Enzyme actionAQA A-Level Biology: Revision notes
Section 1
Enzymes and activation energy
Enzymes are biological catalysts, almost all of them globular proteins. They speed up reactions without being used up. Every reaction needs a minimum input of energy to get started, the activation energy. Each enzyme lowers the activation energy of the reaction it catalyses, so more molecules have enough energy to react at body temperature and the reaction is faster.
Enzymes do not change the products or the overall energy change of a reaction. They only provide an easier route.
Enzymes do not 'provide energy' for reactions. They lower the energy needed to start them.
Section 2
The active site and tertiary structure
An enzyme's active site is a small region, often a cleft or pocket, where the substrate binds. Its shape depends on the enzyme's tertiary structure, the 3D folding of the polypeptide held by hydrogen bonds, ionic bonds and disulfide bridges. The sequence of amino acids (the primary structure) decides this folding, so it also decides which R groups line the active site.
The substrate binds to the active site to form an enzyme-substrate complex. The reaction occurs, the enzyme-product complex forms, the products leave and the enzyme is free to act again.
Link the chain: primary structure, tertiary structure, active site shape, specificity. A change in amino acid sequence can change the active site.
Section 3
Specificity
Enzymes are specific: each enzyme catalyses only one reaction, or a very small number of similar reactions. This is because the active site has a particular shape and arrangement of R groups that is complementary only to its own substrate(s). Other molecules do not fit, so no enzyme-substrate complex forms and no catalysis occurs.
Specificity is why thousands of reactions can occur in one cell without interfering with each other, and why enzymes such as trypsin (extracellular, protein digestion) and catalase (intracellular, breaks down hydrogen peroxide) do different jobs.
Section 4
Models of enzyme action
The first model, the lock and key model (Fischer, 1894), proposed a rigid active site with a shape exactly complementary to the substrate. It explained specificity.
The induced-fit model (Koshland, 1958) proposes that the active site is flexible. It is not exactly complementary at first: as the substrate binds, the active site changes shape and moulds around it. This puts strain on bonds in the substrate, lowering the activation energy. Evidence for the newer model includes enzymes acting on substrates of slightly different shapes and X-ray crystallography showing enzymes changing shape on binding substrate.
This is a good example of how a scientific model is modified as new evidence and techniques appear.
In induced fit it is the active site that changes shape, not the substrate. The enzyme returns to its original shape afterwards.
Section 5
Enzymes in cells and organisms
Enzymes catalyse a wide range of reactions. Intracellular enzymes work inside cells, for example catalase breaking down hydrogen peroxide, or the enzymes of respiration. Extracellular enzymes are secreted and work outside the cells that made them, for example amylase and trypsin in digestion, or lactase on the surface of the intestine lining.
Because enzymes control which reactions happen and how fast, they determine structures and functions at every level, from the cell (making ATP, copying DNA) to the whole organism (digesting food). A mutation that changes an enzyme's active site can therefore affect the whole organism.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Enzyme action
- Catalase, found inside liver cells, catalyses the breakdown of hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen. Trypsin is made by the pancreas and secreted into the small intestine, where it catalyses the hydrolysis of proteins to shorter polypeptides.Explain why trypsin does not catalyse the breakdown of hydrogen peroxide.2 marks
- In 1894 Emil Fischer proposed that an enzyme and its substrate fit together like a lock and key. In 1958 Daniel Koshland proposed a modified model after it was found that some enzymes can act on substrates of slightly different shapes and that the shape of an enzyme can change when its substrate binds.Explain how the induced-fit model accounts for an enzyme lowering the activation energy of a reaction.2 marks
- Lactase is an enzyme made by cells lining the small intestine. It is anchored in the cell surface membrane with its active site facing the gut lumen, where it hydrolyses the disaccharide lactose to glucose and galactose. Some adults make too little lactase, and can take a tablet containing lactase from a fungus with dairy foods.A mutation in a different person replaces one amino acid in the active site of lactase with an amino acid with a different R group. The enzyme no longer hydrolyses lactose. Explain why.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).