The metabolism of C2 compounds in micro-organisms. 3. Synthesis of malate from acetate via the glyoxylate cycle

Publisher Website
Google Scholar
Abstract
Reactions of acetate were studied in cell-free extracts of acetate-grown Pseudomonas KB 1. Such extracts are capable of catalyzing the formation of acetyl-coenzyme A from acetate. Acetyl-coenzyme A thus formed reacts both with oxalacetate, to form citrate, and with glyoxylate to form malate. By the use of extracts from mutant M 22-64 of Escherichia coli, it was shown that these condensations are catalyzed by different enzymes. The aldol fission of isocitrate to succinate and glyoxylate can provide the glyoxylate necessary for the formation of malate. The cell-free extracts catalyze the net anaerobic formation of malate and succinate from acetate and isocitrate. These reactions, in conjunction with the reactions leading from acetate to citrate, from citrate to isocitrate and from malate to oxaloacetate, shown to occur in the bacterial extracts, represent a synthesizing variant of the tricarboxylic acid cycle leading to the formation of 1 molecule of C4 dicarboxylic acid from 2 molecules of acetate. The rate of formation of malate from acetate and isocitrate is at least 3[mu] moles/hour/extract from 1 mg dry weight of bacteria. Since malate is known to be a precursor of most cell constituents, this rate is sufficiently high to account for the observed rates of growth of Pseudomonas KB 1 on ammonium acetate. The bacterial extracts also catalyze an interconversion of malate with pyruvate and CO2 that is linked with triphosphopyridine nucleotide.