Biosynthesis of amino acids from sucrose and Krebs cycle metabolites by Rhizobium lupini bacteroids

Abstract

The possibility of amino acids biosynthesis from sucrose, metabolites of Krebs cycle or glyoxylate and ammonium by intact bacteroids has been studied. The suspension of intact Rhizobium lupini bacteroids in phosphate buffer solution pH 7.8 was shown to catalyse the biosynthesis from sucrose and ammonium of some amino acids, such as alanine, aspartic and glutamic acids, glycine and serine. The yield of alanine and aspartic acid was 2.5-3 times higher than that of other amino acids, which were formed in almost equal quantities. Intact bacteroids were also found to catalyse the biosynthesis of aspartic and glutamic acids, alanine and glycine from ammonium and Krebs cycle metabolites such as fumaric acid (FA), oxaloacetic acid (OAA), pyruvic acid (PA), alpha-ketoglutaric acid (alpha-KGA), malic acid (MA), as well as from glyoxylic acid (GOA). The biosynthesis of aspartic acid from fumaric acid was dominant. Besides that, the suspension of intact bacteroids catalysed transamination of aspartic and glutamic acids, the transamination of aspartic acid being especially intense with alpha-KGA and GOA. Aspartic acid was synthesized most efficiently through the amination of fumaric acid, while glutamic acid was better synthesized through the transamination of aspartic acid with alpha-KGA than through reductive amination of alpha-KGA. The experimental data proved that intact bacteroids possess Krebs cycle enzymes and primary ammonia assimilation enzymes. This enzyme complex permits bacteroids to detoxify ammonia, which they produce using sucrose and metabolites of Krebs cycle as the sources of carbon. The data obtained are of great interest as they prove the importance of bacteroids in the synthesis of amino acids from ammonium which is formed in the course of N2-fixation, and sucrose available from leaves.

Aspartate aminotransferase of Rhizobium leguminosarum has extended substrate specificity and metabolizes aspartate to enable N2 fixation in pea nodules

“…As the phenotype of aatA mutants seems to differ based on the physiological status of the plant, AatA is likely involved in balancing the endogenous aspartate, and by extension, wider amino acid pools in response to the host physiological status. As amino acids are not only secreted by the bacteroids (including aspartate and alanine) [16,[19][20][21]47] but also received from the plant [23,24], AatA is likely involved in integrating the symbiont into the host metabolism. Metabolomic analysis of the mutant in free-living conditions but also from bacteroids may elucidate the exact role of AatA on the wider metabolic landscape under these conditions.…”

“…In addition to alanine and ammonium, aspartate is also released by bacteroids to host plants [15,[18][19][20][21]. As bacteroids show reduced flux of carbon towards α-ketoglutarate [15], the assimilation of ammonium into glutamate is restricted.…”