5) Enzymes

5) Enzymes


Catalyst: is a substance that increases the rate of a chemical reaction and is not changed by the reaction.

Enzyme: is a protein that functions as a biological catalyst.

Enzymes, like catalysts, can be used over and over again because they are not used up during the reaction and only a small amount is needed to speed the reaction up.

 The enzymes and substrate molecules have

complementary shapes (like adjacent pieces of a jigsaw) so they fit together.


Substrate: the substance on which the enzyme acts.

Products: the molecules produced.

Anabolic reaction: reactions in which large molecules are built up from smaller molecules.

Catabolic reaction: reactions that splits large molecules into smaller ones.

Enzyme-substrate complex: is formed temporarily when the enzyme combines with the substrate.


Enzymes are specific, this means simply that an enzyme which normally acts on one substance will not act on a different one.

The enzyme has a shape called the active site, which exactly fits the substances on which it acts.

The shape of the active site of the enzyme molecule and the substrate molecules are complementary.


Effect of temperature:

The rate of an enzyme-catalysed reaction increases as the temperature increases. However, at high temperatures the rate decreases again because the enzyme becomes denatured and can no longer function as a biological catalyst.


As the temperature is increased, the molecules gain more kinetic energy, so they move faster and there is a greater chance of collisions happening. Therefore the rate of reaction increases.

Above the optimum temperature the reaction slows down. This is because enzyme molecules are proteins. Proteins molecules start to lose their shape at higher temperatures, so the active site becomes deformed.

Substrate molecules cannot fit together with the enzyme, stopping the reaction. (Denaturation)


Effects of pH:

Changes in pH alter the shape of an enzyme’s active site. Different enzymes work best at different pH values.

The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5, but stomach enzymes have an optimum pH of about 2.


Each enzyme has an optimum pH, if this pH changes, the shape of the active site of the enzyme is changed (Denature), thus the substrate will not be able to fit in it, and the enzyme becomes useless

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