1.8 Enzymes

Why are enzymes catalysts?

• They increase the rate of reaction

• They are not used up by the reaction

Intracellular enzymes

Catalyse reactions inside the cell

Examples of intracellular enzymes

Catalase, DNA polymerase, any enzyme in respiration

Extracellular enzymes

Secreted from cells and catalyse reactions outside the cell

Examples of extracellular enzymes

Amylase, protease and lipase in digestion

Why can an enzyme be reused?

• None of the enzyme is changed or used up in the reaction

• Only the substrate is changed into the product

Anabolic reaction

• Building large molecules from smaller ones

• It requires energy

Examples of anabolic reactions

• Condensation reactions

• Protein ATP and starch synthesis

Catabolic reactions

• Breaks large molecules into smaller ones

• Releases energy

Examples of catabolic reactions

• Hydrolysis reactions

• Digestion of lipids, starch and proteins

Activation energy

The minimum energy to start a reaction

The induced fit model

• The substrate is complementary to the active site

• The active site is flexible not completely rigid

• Binding causes a slight change in the active site

• This change helps the reaction occur, by placing strain on substrate bonds

Temperature affecting rate of reaction

• At low temperatures, molecules have less kinetic energy and collide less frequently

• As temperature rises, collision frequency and successful collisions increase

• Above the optimum, bonds maintaining the tertiary structure break

• The active site changes shape and fewer ESC are made

pH affecting rate of reaction

• Changes in hydrogen ion concentration can disrupt ionic and hydrogen bonds, as well as disulphide bridges

• Tertiary structure and active site will change

• Each enzyme has its optimum pH where the rate is the highest

What happens with increasing substrate concentration

• Initial rate increases

• More frequent collisions and more ESC formed

• Then rate plateaus

• All active sites are filled, enzyme is a limiting factor

What happens with increasing enzyme concentration:

• Rate increases if substrate is in excess

• More active sites are available so more ESC per unit time

• When substrate becomes limiting rate no longer rises

• There are insufficient substrate molecules to occupy the additional active sites

Competitive inhibitors

• Bind at the active site

• Occupies the site but doesn’t change its shape

• Competes with the substrates

• If more substrate is added, substrate is more likely to bind and reduces inhibitor effect

Non-competitive inhibitors:

• Bind at the allosteric site

• Changes tertiary structure and active site

• Doesn’t compete with substrates

• Adding more substrate won’t affect it, because the active site has already changed shape