Methylglyoxal Bypass Identified as Source of Chiral Contamination in l(+) and d(−)-lactate Fermentations by Recombinant Escherichia coli

Abstract
Two new strains of Escherichia coli B were engineered for the production of lactate with no detectable chiral impurity. All chiral impurities were eliminated by deleting the synthase gene (msgA) that converts dihydroxyacetone-phosphate to methylglyoxal, a precursor for both l(+)- and d(−)-lactate. Strain TG113 contains only native genes and produced optically pure d(−)-lactate. Strain TG108 contains the ldhL gene from Pediococcus acidilactici and produced only l(+)-lactate. In mineral salts medium containing 1 mM betaine, both strains produced over 115 g (1.3 mol) lactate from 12% (w/v) glucose, >95% theoretical yield.