sentences:  The reduction of activation energy ("E"a) increases the fraction of reactant molecules that can overcome this barrier and form the product.An important principle is that since they only reduce energy barriers between products and reactants, enzymes always catalyze reactions in both directions, and cannot drive a reaction forward or affect the equilibrium position - only the speed with which is it achieved.As with other catalysts, the enzyme is not consumed or changed by the reaction (as a substrate is) but is recycled such that a single enzyme performs many rounds of catalysis.