Efficient synthesis of L-lactic acid from glycerol by metabolically engineered Escherichia coli.
BACKGROUND
Due to its abundance and low-price, glycerol has become an attractive carbon source for the industrial production of value-added fuels and chemicals. This work reports the engineering of E. coli for the efficient conversion of glycerol into L-lactic acid (L-lactate).


RESULTS
Escherichia coli strains have previously been metabolically engineered for the microaerobic production of D-lactic acid from glycerol in defined media by disrupting genes that minimize the synthesis of succinate, acetate, and ethanol, and also overexpressing the respiratory route of glycerol dissimilation (GlpK/GlpD). Here, further rounds of rationale design were performed on these strains for the homofermentative production of L-lactate, not normally produced in E. coli. Specifically, L-lactate production was enabled by: 1), replacing the native D-lactate specific dehydrogenase with Streptococcus bovis L-lactate dehydrogenase (L-LDH), 2) blocking the methylglyoxal bypass pathways to avoid the synthesis of a racemic mixture of D- and L-lactate and prevent the accumulation of toxic intermediate, methylglyoxal, and 3) the native aerobic L-lactate dehydrogenase was blocked to prevent the undesired utilization of L-lactate. The engineered strain produced 50 g/L of L-lactate from 56 g/L of crude glycerol at a yield 93% of the theoretical maximum and with high optical (99.9%) and chemical (97%) purity.


CONCLUSIONS
This study demonstrates the efficient conversion of glycerol to L-lactate, a microbial process that had not been reported in the literature prior to our work. The engineered biocatalysts produced L-lactate from crude glycerol in defined minimal salts medium at high chemical and optical purity.
Department of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, MS-362, Houston, TX 77005, USA.
Suman
S
Mazumdar
Matthew D
MD
Blankschien
James M
JM
Clomburg
Ramon
R
Gonzalez
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism*; D005990:Glycerol / Q000378:metabolism*; D005991:Glycerol Kinase / Q000235:genetics / Q000378:metabolism; D005993:Glycerolphosphate Dehydrogenase / Q000235:genetics / Q000378:metabolism; D007700:Kinetics; D007770:L-Lactate Dehydrogenase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D019344:Lactic Acid / Q000096:biosynthesis*; D060847:Metabolic Engineering; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000235:genetics / Q000378:metabolism; D011765:Pyruvaldehyde / Q000378:metabolism; D013237:Stereoisomerism; D016985:Streptococcus bovis / Q000201:enzymology; D013401:Sugar Alcohol Dehydrogenases / Q000235:genetics / Q000378:metabolism
D001426:Bacterial Proteins; D019344:Lactic Acid; D011765:Pyruvaldehyde; D005993:Glycerolphosphate Dehydrogenase; D013401:Sugar Alcohol Dehydrogenases; D007770:L-Lactate Dehydrogenase; C020672:glycerol dehydrogenase; D017853:Phosphotransferases (Alcohol Group Acceptor); C023132:glycerone kinase; D005991:Glycerol Kinase; D005990:Glycerol