The specificity of enzymes The effects of the following factors on the…

The idea

Enzyme specificity arises from the complementary fit between a substrate and the active site. Rate depends on collision frequency and enzyme-substrate complex formation. Increasing enzyme or substrate concentration increases rate until the other becomes a limiting factor, causing a plateau. Temperature increases kinetic energy and collision rate up to an optimum; above this, or at extreme pH, hydrogen and ionic bonds break. This alters the tertiary structure, denaturing the active site so it is no longer complementary. Competitive inhibitors bind the active site, which is overcome by adding substrate. Non-competitive inhibitors bind allosteric sites, changing the active site shape so fewer enzyme-substrate complexes form; this inhibition is typically reversible, though some inhibitors can bind irreversibly.

What earns the marks

• Enzyme specificity is determined by the tertiary structure of the active site being complementary to the substrate.
• At high temperatures or extreme pH, hydrogen and ionic bonds break, altering the tertiary structure and denaturing the enzyme.
• A competitive inhibitor has a similar shape to the substrate and binds to the active site, but its effect is reduced by increasing substrate concentration.
• A non-competitive inhibitor binds to an allosteric site, changing the active site shape so fewer enzyme-substrate complexes form; this inhibition is typically reversible, not necessarily permanent.

Common mistakes and how to put them right

• Stating that low temperatures denature enzymes.

  Put it right: Low temperatures only reduce kinetic energy and collision frequency, whereas high temperatures cause denaturation.

• Claiming that increasing substrate concentration overcomes a non-competitive inhibitor.

  Put it right: Non-competitive inhibitors alter the active site shape, so adding more substrate has no effect.

• Saying a non-competitive inhibitor binds to the active site.

  Put it right: They bind to an allosteric site elsewhere on the enzyme.

• Describing non-competitive inhibition as always permanent.

  Put it right: Non-competitive inhibition is typically reversible, so the active site shape change is not necessarily permanent.

Exam tips

• When describing a plateau on a graph, explicitly name the limiting factor at that point rather than just describing the shape of the curve.
• Use the term 'denatured' only for high temperatures or extreme pH, and always specify that hydrogen and ionic bonds break to alter the tertiary structure.
• For non-competitive inhibition, say the inhibitor binds an allosteric site and changes the active site shape; avoid claiming the change is permanent unless the question specifies an irreversible inhibitor.