Previously, odd-chain FAs were produced by addition of
propionate to the medium for proof-of-concept, but this
approach would generally be too expensive for commercial
applications. In other work, propionyl-CoA was produced
from phosphoenolpyruvate (derived from glucose or
glycerol) via threonine in nine steps with an energy cost
of three ATP per propionyl-CoA (Tseng & Prather, 2012).
Lengthy pathways are more difficult to engineer and ATP
consumption generally reduces product yield (particularly
under anaerobic conditions) as some feedstock must be
consumed to supply the needed energy. Thus, the B12-
dependent pathway used here (in combination with a
previously described pathway for 1,2-propanediol production
from glucose; Clomburg & Gonzalez, 2011)