What do you remember about enzymes?

Create a mind map Include What they are made of How they work What factors affect them Any good examples

All enzymes are proteins Enzymes are similar in many ways: Globular proteins with a specific 3D shape (tertiary structure) They act as catalysts Specific Active site (pocket or cleft area) Activity affected by temperature and pH A catalyst is a molecule (or element) that speeds up a chemical reaction but does not get used up in the reaction. At the end of the reaction the catalyst remains unchanged.

Enzymes sucrose + water glucose + fructose substrate(s) – substance(s) used up in an enzyme-controlled reaction product(s) – substance(s) formed in an enzyme-controlled reaction Sucrase – the enzyme

Active site area on an enzyme to which substrate binds (not bonds or joins) Tiny part of enzyme Very few (often fewer than 10) amino acids form the actual active site Individual cell may contain as many as 1000 different enzymes

Inside and out Intracellular (enzymes catalyse reactions inside the cell)

Extracellular enzymes: released from cells that make them and their action takes place outside cells eg Salivary amylase Fungal hyphae secrete digestive enzymes

Why do reactions need enzymes? Increase rate of reaction Enzymes increase the rate of reaction by 107 (10 million times faster!)

Reactions need a level of energy before they can proceed = activation energy

Lowering the amount of energy required to get reaction started makes the reaction more likely to occur

Enzymes control metabolic processes? E.g. Photosynthesis Respiration

Draw a similar diagram to the one above and explain how the end product can control the rate of the reaction

Specificity Due to the 3D shape of the active site Active site shape is determined by tertiary (3D) structure of enzyme Due to the 3D shape of the active site Specificity Determined by the primary structure of the protein which forms the enzyme The active site is only complementary to a specific substrate Each enzyme can only catalyse one reaction Formed during protein synthesis and coded for by DNA

Enzyme action

Theories of Enzyme Action Lock and Key Hypothesis Induced Fit Hypothesis

Lock and key hypothesis Lock = Enzyme Key = Substrate This theory relies on the complementary shapes of the substrate and active site creating enzyme-substrate complexes Textbook p126-127

Induced Fit Hypothesis substrate molecule changes shape to fit enzyme molecule more closely as they bind, putting a strain on the substrate bonds. (Think of it as a hand in a glove) Charges on amino acids at active site also hold substrate in place. Enzyme-substrate complexes are formed as before. Products no longer fit the active site so they leave. Animation – hydrolysis of sucrose

Examiners tip: Don’t write: “the enzymes shape is complementary to the substrate” Do write: “the shape of the enzymes active site is complementary to the shape of the substrate molecule”

Enzyme action Enzyme and substrate molecules have their own kinetic energy and are moving – they collide randomly

Enzyme action 2 substrate molecule binds to active site of enzyme forming an enzyme-substrate complex (lock and key / induced fit)

Enzyme action 3 Enzymes reduce activation energy by holding substrate in a way which causes reaction to occur more easily forming enzyme-product complex

Enzyme action 4 The products no longer fit active site and move out – enzyme is free to catalyse same reaction with another substrate molecule

Enzyme action ‘a single enzyme molecule typically acts on a thousand substrate molecules per second’

Straw is plentiful and it could be used for the production of many organic substances. The first step is the conversion of cellulose to glucose. It has been suggested that an enzyme could be used for this process. There is a difficulty - the lignin that covers the cellulose would protect the cellulose from attack by the cellulose-digesting enzyme. Use your knowledge of the way in which enzymes work to explain why cellulose-digesting enzymes do not digest the lignin. (2 marks) This is a straightforward question relating to enzyme specificity and its accompanying mark scheme. Rather more is needed in a full answer than a superficial statement about enzymes being specific. Enzymes are specific; Shape of lignin molecules will not fit active site of enzyme;

Enzyme action - Catalase

1 2 Catalase is a globular protein made from four polypeptide chains (an example of a protein with a quaternary structure) 3 4 One molecule of catalase can break down 83 000 molecules of hydrogen peroxide every second!

Catalase is found in living cells and breaks down toxic hydrogen peroxide into harmless water and oxygen 2H2O2 2H2O + O2 catalase Enzyme structure and action

Investigating the time course for an enzyme controlled reaction Enzyme: catalase Substrate: H2O2 (3 different sources – peas, potato, yeast ) Independent Variables Time, Source of catalase Dependent Variable Volume of oxygen Controlled variables: Method: Prepare measuring cylinder(full of water) Add enzyme source to test-tube Add 10cm3 hydrogen peroxide to test-tube (use syringe) Quickly place bung into top of test-tube Ensure delivery tube carries gas into cylinder Measure volume of O2 produced every 30 seconds for 10 minutes apparatus temperature Volume of H2O2

Follow the progress of an enzyme-catalysed reaction measuring cylinder – select an appropriate size Prepare a results table Set up the apparatus (minus the hydrogen peroxide) as shown Add the hydrogen peroxide, return bung and immediately start a stop clock Record volume of O2 collected in measuring cylinder every 10s until reaction stops If reaction is too fast / slow – adjust volume / surface area of catalase source/ size of measuring cylinder/ time intervals delivery tube clampstand bung water boiling tube source of catalase – adjust size etc as needed 10cm³ hydrogen peroxide – add last using a syringe plastic tub

Total Volume O2 produced (cm3) Results table: Total Volume O2 produced (cm3) Time (s) Potato Peas Yeast 30 60 90 120 150 180 210 240 270 300 330 360

390 420 450 480 510 540 570 600

Graph: Graph to show the effect of enzyme source on the time course of an enzyme controlled reaction Volume of O2 produced (cm3) Time (s)

Calculating initial rate of reaction Comparing the initial rate of an enzyme controlled reaction is useful as it should be the fastest point of a reaction. Tangents taken later on the reaction curve allow you to calculate the rate at later stages. You can also use these calculations to plot a graph of rate against time Volume of O2 produced (cm3) b a Time (s) Volume O2 Initial rate = cm3 s-1 Time taken (s) a cm3 s-1 b Page 142-143

Calculating Mean Rates of a Reaction  

Factors Affecting Enzyme Activity Temperature pH Concentration of the substrate Concentration of the enzyme Presence of inhibitors