Abstract
BackgroundIsobutanol is an important biorefinery target alcohol that can be used as a fuel, fuel additive, or commodity chemical. Baker’s yeast, Saccharomyces cerevisiae, is a promising organism for the industrial manufacture of isobutanol because of its tolerance for low pH and resistance to autolysis. It has been reported that gene deletion of the pyruvate dehydrogenase complex, which is directly involved in pyruvate metabolism, improved isobutanol production by S. cerevisiae. However, the engineering strategies available for S. cerevisiae are immature compared to those available for bacterial hosts such as Escherichia coli, and several pathways in addition to pyruvate metabolism compete with isobutanol production.ResultsThe isobutyrate, pantothenate or isoleucine biosynthetic pathways were deleted to reduce the outflow of carbon competing with isobutanol biosynthesis in S. cerevisiae. The judicious elimination of these competing pathways increased isobutanol production. ILV1 encodes threonine ammonia-lyase, the enzyme that converts threonine to 2-ketobutanoate, a precursor for isoleucine biosynthesis. S. cerevisiae mutants in which ILV1 had been deleted displayed 3.5-fold increased isobutanol productivity. The ΔILV1 strategy was further combined with two previously established engineering strategies (activation of two steps of the Ehrlich pathway and the transhydrogenase-like shunt), providing 11-fold higher isobutanol productivity as compared to the parent strain. The titer and yield of this engineered strain was 224 ± 5 mg/L and 12.04 ± 0.23 mg/g glucose, respectively.ConclusionsThe deletion of competitive pathways to reduce the outflow of carbon, including ILV1 deletion, is an important strategy for increasing isobutanol production by S. cerevisiae.
Highlights
Isobutanol is an important biorefinery target alcohol that can be used as a fuel, fuel additive, or commodity chemical
Strategy to reduce the competitive outflow of carbon during isobutanol biosynthesis Several enzymes have broad substrate specificities; for example, aldehyde dehydrogenase can catalyze the oxidation of several kinds of aldehydes such as acetaldehyde, isobutyraldehyde, isopentaldehyde, and 2-methyl-butyraldehyde [23,24]
It is expected that the prevention of isoleucine biosynthesis would stop the competitive outflow of carbon from the pyruvate pathway to the isoleucine pathway, and should consolidate the activities of three enzymes (ALS, ketol-acid reductoisomerase (KARI) and dihydroxyacid dehydratase (DHAD)) into valine and isobutanol biosynthesis
Summary
Isobutanol is an important biorefinery target alcohol that can be used as a fuel, fuel additive, or commodity chemical. Engineered microbial strains for producing isobutanol have been developed by introducing parts of the Ehrlich pathway into bacterial hosts such as Escherichia coli, Corynebacterium glutamicum, Clostridium cellulolyticum, and Bacillus subtilis [3,4,5,6,7,8]. In these recombinant strains, an intermediate of valine biosynthesis, 2-ketoisovalerate, is converted into isobutanol through isobutyraldehyde by two steps of the Ehrlich pathway involving 2-keto acid decarboxylase (2-KDC). In E. coli in particular, additional metabolic modifications, such as deletion of competing pathways and resolving cofactor imbalance, have provided quite high yields of isobutanol (21.2 g/L and 13.4 g/L; 76% and 100% of theoretical maximum yields, respectively) [9,10]
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