chapter  1
18 Pages

Introduction to enzymes

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Specicity and catalytic power are two characteristics of an enzyme. Most enzymes can be extremely specic for their substrates and catalyze reactions under mild conditions by lowering the free energy requirement of the transition state without altering the equilibrium condition. The enzyme specicity depends on the conformation of the active site. The enzyme-substrate binding is generally explained by lock-and-key model (conformational perfect t) or induced t model (enzyme conformation change such as closing around the substrate). The lock-and-key model has been modied due to the exibility of enzymes in solution. The binding of the substrate to the enzyme results in a distortion of the substrate into the conformation of the transition state, and the enzyme itself also undergoes a change in conformation to t the substrate. Many enzymes exhibit stereochemical specicity in that they catalyze the reactions of one conformation but not the other. Catalytic power is increased by use of binding energy, induced-t, proximity effect, and stabilization of charges in hydrophobic environment. The catalytic activity of many enzymes depends on the presence of cofactor for catalytic activity. If the organic compound as cofactor is loosely attached to enzyme, it is called a coenzyme. It is called a prosthetic group when the organic compound attaches rmly to the enzyme by covalent bond. Metal ion activators such as Ca++, Cu++, Co++, Fe++, Fe+++, Mn++, Mg++, Mo+++,and Zn++ can be cofactors. An enzyme without its cofactor is called an apoenzyme. An enzyme with a cofactor is referred as a haloenzyme. Enzymes catalyze the reactions by covalent catalysis or general acid/base catalysis.