Enzymes

1. Enzymes • explain the induced fit model of enzyme action • evaluate the strengths of the induced fit model against the lock and key theory • apply knowledge of tertiary structure to explain enzyme specificity and the formation of enzyme-substrate complexes.

2. Structure of Enzymes: All enzymes are tertiary globular proteins, where the protein chain is folded back on itself into a spherical or globular shape. Each enzyme has its own sequence of amino acids and is held in its tertiary structure by hydrogen bonds, disulfide bridges and ionic bonds. This complex 3D shape gives the enzyme many of its properties.

3. How Enzymes Work: Enzymes are biological catalysts - they speed up the rate of metabolic reactions, without being used up themselves. These reactions can be of two types: Reactions where larger molecules are broken down into smaller molecules. 2. Reactions where small molecules are built up into larger, more complex, molecules. • Enzymes react with another molecule called the SUBSTRATE. • Each enzyme has its own special shape, with an area, the ACTIVE SITE, onto which the substrate molecules bind. • This is known as the lock and key theory. You will have seen this at GCSE.

4. Models of enzyme action: lock-and-key

5. How Enzymes Work: Modern interpretations of the lock and key theory suggest that in the presence of the substrate, the active site may change in order to fit the substrate’s shape. This is called the INDUCED FIT HYPOTHESIS. Enzyme + Substrate Enzyme-Substrate complex Enzyme + Product

6. Models of enzyme action: induced fit

7. Properties of Enzymes: Enzymes are specific i.e. each enzyme will catalyse only one particular reaction, for example, sucrASE acts on the sugar, sucrOSE. Chemical reactions need energy to start them off - this is called ACTIVATION ENERGY. • This energy is needed to break the existing chemical bonds inside molecules. • In the body, enzymes lower the activation energy of reactions and so reduce the input of energy needed, allowing reactions to take place at lower temperatures. Enzymes are very efficient and have a high TURNOVER NUMBER. • This means that they can convert many molecules of substrate per unit time, for example, catalase, which breaks down the waste product hydrogen peroxide in the body, has a turnover number of several million per second!

8. Why do enzymes increase the rate?

9. Past Paper Question (5 min)

10. Mark Scheme – Peer mark! • a) 3 marks max • Enzyme/active site has a (specific) tertiary structure ü • Only glucose has the correct shape/ is complementary/ will fit ü • To active site ü • (Forming) enzyme-substrate complexü • DO NOT allow ‘same’ shape! û • DO NOT allow active site is on the substrate û

11. Mark Scheme – Peer mark! • b) 2 marks max • (Only detects glucose whereas) Benedict’s detects (all) reducing sugars/ example(s) ü • Provides a reading/ is quantitative/ Benedict’s only gives a colour/ doesn’t measure concentration/ is qualitative/semiquantitativeü • Is more sensitive/ detects low concentration ü • Red colour/ colour of blood masks result ü • Can monitor blood glucose conc. Continuouslyü • DO NOT credit ‘quicker’/ ‘more accurate’û