(rī'byū-lās bis-fos'fāt kar-boks'il-ās), A dimerizing carboxy-lyase; an enzyme that catalyzes the addition of carbon dioxide to d-ribulose 1,5-bisphosphate and the hydrolysis of the addition product to two molecules of 3-d-phosphoglyceric acid, a key reaction in the fixation of CO2 in photosynthesis. Synonym(s): carboxydismutase