An enzyme called carbonic anhydrase catalyzes both the forward and reverse reactions for the hydration of co2 according to

conversion of carbon dioxide and water to bicarobonates and protons and vice versa.This enzyme is playing a vital role in organizms. It helps to maintain acid base balance in tissues and blood of animals. Its actions helps to drive the carbon dioxed produced due to respiration out of cell.

Carbon dioxide is the correct answer