Structural requirements for the binding of AMP to the allosteric site of NAD-specific isocitrate dehydrogenase from bakers' yeast

Regulation of NAD(+)-dependent isocitrate dehydrogenase in the citrate producing yeast Yarrowia lipolytica

The mechanism of the increased accumulation (overproduction) of citric acids in the yeast Yarrowia lipolytica while growing in the presence of glucose under nitrogen deficiency was investigated. The limitation of the yeast growth by the source of nitrogen decreases the total content of nucleotides and increases the ratios of ATP/AMP and NADH/NAD+. NAD+-Dependent isocitrate dehydrogenase, an enzyme of the tricarboxylic acid cycle playing a key role in the regulation of biosynthesis of citric and isocitric acids, was isolated from Y. lipolytica. The molecular weights of the native enzyme and its subunits were found to be 412 and 52 kD, respectively. It is concluded that the enzyme is a homo-oligomer consisting of eight subunits. Investigation of the effect of some intermediates of the tricarboxylic acid cycle on the activity of this enzyme suggests that the enhanced excretion of citric acids can be caused by the inhibition of NAD+-dependent isocitrate dehydrogenase due to the decrease in the content of AMP and increase in the NADH/NAD+ ratio in the cells of Y. lipolytica under depletion of nitrogen.

A dual cofactor-specific isocitrate dehydrogenase from Pythium ultimum

Isocitrate dehydrogenase is considered to be one of the key regulatory enzymes in the conversion of glucose into fatty acids by oleaginous microorganisms. A dual coenzyme-specific isocitrate dehydrogenase (EC 1.1.1.41) (IDH) was isolated from the primitive fungus Pythium ultimum and purified by 211-fold by sequential ion-exchange, affinity, and gel filtration chromatographies. Specific activity of the partially purified enzyme was 76.2 mumol/(min.mg protein) with NAD+ and 40% less active with NADP+. Optimum pH for activity was 8.5-9.5. K(m) values for threo-D-isocitrate and NAD+ were 0.031 and 0.55 mM, respectively. The estimated molecular mass of the IDH was 96 kDa under nondenaturing conditions and 48 kDa under denaturing conditions, suggesting that the enzyme is composed of two subunits of the same size. The enzyme was relatively stable up to 55 degrees C, but no activity was detected after exposure to 65 degrees C for 15 min. Mg2+ or Mn2+ were required for activity.

Kinetic analysis of NAD(+)-isocitrate dehydrogenase with altered isocitrate binding sites: contribution of IDH1 and IDH2 subunits to regulation and catalysis

NAD(+)-dependent isocitrate dehydrogenase from Saccharomyces cerevisiae is an allosterically regulated enzyme that exists as an octamer composed of two nonidentical subunits, designated IDH1 and IDH2. To determine the contribution of each subunit to regulation and catalysis, a conserved serine residue at the proposed active site of each subunit was mutated to alanine. This mutation in IDH1 resulted in a 6-fold decrease in Vmax and a decrease in cooperativity, but little change in S0.5 for isocitrate. The mutant IDH2, in contrast, exhibited a 60-fold decrease in maximal velocity and a 2-fold reduction in S0.5 for isocitrate, but the cooperativity was unaffected. Responses to the allosteric modifier AMP also differed for the two mutant enzymes. The IDH1 mutant enzyme was not activated by AMP, whereas the IDH2 mutant enzyme exhibited an increase in isocitrate affinity in the presence of AMP similar to that observed with the wild-type enzyme. On the basis of these kinetic results, a model is presented which proposes that IDH1 functions as a regulatory subunit while IDH2 functions in catalysis. To determine if IDH1 or IDH2 alone is catalytically active, we also expressed the individual subunits in yeast strains in which the gene encoding the other subunit had been disrupted. Mitochondrial extracts from strains overexpressing solely IDH1 or IDH2 contained no detectable activity in the presence or absence of AMP. Gel filtration of these extracts showed that both IDH1 and IDH2 behaved as monomers, suggesting that the major subunit interactions within the octamer are between IDH1 and IDH2.