14-01: Repurposing antibiotic-producing polyketide synthases to produce biofuels

Multimodular polyketide synthases (PKSs) are among the largest (2-7 MDa) and most complex enzymes known.  They carry out a programmed, step-wise process that leads to the generation of poly b-ketones with varying degrees of reductions at the b-carbonyl centers and with or without methyl groups at the a-carbons.  These products are called polyketides and are widely used as antibiotics and other pharmaceutical agents.  Multimodular PKSs utilize a wide range of starter units in the assembly of their products including branched acyl-CoAs such as 2-methylpropionyl (isobutyryl)-CoA, 2,2-dimethylpropionyl (pivaloyl)-CoA, 2-methylbutyryl-CoA, and 3-methylbutyryl (isovaleryl)-CoA.  Here we report an engineered PKS that starts synthesis using the above acyl-CoAs and extends once with (2S)-methylmalonyl-CoA in vitro.  The products are 3-hydroxy-2,4-dimethylpentanoate, 3-hydroxy-2,4,4-trimethylpentanoate, 3-hydroxy-2,4-dimethylhexanoate, and 3-hydroxy-2,5-dimethylhexanoate, respectively, which can be chemically reduced to 2,4-dimethylpentane, 2,2,4-trimethylpentane (isooctane), 2,4-dimethylhexane, and 2,5-dimethylhexane.  The octane ratings of these alkanes are 83.8, 100 (by definition), 69.9, 55.7, respectively.  We also report engineered Escherichia coli strains that can produce isobutyryl-CoA, 2-methylbutyryl-CoA, isovaleryl-CoA, or (2S)-methylmalonyl-CoA from glucose as a sole carbon source.  This work may open a way to produce a variety of gasoline precursors in E. coli from renewal carbon sources.