Catalyses a step in the 3-hydroxypropanoate/4-hydroxybutanoate cycle, an autotrophic CO2 fixation pathway found in some thermoacidophilic archaea [1,2].The enzymes from Metallosphaera sedula and Sulfolobus tokodaii can also use propionate, acrylate, acetate, and butanoate as substrates [2], and are thus different from EC 6.2.1.17 (propionate---CoA ligase), which does not accept acetate or butanoate.