Defects of glucose-6-phosphate and 6-phosphogluconate dehydrogenases in Neurospora and their pleiotropic effects

Isolation and characterization of the glucose-6-phosphate dehydrogenase encoding gene (gsdA) from Aspergillus niger

Genomic and cDNA clones encoding glucose-6-phosphate dehydrogenase (G6PD) were isolated from the fungus Aspergillus niger. Sequence analysis of the glucose-6-phosphate dehydrogenase gene (gsdA) revealed an open reading frame of 1530 bp, encoding a protein of 58,951 kDa. The gsdA gene is interrupted by nine introns the most proximal of which is exceptionally large (348 bp). The region upstream of the ATG contains several C+T-rich stretches. The two major and one minor transcription start points are all located within these regions. In the upstream region several direct and inverted repeats, but no clear TATA or CCAAT boxes can be found. A. niger strains overproducing G6PD were constructed by cotransformation of gsdA subclones. Overexpression of G6PD was shown to be deleterious for the fungus, especially when cotransformants were grown on media containing ammonia. Attempts to construct a gsdA null mutant by gene disruption were unsuccessful.

Isolation and characterization of the ZWF1 gene of Saccharomyces cerevisiae, encoding glucose-6-phosphate dehydrogenase

Glucose-6-phosphate dehydrogenase (G6PD) catalyzes the first step of the pentose phosphate pathway, a reaction that generates NADPH. We have isolated ZWF1, the Saccharomyces cerevisiae gene that encodes G6PD, and identified its transcript and transcription start point. Expression of ZWF1 appears not to be regulated, consistent with its 'housekeeping' role. Null mutants lacking G6PD appear to grow normally, but are more sensitive than wild type to oxidizing agents that presumably reduce the level of NADPH. This suggests that G6PD has a major role in NADPH production in yeast. Regulation of GAL1 expression appears normal in zwf1 mutants, suggesting that the pentose phosphate pathway is not involved in glucose repression. The predicted amino acid sequence of yeast G6PD is highly similar to the sequence of the Drosophila, human, and rat enzymes, except near its N terminus, where the yeast and Drosophila sequences diverge from that of human and rat.

Relationship between deficiency of phosphoglucose isomerase in Coprinus macrorhizus and fruiting body formation

A mutant (pgi) of Coprinus macrorhizus deficient in phosphoglucose isomerase did not grow on fructose and grew poorly on glucose. The pgi mutation inhibited the formation of monokaryotic and dikaryotic fruiting bodies.

The regulation of hexokinase and phosphoglucomutase activity in Aspergillus nidulans

The levels of glucose-6-phosphate and 6-phosphogluconate dehydrogenase in wildtype cells of Aspergillus nidulans varied with the carbon and nitrogen source. In general, hexokinase activity did not vary with carbon or nitrogen source. The ammonium derepressed mutant amrA1 had only 50% of the wildtype level of hexokinase. Phosphoglucomutase activity was low in wildtype cells grown with nitrate, but high in cells grown with ammonium when glucose was the carbon source. A non-inducible mutant, nirA-1, in the regulatory gene for nitrate reductase, had high phosphoglucomutase activity when grown with nitrate or ammonium. A constitutive mutant nirAc1, in the regulatory gene for nitrate reductase had low phosphoglucomutase activity when grown with nitrate or ammonium. The mutants nir-1 and nirAc1 are recessive and semi-dominant respectively for abnormal phosphoglucomutase activity.

Selective denaturation of several yeast enzymes by free fatty acids

The denaturation of eight purified yeast enzymes, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, alcohol dehydrogenase, beta-fructosidase, hexokinase and glucose-6-phosphate isomerase, promoted under controlled conditions by the free fatty acids myristic and oleic, is selective. Glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1 oxidoreductase, EC 1.1.1.49) is extremely sensitive to destabilization and was studied in greater detail. Results show that chain length and degree of unsaturation of fatty acids are important to their destabilizing effect, and that ligands of the enzyme can afford protection. The denaturation process results in more than one altered form. These results can be viewed in the perspective of the possibility that amphipathic substances, and in particular free fatty acids, may play a role for enzyme degradation in vivo, by initiating steps of selective denaturation.

Characterization of the fatty acid-sensitive glucose 6-phosphate dehydrogenase from Pseudomonas cepacia

The adenosone 5'-triphosphate-insensitive glucose 6-phosphate dehydrogenase from Pseudomonas cepacia has been found to be strongly inhibited by long-chain fatty acids and their acyl coenzyme A esters, suggesting that an important role of this isoenzyme might be to provide reduced nicotinamide adenine dinucleotide phosphate for reductive steps in fatty acid synthesis. The enzyme, which has been redesignated the fatty acid-sensitive glucose 6-phosphate dehydrogenase, has been purified to homogeneity using affinity chromatography with nicotinamide adenine dinulceotide phosphate-substituted Sepharose as a key step in the purification. The purified preparations were used to study the immunological properties and subunit composition of the enzyme and its relationship to the adenosine 5'-triphosphate-sensitive glucose 6-phosphate dehydrogenase present in extracts of P. cepacia. Although both enzymes were found to be composed of similar size subunits of about 60,000 daltons, immunological studies failed to demonstrate any antigenic similarity between them. Studies of the sedimentation behavior of the fatty acid-sensitive enzyme in sucrose gradients indicated that its apparent molecular weight is increased in the presence of glucose 6-phosphate and suggest that it may exist in an aggregated state in vivo. Palmitoyl coenzyme A, which strongly inhibited the enzyme, failed to influence its sedimentation behavior.