Genetic mapping of a mutation affecting pyridine nucleotide transhydrogenase in Escherichia coli

Mitochondrial NADPH, transhydrogenase and disease

Ever since its discovery in 1953 by N. O. Kaplan and coworkers, the physiological role of the proton-translocating transhydrogenase has generally been assumed to be that of generating mitochondrial NADPH. Mitochondrial NADPH can be used in a number of important reactions/processes, e.g., biosynthesis, maintenance of GSH, apoptosis, aging etc. This assumed role has found some support in bacteria but not in higher eukaryotes, a situation which changed dramatically with two recent but separate findings, both using transhydrogenase knockouts, in the nematode C. elegans and the mouse strain C57BL/6J. The latter, which is due to a spontaneous deletion mutation in the Nnt gene, was serendipitously found during investigations of the diabetic properties of these mice. The implications of these findings for the overall role of transhydrogenase in cell metabolism and disease are discussed.

The mechanism of hydride transfer between NADH and 3-acetylpyridine adenine dinucleotide by the pyridine nucleotide transhydrogenase of Escherichia coli

The pyridine nucleotide transhydrogenase of Escherichia coli catalyzes the reversible transfer of hydride ion equivalents between NAD+ and NADP+ coupled to translocation of protons across the cytoplasmic membrane. Recently, transhydrogenation of 3-acetylpyridine adenine dinucleotide (AcPyAD+), an analog of NAD+, by NADH has been described using a solubilized preparation of E. coli transhydrogenase [Hutton, M., Day, J.M., Bizouarn, T., and Jackson, J.B. (1994) Eur. J. Biochem. 219, 1041-1051]. This reaction depended on the presence of NADP(H). We show that (a) this reaction did not require NADP(H) at pH 6 in contrast to pH 8; (b) the reaction occurred at pH 8 in the absence of NADP(H) in the mutant beta H91K and in a mutant in which six amino acids of the carboxy-terminus of the alpha subunit had been deleted; (c) the mutant transhydrogenases contained bound NADP+ and were in a conformation in which the beta subunit was digestible by trypsin; (d) the conformation of the beta subunit of the wild-type enzyme was made susceptible to trypsin digestion by NADP(H) or by placing the enzyme at pH 6 in the absence of NADP(H). It is concluded that reduction of AcPyAD+ by NADH does not involve NADPH as an intermediate and that the role of NADP(H) in this reaction at pH 8 is to cause the transhydrogenase to adopt a conformation favouring transhydrogenation between NADH and AcPyAD+.

A mutation at Gly314 of the beta subunit of the Escherichia coli pyridine nucleotide transhydrogenase abolishes activity and affects the NADP(H)-induced conformational change

Escherichia coli RH1 contains a mutation causing complete loss of pyridine nucleotide transhydrogenase activity. A single base change in the chromosomal DNA resulted in the replacement of Gly314 of the beta subunit by a Glu residue. The mutant enzyme was partially purified and its trypsin cleavage products examined. The distinct pattern of polypeptides given by proteolysis of the normal transhydrogenase in the presence of NADP(H) was absent when the mutant enzyme was treated with trypsin. However, the beta subunit of the mutant enzyme retained its ability to bind to NAD-agarose. Further substitutions were made at Gly314 converting it to Ala, Val or Cys by the use of site-directed mutagenesis. All substitutions for Gly314 abolished the activity completely. The enzyme containing the Gly314----Ala mutation was studied in detail and behaved exactly as the enzyme containing the Gly314----Glu mutation. It is concluded that the mutation in the beta subunit abolished the NADP(H)-induced conformational change in the mutant enzyme. This conformational change, caused by NADP(H) binding, is required to cleave the normal beta subunit at Arg265 by trypsin. The genes encoding the pyridine nucleotide transhydrogenase were completely resequenced and several corrections have been made to the previously published sequence [Clarke et al. (1986) Eur. J. Biochem. 158, 647-653].

Coregulation of oxidized nicotinamide adenine dinucleotide (phosphate) transhydrogenase and glutamate dehydrogenase activities in enteric bacteria during nitrogen limitation

The relationship between oxidized nicotinamide adenine dinucleotide (phosphate) [NAD(P)+] transhydrogenase (EC 1.6.1.1) and NAD(P)+ glutamate dehydrogenase in several enteric bacteria which differ slightly in their regulation of nitrogen metabolism was studied. Escherichia coli strain K-12 was grown on glucose and various concentrations of NH4Cl as the sole nitrogen source. In the range of 0.5 to 20 mM NH4Cl, the energy-independent transhydrogenase increased two to threefold. Comparable changes occurred in NAD(P)+-linked glutamate dehydrogenase. NH4Cl concentrations of 20 to 60 mM resulted in relatively constant specific activities for both enzymes. Higher exogenous NH4Cl, however, led to a decline in both activities. Isocitrate dehydrogenase, another potential source of cellular NADPH, was insensitive to NH4Cl limitation. Similar studies in the presence of glutamate and different exogenous NH4Cl concentrations again showed concerted effects on both enzymes. Growth on glutamate as the sole nitrogen source led to severe repression of both transhydrogenase and glutamate dehydrogenase. In Salmonella typhimurium, both enzymes were unaffected by limiting NH4Cl or growth on glutamate as the sole nitrogen source. Both were, however, repressed by growth on aspartate, a potential source of cellular glutamate. Coordinate changes in glutamate dehydrogenase and transhydrogenase were also evident in Klebsiella aerogenes, particularly under conditions in which glutamate dehydrogenase was regulated inversely to glutamate synthetase. Coordinate changes in glutamate dehydrogenase and transhydrogenase in enteric bacteria are discussed in terms of the possible involvement of the latter enzyme as a direct source of NADPH in the ammonia assimilation system.

Improved mapping of the tyrS locus in Escherichia coli

A tyrosyl-tRNA synthetase mutant of Escherichia coli was isolated and the tyrS gene assigned a map position between man and pdxH at 36.0 min on the chromosome. The tyrS mutant grew badly on broth as did previously described tyrS mutants. This sensitivity to broth was suppressed by tyrR mutations. F-prime factors were found to complement the tyrS mutation.

Effects of an insertion mutation in a locus affecting pyridine nucleotide transhydrogenase (pnt::Tn5) on the growth of Escherichia coli

The effects of a pnt::Tn5 insertion mutation on the growth of strains lacking phosphoglucoisomerase or glucose 6-phosphate dehydrogenase were examined. The results support the idea that the energy-linked transhydrogenase is an important source of reduced nicotinamide adenine dinucleotide phosphate for Escherichia coli under some conditions.