inhibition as the definition for non-competitive inhibition. Mechanism Non-competitive inhibition models a system where the inhibitor and the substrate may both be bound to the enzyme at any given time. When both the substrate and the inhibitor are bound, the enzyme-substrate-inhibitor complex cannot form product and can only be converted back to the enzyme-substrate complex or the enzyme-inhibitor complex. Non-competitive inhibition is distinguished from general mixed inhibition in that the inhibitor has an equal affinity for the enzyme and the enzyme-substrate complex. For example, in the enzyme-catalyzed reactions of glycolysis, accumulation phosphoenol is catalyzed by pyruvate kinase into pyruvate. Alanine is an amino acid which is synthesized from pyruvate also inhibits the enzyme pyruvate kinase during glycolysis. Alanine is a non-competitive inhibitor, therefore it binds away from the active site to the substrate in order for it to still be the final product. Another example of non-competitive inhibition is given by glucose-6-phosphate inhibiting hexokinase in the brain. Carbons 2 and 4 on glucose-6-phosphate contain hydroxyl groups that attach along with the phosphate at carbon 6 to the enzyme-inhibitor complex. The substrate and enzyme are different in their group combinations that an inhibitor attaches to. The ability of glucose-6-phosphate to bind at different places at the same time makes it a non-competitive inhibitor. The most common mechanism of non-competitive inhibition involves reversible binding of the inhibitor to an allosteric site, but it is possible for the inhibitor to operate via other means including direct binding to the active site. It differs from competitive inhibition in that the binding of the inhibitor does not prevent binding of substrate, and vice versa, but simply prevents product formation for a limited time. This type of inhibition reduces the maximum rate of a chemical reaction without changing the apparent binding affinity of the catalyst for the substrate (Kmapp – see Michaelis-Menten kinetics). When a non-competitive inhibitor is added the Vmax is changed, while the Km remains unchanged. According to the Lineweaver-Burk plot the Vmax is reduced during the addition of a non-competitive inhibitor, which is shown in the plot by a change in both the slope and y-intercept when a non-competitive inhibitor is added. The primary difference between competitive and non-competitive is that competitive inhibition affects the substrate's ability to bind by binding an inhibitor in place of a substrate, which lowers the affinity of the enzyme for the substrate. In non-competitive inhibition, the inhibitor