Carbon isotopic fractionation in heterotrophic microbial metabolism

Abstract

Differences in the natural-abundance carbon stable isotopic compositions between products from aerobic cultures of Escherichia coli K-12 were measured. Respired CO2 was 3.4‰ depleted in 13C relative to the glucose used as the carbon source, whereas the acetate was 12.3‰ enriched in 13C. The acetate 13C enrichment was solely in the carboxyl group. Even though the total cellular carbon was only 0.6‰ depleted in 13C, intracellular components exhibited a significant isotopic heterogeneity. The protein and lipid fractions were -1.1 and -2.7‰, respectively. Aspartic and glutamic acids were -1.6 and +2.7‰, respectively, yet citrate was isotopically identical to the glucose. Probable sites of carbon isotopic fractionation include the enzyme, phosphotransacetylase, and the Krebs cycle.