Involved in syntrophic growth of S.wolfei with butyrate, as part of the butyrate oxidation pathway. Probably catalyzes the oxidation of 3-hydroxybutyryl-CoA to acetoacetyl-CoA. 3-hydroxybutanoyl-CoA + NAD(+) = acetoacetyl-CoA + H(+) + NADH a (3S)-3-hydroxyacyl-CoA + NAD(+) = a 3-oxoacyl-CoA + H(+) + NADH Lipid metabolism; butanoate metabolism. Highly expressed during syntrophic growth with butyrate (at protein level). Seems to be constitutively expressed. Belongs to the 3-hydroxyacyl-CoA dehydrogenase family.