Catalyzes the NADPH-dependent reductive carboxylation of crotonyl-CoA ((2E)-butenoyl-CoA) to (2S)-ethylmalonyl-CoA, in the presence of CO2 (PubMed:17548827, PubMed:18819910, PubMed:19458256). This is a key reaction in the ethylmalonyl-CoA pathway for acetyl-CoA assimilation required for R.sphaeroides growth on acetate as sole carbon source (PubMed:17548827, PubMed:22056933). Is also able to accept acryloyl-CoA as an alternative substrate, yielding (2S)-methylmalonyl-CoA (PubMed:18819910, PubMed:19458256). To a lesser extent, when CO2 is absent, the enzyme also catalyzes the reduction of crotonyl-CoA to butanoyl-CoA (PubMed:17548827, PubMed:19458256). (2S)-ethylmalonyl-CoA + NADP(+) = (2E)-butenoyl-CoA + CO2 + NADPH (S)-methylmalonyl-CoA + NADP(+) = acryloyl-CoA + CO2 + NADPH butanoyl-CoA + NADP(+) = (2E)-butenoyl-CoA + H(+) + NADPH Despite some sequence similarity to zinc-containing alcohol dehydrogenases, this enzyme does not bind any metals. kcat is 104 sec(-1) for the reductive carboxylation of (2E)-butenoyl-CoA (PubMed:17548827). Acryloyl-CoA is accepted as an alternative substrate analog by the enzyme with 40% relative activity (compared with Vmax of crotonyl-CoA carboxylation) (PubMed:19458256). Optimum pH is 7.5-8.0. Homodimer. Down-regulated in cells grown photoheterotrophically with succinate. No effect during photoheterotrophic (anaerobic/light) growth on succinate or 3-hydroxypropionate, no growth on acetate under the same conditions. Belongs to the zinc-containing alcohol dehydrogenase family. Crotonyl-CoA carboxylase/reductase subfamily.