Chapter 7 - Enzymes

Abstract

Enzymes are protein or RNA molecules that catalyze biotransformations. How do enzymes work? Enzymes bind substrate molecules and reduce the activation energy of the reaction catalyzed. Some protein enzymes require a nonprotein group for their activity as a cofactor. What is Michaelis-Menten equation? Why does Michaelis-Menten equation even work for an enzyme catalysis? Enzymatic kinetics is usually modeled after FES (fast-equilibrium steps). While FES produces simplest rate expression, PSSH (pseudosteady state hypothesis) is more general which puts reaction networks in direct analogy with electric circuits. The Michaelis-Menten rate expression is an asymptote. Allosteric enzymes have additional active centers although not to substrate molecules but to cofactors or effectors. An effector that results in a decrease in the catalytic rate is called an inhibitor. Enzyme inhibition may be competitive, or allosteric. When enzyme-substrate complex changes conformal structure at a rate similar to the catalytic rate, Michaelis-Menten equation breaks down. Enzymes require optimal conditions (pH, temperature, ionic strength) for their maximum activity. The effect of pH is similar to that of an effector or that of protein ionization. Enzymes can be used in suspension or in immobilized form. Enzymes can be immobilized by entrapment in a porous matrix or between membranes, or by adsorption onto a solid-support surface. Enzyme immobilization provides enzyme reutilization, and may result in increased activity by providing a more suitable microenvironment for the enzyme. Immobilization may also cause enzyme instability, loss of activity and shift in optimal conditions. Enzymes are widely used in industry and have significant medical applications. Reactor performance for enzymatic reactions can be carried out in the same manner as other chemical reactors. Enzymatic reactions are commonly carried out in batch reactors, while flow reactors can be used when enzymes are immobilized.