According to the most widely accepted mechanism of enzyme action, the enzyme molecule binds substrate molecule to form a temporary transition state (label A) which rapidly degrades into enzyme molecule and product. The transition state has higher free energy than the substrate (label B) or product molecule. Thus a substrate has to pass through the energy barrier in order to form the product molecule. An uncatalyzed reaction (label C) has to cross a higher barrier as compared to a reaction catalysed by an enzyme (label D). Thus, enzymes work by reducing the free energy barrier between substrates and the product molecules.