Plays a role in autotrophic carbon fixation via the 3-hydroxypropionate/4-hydroxybutyrate cycle. Catalyzes the reversible dehydration of 3-hydroxypropionyl-CoA to form acryloyl-CoA, and the reversible dehydration of (S)-3-hydroxybutyryl-CoA to form crotonyl-CoA. Inactive towards (R)-3-hydroxybutyryl-CoA. 3-hydroxypropanoyl-CoA = acryloyl-CoA + H2O Optimum pH is 8.1 at room temperature. Retains half maximum activity at pH 6.5 and pH 9.5 at room temperature. Monomer. Belongs to the enoyl-CoA hydratase/isomerase family.