Aconitase and Isocitric Dehydrogenases of Aspergillus niger in Relation to Citric Acid Production

Abstract

SUMMARYAconitase and NAD-and NADP-linked isocitric dehydrogenases were examined in two strains of Aspergillus niger. The mutant strain 72-44 produced higher yields of citric acid on the culture medium used than did the parent strain 7 2 4 . After growth for 22 hr on a medium in which citric acid did not accumulate, the amounts of each enzyme in the parent strain were approximately the same as those in the mutant strain. The enzymes were also found, though in lower amounts, throughout the incubation period of 8 days in both strains grown on a medium in which citric acid accumulated. A 20-fold increase in the concentration of iron in this medium doubled the activity of aconitase in extracts from the mutant strain, though citric acid accumulation was only decreased by 25 %. The addition of monofluoroacetate to the culture medium was toxic to the mould and did not stimulate citric acid yields. It is suggested that during the incubation, some recycling of citric acid may take place. The difference in citric acid yields from the parent and mutant strains was not accounted for on the basis of the degrees of aconitase or isocitric dehydrogenase activity.

Overview of citric acid production from Aspergillus niger

“…These enzymes are known as NADP + -dependent forms (one exists in mitochondria and another in the cytoplasm) and NAD + -dependent isocitrate dehydrogenases (which exists only in mitochondria). The NAD + -dependent enzymes are found in lower quantities than the NADP + -dependent enzymes (La Nauze 1966;Jernejc et al 1992). …”

Organic Acid Production by Filamentous Fungi

“…Aconitase inhibition would theoretically lead to an accumulation of citrate by blocking subsequent flux through the citric acid cycle. However, there were no significant differences in aconitase or isocitrate dehydrogenase activity levels in the A. niger parent strain and in mutant strains producing higher yields of citric acid (La Nauze, 1966). Furthermore, aconitase has been shown to be active even under citric acid production conditions .…”

Biotechnological production of citric acid

“…Finally, there are three different isozymes of isocitrate dehydrogenase in A. niger , a NAD + -dependent form present in mitochondria at low concentrations, and two NADP + -dependent forms, one in the mitochondria and the other in the cytoplasm (40, 51). All require Mg 2+ or Mn 2+ and the NADP + -dependent form is inhibited by citric and α-ketoglutaric acids (40, 61), which ensures the accumulation of citric acid.…”