Isobutanol production in engineered Saccharomyces cerevisiae by overexpression of 2-ketoisovalerate decarboxylase and valine biosynthetic enzymes

Production of 2,3-butanediol by engineered Saccharomyces cerevisiae

In order to produce 2,3-butanediol (2,3-BD) with a high titer, it is necessary to engineer Saccharomyces cerevisiae by deleting the competing pathway and overexpressing the 2,3-BD biosynthetic pathway. A pyruvate decarboxylase (Pdc)-deficient mutant was constructed and evolved for rapid glucose consumption without ethanol production. Genome re-sequencing of the evolved strain (SOS4) revealed a point mutation (A81P) in MTH1 coding for a transcriptional regulator involved in glucose sensing, unlike the previously reported Pdc-deficient mutant which had internal deletion in MTH1. When alsS and alsD genes from Bacillus subtilis, and endogenous BDH1 gene were overexpressed in SOS4, the resulting strain (BD4) not only produced 2,3-BD efficiently, but also consumed glucose faster than the parental strain. In fed-batch fermentation with optimum aeration, 2,3-BD concentration increased up to 96.2 g/L. These results suggest that S. cerevisiae might be a promising host for producing 2,3-BD for industrial applications.

A novel pathway to produce butanol and isobutanol in Saccharomyces cerevisiae

BACKGROUND

The sustainable production of biofuels remains one of the major issues of the upcoming years. Among the number of most desirable molecules to be produced, butanol and isobutanol deserve a prominent place. They have superior liquid-fuel features in respect to ethanol. Particularly, butanol has similar properties to gasoline and thus it has the potential to be used as a substitute for gasoline in currently running engines. Clostridia are recognized as natural and good butanol producers and are employed in the industrial-scale production of solvents. Due to their complex metabolic characteristics and to the difficulty of performing genetic manipulations, in recent years the Clostridia butanol pathway was expressed in other microorganisms such as Escherichia coli and Saccharomyces cerevisiae, but in yeast the obtained results were not so promising. An alternative way for producing fusel alcohol is to exploit the degradation pathway of aminoacids released from protein hydrolysis, where proteins derive from exhausted microbial biomasses at the end of the fermentation processes.

RESULTS

It is known that wine yeasts can, at the end of the fermentation process, accumulate fusel alcohols, and butanol is among them. Despite it was quite obvious to correlate said production with aminoacid degradation, a putative native pathway was never proposed. Starting from literature data and combining information about different organisms, here we demonstrate how glycine can be the substrate for butanol and isobutanol production, individuating at least one gene encoding for the necessary activities leading to butanol accumulation. During a kinetic of growth using glycine as substrate, butanol and isobutanol accumulate in the medium up to 92 and 58 mg/L, respectively.

CONCLUSIONS

Here for the first time we demonstrate an alternative metabolic pathway for butanol and isobutanol production in the yeast S. cerevisiae, using glycine as a substrate. Doors are now opened for a number of optimizations, also considering that starting from an aminoacid mixture as a side stream process, a fusel alcohol blend can be generated.

Microbial synthesis of n-butanol, isobutanol, and other higher alcohols from diverse resources

Microbial production of fuel and chemical feedstock is a promising approach to solving energy and environmental problems. n-Butanol, isobutanol and other higher alcohols are of particular interest because they can serve as both fuel and chemical feedstock. Alternative resources such as CO2, syngas, waste protein, and lignocellulose are currently being investigated for their potential to produce these compounds. Except for lignocellulose, utilization of such alternative resource has not been examined extensively. This review aims to summarize the development of metabolic pathways for efficient synthesis of these higher alcohols and the current status of microbial strain development for the conversion of diverse resources into higher alcohols.

Compartmentalization of metabolic pathways in yeast mitochondria improves the production of branched-chain alcohols

Efforts to improve the production of a compound of interest in Saccharomyces cerevisiae have mainly involved engineering or overexpression of cytoplasmic enzymes. We show that targeting metabolic pathways to mitochondria can increase production compared with overexpression of the enzymes involved in the same pathways in the cytoplasm. Compartmentalization of the Ehrlich pathway into mitochondria increased isobutanol production by 260%, whereas overexpression of the same pathway in the cytoplasm only improved yields by 10%, compared with a strain overproducing enzymes involved in only the first three steps of the biosynthetic pathway. Subcellular fractionation of engineered strains revealed that targeting the enzymes of the Ehrlich pathway to the mitochondria achieves greater local enzyme concentrations. Other benefits of compartmentalization may include increased availability of intermediates, removing the need to transport intermediates out of the mitochondrion and reducing the loss of intermediates to competing pathways.