Most of the chemical reactions in the body, if carried out in a test tube with only reactants and products present, would proceed at very low rates because they have high activation energies. In order to achieve the high reaction rates observed in living organisms, catalysts are required to lower the activation energies. These particular catalysts are called enzymes (meaning "in yeast" since the first enzymes were discovered in yeast cells). Enzymes are protein molecules, so an enzyme can be defined as a protein catalyst. (Although some RNA molecules possess catalytic activity, the number of reactions they catalyze is very small, and we shall restrict the term "enzyme" to protein catalysts.)

To function, an enzyme must come into contact with reactants, which are called substrates in the case of enzyme-mediated reactions. The substrate becomes bound to the enzyme, forming an enzyme-substrate complex, which breaks down to release products and

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR

enzyme. The reaction between enzyme and substrate can be written:

TABLE 4-4 Characteristics of Enzymes



ES Enzymesubstrate complex

P + E Product Enzyme

At the end of the reaction, the enzyme is free to undergo the same reaction with additional substrate molecules. The overall effect is to accelerate the conversion of substrate into product, with the enzyme acting as a catalyst. Note that an enzyme increases both the forward and reverse rates of a reaction and thus does not change the chemical equilibrium that is finally reached.

The interaction between substrate and enzyme has all the characteristics described previously for the binding of a ligand to a binding site on a protein— specificity, affinity, competition, and saturation. The region of the enzyme to which the substrate binds is known as the enzyme's active site (a term equivalent to "binding site"). The shape of the enzyme in the region of the active site provides the basis for the enzyme's chemical specificity since the shape of the active site is complementary to the substrate's shape (Figure 4-8).

There are approximately 4000 different enzymes in a typical cell, each capable of catalyzing a different chemical reaction. Enzymes are generally named by adding the suffix -ase to the name of either the substrate or the type of reaction catalyzed by the enzyme.




active site


Enzyme-substrate complex

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  • samuel
    What are the characteristics of enzyme in the body?
    8 years ago

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