Factors Affecting Enzyme Activity 2
Biology: Cambridge International A Level, topic 3.1.3
Inhibition of Enzyme Activity
Reaction rate is influenced by the presence of competitive and non-competitive inhibitors.
Function of competitive inhibitors
- Inhibitors are chemicals that slow down the rate or stop the reaction altogether.
- Enzyme-substrate complexes cannot be formed or are formed at a much lower rate.
Structure of competitive inhibitors
- Competitive inhibitors are similar in shape to the usual substrate and affect the active site directly, blocking access for the formation of ES complexes.
- Increasing the substrate concentration can compensate for the effects of a competitive inhibitor as there is no permanent damage to the shape of the active site.
- Malonate ions are similar in shape to succinate ions and act as a competitive inhibitor of succinate dehydrogenase, an important enzyme in the Krebs cycle.
Function of non-competitive inhibitors
- Some non-competitive inhibitors have reversible effect but others are irreversible and denature the enzyme.
- E.g. Lead denatures a number of enzymes required to synthesise haemoglobin.
Structure of non-competitive inhibitors
- Non-competitive inhibitors affect another part of the enzyme molecule causing a change to the shape of the active site.
- The active site is no longer complementary to the substrate molecules.
Inactive precursors
- A zymogen is an inactive precursor to an enzyme.
- A zymogen needs to be cleaved to be activated, and the activation of the enzyme is irreversible.
- Inactive precursors are a way of regulating enzyme activity.
- An example of a zymogen is chymotrypsin, which is the precursor to trypsin, a digestive enzyme that works in the small intestine.
Coenzymes
Some enzymes cannot catalyse a reaction without an additional molecule. These molecules include coenzymes, cofactors and prosthetic groups and they influence enzyme activity.
Cofactors
- Cofactors are small molecules that increase the activity of enzymes.
- When a cofactor binds to an enzyme, it undergoes a conformational change. The enzyme can now catalyse its reaction.
- Cofactors can be organic or inorganic molecules.
- E.g. Cl− is an inorganic cofactor to the enzyme amylase. When Cl− binds to amylase, it breaks down starch into sugars.
Coenzymes
- Coenzymes are organic molecules that bind to enzymes to increase the speed of reaction.
- Coenzymes are a type of cofactor.
- E.g. Vitamins are an important source of coenzymes but they are not naturally produced in the body and must be supplied in the diet. If an individual's diet is low in vitamins, this can cause health problems because the enzymes cannot catalyse the necessary reactions.
Prosthetic groups
- Prosthetic groups are inorganic molecules that permanently bind to enzymes to form part of the structure and increase the speed of reaction.
- Prosthetic groups are a type of cofactor.
- E.g. Zn2+ is a prosthetic group that forms part of carbonic anhydrase.
Test yourself on Factors Affecting Enzyme Activity 2
How can the effects of a competitive inhibitor be compensated?
Which of the following statements about enzyme inhibition are true?
Which of these is NOT a feature of coenzymes?
Which of the following statements about coenzymes are true?