At a fixed substrate concentration, an increase in enzyme concentration increases the rate of an enzyme catalyzed reaction until the substrate concentration becomes the limiting factor.

When the substrate concentration is not limiting, the rate of an enzyme catalyzed reaction is directly proportional to the enzyme concentration. The rate slows down when [s] starts becoming limiting factor. The rate remains unchanged at high [E] when [s] is the limiting factor.

Effect of Substrate Concentration

At a fixed enzyme concentration, an increase in f substrate concentration increases the rate of an enzyme § catalyzed reaction until the enzyme concentration becomes the limiting factor.

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When the enzyme concentration is not limiting, the rate of an enzyme catalyzed reaction is directly proportional to the substrate concentration. on increasing [s] when [E] starts becoming limiting factor, the rate of the enzyme catalyzed reaction slows down. At high [s], the rate remains unchanged because [s] becomes limiting factor.

Effect of pH

Enzymes, being proteins, are affected by the changes in the pH of the reaction medium. Most of the enzymes are, in fact, active only within a narrow range of pH, typically pH 5 to pH 9. The most favorable pH at which an enzyme activity is the maximum is known as the optimum pi I for the enzyme.

The optimum pH value varies from enzyme to enzyme. The pH affects the£ degree of ionization of the side chains of then amino acid residues of proteins and thereby their three dimensional structure. Furthermore, ionization state of amino acid side chains present in the active site of enzymes are responsible for the catalytic activity of enzymes (see later in this chapter).

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The pH also affects the ionization characteristics of the substrates and coenzymes, and thereby, their binding with the enzyme. Thus pH of the reaction medium affects the catalytic activity of the enzyme. The catalytic activity of the enzyme as a function of pH usually appears as a bell-shaped curve.

Effect of Temperature

Like most of the chemical reactions, the rate of an enzyme catalyzed reaction increases with the increase in temperature of the reaction medium.

However, enzymes-being proteins are gradually denatured and lose their activity at the temperature beyond 40 – 50°C. Some enzymes, however, remain active even at the temperature as high as 100oC. Each enzyme has an optimum temperature at which its activity is the maximum.

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Effect of Inhibitors and Activators

Many molecules and ions, when present in the reaction medium; affect the rate of enzyme-catalyzed reactions. These substances bind to the enzyme or the enzyme-substrate complex, thereby, affecting the rate.

Substances which lower the reaction rate are called enzyme inhibitors and substances which increase the rate are as enzyme activators. Inhibitors and activators are very important in the cellular regulation of enzymes.

Inhibitors can be classified as reversible or irreversible. Reversible inhibitors bind with the enzyme by weak noncovalent bonds and can be removed by dialysis restore the active enzyme. On the other hand irreversible inhibitors bind with enzyme by covalent bonds and cannot be removed from the enzyme by dialysis.

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Mechanism of Enzyme Action

Enzymes, as explained before accelerate the rates of biochemical reactions show great deal of substrate specificity. Therefore, any theory on the mechanic enzymatic reaction must take these two facts into account.

Activation Energy

The conversion of reactants to products in any chemical reaction is accompanied by continuous change in energy. During the reaction pathway, the reactants p through a transition state.

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This transition state of the reactants has higher free energy than either the ground state reactants or products.

The difference in free energy between transition state and ground state reactants is called the activation energy. Activation energy, thus, is the energy required to activate the reactants to move them to transition state.

In the activation pathway, the transition state has the maximum energy. As the reaction continues, the energy of the system decreases until it reaches a new minimum in the products. The energy changes for a simple chemical reaction as the function of the progress of the reaction (or reaction pathway) is graphically represented in.

The magnitude of the activation energy influences the rate of the chemical reaction – the higher the activation energy (or energy barrier), the slower the reaction rate. Catalysts speed up the reaction rate by lowering the energy of Plot shows the requirement of energy of activation required for the reactants to activation for the enzyme catalyzed and uncatalyzed reactions.

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Enzymes reduce the reach the transition state. Enzyme catalyzed reactions obey the same laws of chemistry as the corresponding uncatalyzed reactions. Similar to ordinary chemical catalysts, enzymes also speed up the rate of biochemical reactions by lowering the energy of activation of the substrates to reach the transition state. An enzyme lowers the activation energy by forming an enzyme- substrate complex and thereby increasing the reaction rate.

As shown in, the difference in energy between the substrate and the product is not altered by the enzyme. This difference in energy between the substrate and the product determines the equilibrium point of the reaction. As with the chemical catalysts, enzymes do not alter the equilibrium constant of the reaction they catalyze.