Enzymes as Catalysts

I. Enzymes are biological catalysts.

A. Catalysts increase the rate of chemical reactions.

1. A catalyst is not altered by the reaction.

2. A catalyst does not change the final result of a reaction.

B. Catalysts lower the activation energy of chemical reactions.

1. The activation energy is the amount of energy needed by the reactant molecules to participate in a reaction.

2. In the absence of a catalyst, only a small proportion of the reactants possess the activation energy to participate.

3. By lowering the activation energy, enzymes allow a larger proportion of the reactants to participate in the reaction, thus increasing the reaction rate.

II. Most enzymes are proteins.

A. Protein enzymes have specific three-dimensional shapes that are determined by the amino acid sequence and, ultimately, by the genes.

B. The reactants in an enzyme-catalyzed reaction—called the substrates of the enzyme—fit into a specific pocket in the enzyme called the active site.

C. By forming an enzyme-substrate complex, substrate molecules are brought into proper orientation and existing bonds are weakened. This allows new bonds to be formed more easily.

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