Engineering Acetyl Coenzyme A Supply: Functional Expression of a Bacterial Pyruvate Dehydrogenase Complex in the Cytosol of Saccharomyces cerevisiae
Authors: Kozak Barbara U. , van Rossum Harmen M. , Luttik Marijke A. H. , Akeroyd Michiel , Benjamin Kirsten R. , Wu Liang , de Vries Simon , Daran Jean-Marc , Pronk Jack T. , van Maris Antonius J. A. 
Published: October 2014

ABSTRACTThe energetic (ATP) cost of biochemical pathways critically determines the maximum yield of metabolites of vital or commercial relevance. Cytosolic acetyl coenzyme A (acetyl-CoA) is a key precursor for biosynthesis in eukaryotes and for many industrially relevant product pathways that have been introduced intoSaccharomyces cerevisiae, such as isoprenoids or lipids. In this yeast, synthesis of cytosolic acetyl-CoA via acetyl-CoA synthetase (ACS) involves hydrolysis of ATP to AMP and pyrophosphate. Here, we demonstrate that expression and assembly in the yeast cytosol of an ATP-independent pyruvate dehydrogenase complex (PDH) fromEnterococcus faecaliscan fully replace the ACS-dependent pathway for cytosolic acetyl-CoA synthesis.In vivoactivity ofE. faecalisPDH required simultaneous expression ofE. faecalisgenes encoding its E1α, E1β, E2, and E3 subunits, as well as genes involved in lipoylation of E2, and addition of lipoate to growth media. A strain lacking ACS that expressed theseE. faecalisgenes grew at near-wild-type rates on glucose synthetic medium supplemented with lipoate, under aerobic and anaerobic conditions. A physiological comparison of the engineered strain and an isogenic Acs+reference strain showed small differences in biomass yields and metabolic fluxes. Cellular fractionation and gel filtration studies revealed that theE. faecalisPDH subunits were assembled in the yeast cytosol, with a subunit ratio and enzyme activity similar to values reported for PDH purified fromE. faecalis. This study indicates that cytosolic expression and assembly of PDH in eukaryotic industrial microorganisms is a promising option for minimizing the energy costs of precursor supply in acetyl-CoA-dependent product pathways.IMPORTANCEGenetically engineered microorganisms are intensively investigated and applied for production of biofuels and chemicals from renewable sugars. To make such processes economically and environmentally sustainable, the energy (ATP) costs for product formation from sugar must be minimized. Here, we focus on an important ATP-requiring process in baker’s yeast (Saccharomyces cerevisiae): synthesis of cytosolic acetyl coenzyme A, a key precursor for many industrially important products, ranging from biofuels to fragrances. We demonstrate that pyruvate dehydrogenase from the bacteriumEnterococcus faecalis, a huge enzyme complex with a size similar to that of a ribosome, can be functionally expressed and assembled in the cytosol of baker’s yeast. Moreover, we show that this ATP-independent mechanism for cytosolic acetyl-CoA synthesis can entirely replace the ATP-costly native yeast pathway. This work provides metabolic engineers with a new option to optimize the performance of baker’s yeast as a “cell factory” for sustainable production of fuels and chemicals.