Students should be able to: appreciate how models of enzyme action have…

The idea

Enzyme models have evolved with new evidence. The lock-and-key model proposed a rigid active site perfectly complementary to the substrate. Later evidence supported the induced fit model, where the active site is flexible and moulds around the substrate, straining bonds to lower activation energy. Enzymes catalyse diverse reactions determining structure and function across all levels. Intracellularly, enzymes like DNA polymerase determine cellular structure and function by synthesising DNA. Extracellularly, amylase hydrolyses starch into maltose in the gut, aiding whole-organism digestive function. Other enzymes determine whole-organism structure, such as those synthesising structural proteins like collagen.

What earns the marks

• Describe the lock-and-key model as featuring a rigid active site that is already complementary to the substrate.
• Describe the induced fit model as the active site changing shape to mould around the substrate, straining bonds to lower activation energy.
• Explain that models changed because later evidence showed the active site is flexible rather than rigid.
• Provide an example of an enzyme determining function, such as extracellular amylase hydrolysing starch to maltose for digestion.
• Provide an example of an enzyme determining structure, such as intracellular enzymes synthesising structural molecules like cellulose or collagen.

Common mistakes and how to put them right

• Stating amylase breaks starch directly into glucose.

  Put it right: Amylase hydrolyses starch into the disaccharide maltose.

• Describing the induced fit model as the substrate changing shape to fit the enzyme.

  Put it right: It is the enzyme's active site that changes shape to mould around the substrate.

• Overlooking the 'structure' aspect of enzyme function.

  Put it right: Enzymes also synthesise structural components, such as cellulose for plant cell walls or collagen in animals.

Exam tips

• When comparing enzyme models, explicitly contrast the rigid active site of lock-and-key with the flexible active site of induced fit.
• Ensure you can provide specific examples of enzymes determining both structure (e.g., cellulose synthase) and function (e.g., amylase producing maltose).