Characteristics of Enzymes

Enzymes speed up chemical reactions

Enzymes lower the activation energy required to start a reaction.

Enzymes are required in minute amounts

Since they remain unchanged in the reaction they catalyse, the same enzyme molecules can be used over and over again. Thus, a small amount of enzyme can catalyse a large number of chemical reactions.

Enzymes are affected by temperature

Enzymes shows little activity at very low temperatures. The kinetic energy is low at low temperatures. Hence, chances of substrate molecules colliding with enzymes are very low.

As the temperature rises, enzyme activity increases. The increase in the kinetic energy of molecules increases the frequency of collision between substrate and enzyme molecules. This increases the rate of formation of enzyme-substrate complex.

The optimum temperature is the point when the rate of reaction is highest and the enzyme is most active. The kinetic energy of the enzyme and substrate is very high and so formation of enzyme-substrate complex is very high at optimum temperature.

Higher temp start to break hydrogen bonds and alter active site of enzyme, causing it to lose its specific shape such that it is no longer complementary to shape of substrate, and becomes denatured. Rate of reaction decreases steeply. Enzyme has lost its ability to catalyse the reaction.

Enzymes are affected by pH

Different enzymes have different optimum pHs, at which they are most active.Any pH range that deviates from the optimum pH of that particular enzyme will cause hydrogen bonds in the protein 3D structure to change/be destroyed. This alters the active site, causes it to lose its specific shape such that it is no longer complementary to shape of substrate, and becomes

Denaturation is the change in the 3-D structure of an enzyme activity or any other soluble protein, caused by heat or chemicals such acids or alkalis.

denatured. So rate of reaction at both side of the optimum pH drops steeply.

Enzymes catalyse reversible reactions

Carbonic anhydrase

CO₂+H₂O ⇄ H₂CO₃

Carbon dioxide + water ⇄ carbonic acid

Enzymes are highly specific in action

Enzyme reactions are specific as enzymes act on a specific substrate. They have a unique active site which can only bind to a specific substrate which has a complementary shape. The enzyme is the lock and the substrate is the key. Hence, the substrate binds to the enzyme forming an enzyme-substrate complex.

The ‘lock-and-key’ hypothesis

• An enzyme has a specific 3-D shape which contains an active site.
• Only the substrate with a 3-D shape complementary to that of the active site can fit into the enzyme to form an enzyme-substrate complex.

• Chemical reaction occurs and the substrate is converted into products.
• The products then detach from the active site. The enzyme remains unchanged at the end of the reaction.