Microbial biosynthesis of lactate esters

BACKGROUND

Green organic solvents such as lactate esters have broad industrial applications and favorable environmental profiles. Thus, manufacturing and use of these biodegradable solvents from renewable feedstocks help benefit the environment. However, to date, the direct microbial biosynthesis of lactate esters from fermentable sugars has not yet been demonstrated.

RESULTS

In this study, we present a microbial conversion platform for direct biosynthesis of lactate esters from fermentable sugars. First, we designed a pyruvate-to-lactate ester module, consisting of a lactate dehydrogenase (ldhA) to convert pyruvate to lactate, a propionate CoA-transferase (pct) to convert lactate to lactyl-CoA, and an alcohol acyltransferase (AAT) to condense lactyl-CoA and alcohol(s) to make lactate ester(s). By generating a library of five pyruvate-to-lactate ester modules with divergent AATs, we screened for the best module(s) capable of producing a wide range of linear, branched, and aromatic lactate esters with an external alcohol supply. By co-introducing a pyruvate-to-lactate ester module and an alcohol (i.e., ethanol, isobutanol) module into a modular Escherichia coli (chassis) cell, we demonstrated for the first time the microbial biosynthesis of ethyl and isobutyl lactate esters directly from glucose. In an attempt to enhance ethyl lactate production as a proof-of-study, we re-modularized the pathway into (1) the upstream module to generate the ethanol and lactate precursors and (2) the downstream module to generate lactyl-CoA and condense it with ethanol to produce the target ethyl lactate. By manipulating the metabolic fluxes of the upstream and downstream modules through plasmid copy numbers, promoters, ribosome binding sites, and environmental perturbation, we were able to probe and alleviate the metabolic bottlenecks by improving ethyl lactate production by 4.96-fold. We found that AAT is the most rate-limiting step in biosynthesis of lactate esters likely due to its low activity and specificity toward the non-natural substrate lactyl-CoA and alcohols.

CONCLUSIONS

We have successfully established the biosynthesis pathway of lactate esters from fermentable sugars and demonstrated for the first time the direct fermentative production of lactate esters from glucose using an E. coli modular cell. This study defines a cornerstone for the microbial production of lactate esters as green solvents from renewable resources with novel industrial applications.

Modulating acetate ester and higher alcohol production in Saccharomyces cerevisiae through the cofactor engineering

Flavor production by esters or by higher alcohols play a key role in the sensorial quality of fermented alcoholic beverages. In Saccharomyces cerevisiae cells, the syntheses of esters and higher alcohols are considerably influenced by intracellular CoA levels catalyzed by pantothenate kinase. In this work, we examined the effects of cofactor CoA and acetyl-CoA synthesis on the metabolism of esters and higher alcohols. Strains 12α-BAP2 and 12α+ATF1 where generated by deleting and overexpressing BAP2 (encoded branched-chain amino acid permease) and ATF1 (encoded alcohol acetyl transferases), respectively, in the parent 12α strains. Then, 12α-BAP2+CAB1 and 12α-BAP2+CAB3 strains were obtained by overexpressing CAB1 (encoded pantothenate kinase Cab1) and CAB3 (encoded pantothenate kinase Cab3) in the 12α-BAP2 strain, and 12α-BAP2+CAB1+ATF1 and 12α-BAP2+CAB3+ATF1 were generated by overexpressing ATF1 in the pantothenate kinase overexpression strains. The acetate ester level in 12α-BAP2 was slightly changed relative to that in the control strain 12α, whereas the acetate ester levels in 12α-BAP2+CAB1, 12α-BAP2+CAB3, 12α-BAP2+CAB1+ATF1, and 12α-BAP2+CAB3+ATF1 were distinctly increased (44-118% for ethyl acetate and 18-57% for isoamyl acetate). The levels of n-propanol, methyl-1-butanol, isopentanol, isobutanol, and phenethylol levels were changed and varied among the six engineered strains. The levels of acetate esters and higher alcohols can be modulated by changing the CoA and acetyl-CoA levels. The method proposed in this work supplies a practical means of breeding yeast strains by modulating acetate ester and higher alcohol production.

Regulation of Saccharomyces cerevisiae genetic engineering on the production of acetate esters and higher alcohols during Chinese Baijiu fermentation

Acetate esters and higher alcohols greatly influence the quality and flavor profiles of Chinese Baijiu (Chinese liquor). Various mutants have been constructed to investigate the interactions of ATF1 overexpression, IAH1 deletion, and BAT2 deletion on the production of acetate esters and higher alcohols. The results showed that the overexpression of ATF1 under the control of the PGK1 promoter with BAT2 and IAH1 double-gene deletion led to a higher production of acetate esters and a lower production of higher alcohols than the overexpression of ATF1 with IAH1 deletion or overexpression of ATF1 with BAT2 deletion. Moreover, deletion of IAH1 in ATF1 overexpression strains effectively increased the production of isobutyl acetate and isoamyl acetate by reducing the hydrolysis of acetate esters. The decline in the production of higher alcohol by the ATF1 overexpression strains with BAT2 deletion is due to the interaction of ATF1 overexpression and BAT2 deletion. Mutants with varying abilities of producing acetate esters and higher alcohols were developed by genetic engineering. These strains have great potential for industrial application.