Factors Affecting Enzyme Activity

Learning Outcomes• apply knowledge of proteins to explain the effects on

enzyme activity of:– pH– temperature– substrate concentration– enzyme concentration– competitive inhibitors, and non-competitive inhibitors

including heavy metals• differentiate between the roles of enzymes and coenzymes

in biochemical reactions• identify the role of vitamins as coenzymes

Factors affecting enzyme activity

• Temperature

• pH

• Concentration

• Activation

• Inhibitors

• Co-enzymes

• What happens to the rate of most chemical reactions as the temperature is increased?

• Why?

Temperature

• All enzymes have an optimal temperature

• Up to this temperature, the reaction rate increases as temperature increases

• Above this temperature, denaturation of the enzyme occurs (irreversible)

• What would you expect to be the optimal temperature of enzymes that function in the human body?

Effect of temperature on enzyme activity

Temperature

Enzyme activity

Optimal temperature

• How might a strong acid or base affect a protein?

pH

• All enzymes have an optimal pH

• High or low pH causes reduced enzyme activity and denaturation of the enzyme

Effect of pH on enzyme activity

Enzyme activity

pH

Optimal pH

Fig. 6.8

• What happens if we increase or decrease the concentration of substrate?

Substrate concentration

• Increasing the substrate concentration increases the reaction rate

• only to the point where all enzymes are being used

Effect of substrate concentration

Enzyme activity

Substrate concentration

• What if we add more enzyme?

• How would this happen in the body?

Enzyme concentration

• In cells, enzyme concentration is genetically controlled (control of protein synthesis)

• Increasing the amount of enzyme will increase the reaction rate (as long as substrate is present)

Effect of enzyme concentration

Enzyme activity

Enzyme concentration

Activation

• Enzymes may be “turned on” by the presence of another molecule (ex. The addition of a phosphate group, known as phosphorylation)

Enzyme activation

• How could you prevent a substrate from binding to an enzyme active site?

• Would this ever be a desirable outcome?

Inhibition

• Inhibitors are molecules other than the substrate that bind to the enzyme and inhibit its activity

Competitive inhibition

• Competitive inhibitors compete with the substrate by binding to the active site

Non-competitive inhibition

• Non-competitive inhibitors bind to a different site on the enzyme (allosteric site)

• Change the shape of the enzyme’s active site

• Toxic heavy metals such as lead, cadmium and mercury can bind to enzymes causing denaturation

• Act as non-competitive inhibitors

Feedback inhibition

• Sometimes the product of the pathway acts as an inhibitor

• This stops the pathway when there is enough of the product

Fig. 6.9a

Fig. 6.9b

Enzyme cofactors

• Cofactors or coenzymes are non-protein molecules that assist the enzyme

• May be inorganic ions ex. Copper, zinc, or iron

• Often are organic molecules derived from vitamins

• Function is often to transfer electrons or functional groups (ex. Phosphate) to the substrate

• Vitamin deficiency results in interference with enzyme activity in various metabolic pathways

(so eat your veggies!)