Hypothesized roles of acyl - CoA - active genes of the acetoin dehydrogenase gene cluster in P . carbinolicus . The products of acetoin dehydrogenase, acetaldehyde and acetyl-CoA, might accidentally undergo aldol addition before leaving the active site, forming either isomer of 3-hydroxybutanoyl-CoA. The methylmalonyl-CoA epimerase family protein (Pcar_0333) might convert the (R)-isomer to the (S)-isomer, which 3-hydroxyacyl-CoA dehydrogenase (Pcar_0349) can oxidize. A hydrolase/acyltransferase encoded in the cluster (Pcar_0351) might split 3-oxobutanoyl-CoA into acetate plus acetyl-CoA.