide transhydrogenase from Escherichia coli". FEMS Microbiology Letters. 320 (1): 9–14. doi:10.1111/j.1574-6968.2011.02287.x. PMID 21545646.
2. Sauer U, Canonaco F, Heri S, Perrenoud A, Fischer E (Feb 2004). "The soluble and membrane-bound transhydrogenases UdhA and PntAB have divergent functions in NADPH metabolism of Escherichia coli". The Journal of Biological Chemistry. 279 (8): 6613–9. doi:10.1074/jbc.M311657200. PMID 14660605.

Further reading

• You KS (1985). "Stereospecificity for nicotinamide nucleotides in enzymatic and chemical hydride transfer reactions". CRC Critical Reviews in Biochemistry. 17 (4): 313–451. doi:10.3109/10409238509113625. PMID 3157549.

• Biology

• EC 1.6.1
• NADPH-dependent enzymes
• NADH-dependent enzymes
• Flavoproteins
• Enzymes of known structure