For diagram of reaction click here (another example and another example.
Comments: Catalyses a step in the 3-hydroxypropanoate/4-hydroxybutanoate cycle, an autotrophic CO2 fixation pathway found in some thermoacidophilic archaea . The enzyme from Metallosphaera sedula acts nearly equally as well on (S)-3-hydroxybutanoyl-CoA but not (R)-3-hydroxybutanoyl-CoA .