Often due to changes in gene expression and thus amount of m RNA for the particular enzyme protein in the cell. Does not change amount of enzyme protein. The enzyme is more active in one phosphorylation state than the other. Thus they do not obey classic Michaelis-Menten kinetics. In some cases binding of substrate molecule can increase binding and activity with subsequent substrate molecules e.g., the first CO 2 molecule activates Rubisco enzyme while another CO 2 is used as substrate for carboxylation.
Many enzymes require presence of cofactors for activity.
They are often a way for energy input to endergonic reactions, to make them spontaneous or a source of energy from exergonic reactions. High Energy Compounds. Enzymes only increase the rate at which equilibrium is reached.