Answer:
D. enzymes
Step-by-step explanation:
Enzymes are catalysts that speed up the rate of a chemical reaction in the cell. Enzymes work by lowering the activation energy required by the reactant to get converted into products. Enzymes bind to substrates according to the lock and key model, each enzyme has a specific substrate to which it binds. For example, in the citric acid cycle isocitrate requires the enzyme isocitrate dehydrogenase to get converted to alpha-ketoglutarate. The activity of an enzyme can be stopped by using inhibitors. Genetic mutation in an enzyme can cause metabolic disorder.