期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 108, 期 47, 页码 18920-18925出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1111085108
关键词
bioplastics; biomass conversion
资金
- Department of Energy [DE-FG36-04GO14019, DE-FG36-08GO88142]
- Department of Agriculture
- National Institute of Food and Agriculture [2011-10006-30358]
- State of Florida, University of Florida Agricultural Experiment Station
- Florida Energy Systems Consortium
- Myriant Technologies
Lactic acid, an attractive, renewable chemical for production of biobased plastics (polylactic acid, PLA), is currently commercially produced from food-based sources of sugar. Pure optical isomers of lactate needed for PLA are typically produced by microbial fermentation of sugars at temperatures below 40 degrees C. Bacillus coagulans produces L(+)-lactate as a primary fermentation product and grows optimally at 50 degrees C and pH 5, conditions that are optimal for activity of commercial fungal cellulases. This strain was engineered to produce D(-)-lactate by deleting the native ldh (L-lactate dehydrogenase) and alsS (acetolactate synthase) genes to impede anaerobic growth, followed by growth-based selection to isolate suppressor mutants that restored growth. One of these, strain QZ19, produced about 90 g L-1 of optically pure D(-)-lactic acid from glucose in <48 h. The new source of D-lactate dehydrogenase (D-LDH) activity was identified as a mutated form of glycerol dehydrogenase (GlyDH; D121N and F245S) that was produced at high levels as a result of a third mutation (insertion sequence). Although the native GlyDH had no detectable activity with pyruvate, the mutated GlyDH had a D-LDH specific activity of 0.8 mu moles min(-1) (mg protein)(-1). By using QZ19 for simultaneous saccharification and fermentation of cellulose to D-lactate (50 degrees C and pH 5.0), the cellulase usage could be reduced to 1/3 that required for equivalent fermentations by mesophilic lactic acid bacteria. Together, the native B. coagulans and the QZ19 derivative can be used to produce either L(+) or D(-) optical isomers of lactic acid (respectively) at high titers and yields from nonfood carbohydrates.
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