Section 3Lesson 2– The Catalytic

Cycle

What do enzymes do?

Enzymes lower the activation energy Ea required for a reaction to occur.

Catalytic Cycle of Sucrase

Enzyme Substrate Complex

Lock and Key ModelOn your desks use an example to explain the lock and key model of enzyme activity.

Why is this model misleading?Enzymes are not the rigid structures that the diagrams make them out to be. We already know that they can change their configuration in response to changes in their environmental conditions. They also change in response to interactions with other molecules.

Induced Fit - HexokinaseWhat do kinases do?Kinases catalyse the transfer of a phosphate group onto another molecule. Hexokinase catalyses the transfer of a phosphate from ATP onto glucose.

Catalytic Cycle of HexokinaseCopy Fig 3.1.4 from the monograph into your notes.

Annotate your diagram using the notes found under the diagram in the monograph.

Control of Enzyme ActivityCells have a number of ways to control the activity of their enzymes. Without control the reaction of the cell would be disordered.

Cells can control enzyme activity by:

1. Compartmentalisation – enzymes remain in compartments in the cell such as the mitochondria, chloroplast, Golgi apparatus.

2. pH – the pH affects the activity of enzymes so controlling the pH will change which enzymes are active.

3. Temperature – the temperature affects the activity of enzymes so controlling the temperature will change which enzymes are active.

4. Cofactors – some enzymes require another non-protein molecule to bind to them so that they will work.

5. Altering the shape of the enzyme – this can happen through inhibitors, allosteric effects, covalent modification and end-product inhibition.

InhibitorsThere are 2 types of inhibitors – competitive and non-competitive.Competitive Inhibitors

These are molecules that are similar to the substrate and so are able to compete with the substrate for the active site. If they get there first the enzyme will be unable to catalyse the reaction it is intended for.

Competitive Inhibition in ActionCan you think of an example of competitive inhibition in the nitrogen cycle?O2 is similar enough to N2 to act as a competitive inhibitor. How is this problem overcome?

Non-competitive Inhibitors

These are able to bind to the enzyme in a place other than the active site. By binding they change the active site which reduces the efficiency of the enzyme to catalyse the intended reaction.

Allosteric EnzymesAn allosteric enzyme is a type of enzyme that changes form when a regulating molecule binds to it. The regulating molecule is often called a modulator or effector. (Non-competitive inhibition is an example of an allosteric interaction)There are 2 types – positive or negative.Positive – these stabilise the active form of the enzyme allowing it to work. Negative – these stabilise the inactive form of the enzyme stopping it from working.

In this case the allosteric enzyme is being negatively modulated.

Covalent Modification of Enzymes

Covalent modification involves the addition or removal of molecular units (often phosphate groups) which either activate or inactivate the enzyme.

Protein kinases add phosphate groups and phosphatases remove them.

Some enzymes require phosphorylation to become active while others are inactivated by phosphorylation.

Glycogen phosphorylase

Glycogen phosphorylase is activated by phosphorylation

Allosteric Modification of Glycogen phosphorylase

In addition to the covalent modification of glycogen phosphorylase, it is also modified by glucose and ATP which act as negative modulators. AMP (adenosine monophosphate) acts as a positive modulator.

Having additional allosteric modifiers improves the control of this enzyme. Use your monograph to discover:- Where this enzyme is produces- Why it is produced

Proteolytic CleavageTrypsin is an enzyme which is made in the pancreas and is responsible for the digestion of proteins. When it is produced it is inactive – it has extra amino acids and is called trypsinogen.

Trypsin is VERY important as it is powerful enough to begin self digestion of the pancreas should it be activated!When trypsinogen is secreted in the small intestine it is cut by a protease to remove the additional amino acids. The result is the active trypsin.

End-Product InhibitionSome metabolic pathways are controlled by end-product inhibition. This involves the end product of a series of reactions acting as an inhibitor of the enzyme in one of the earlier reactions.

Your Tasks1. Create a mind map of all the ways that enzymes

can be modified to affect their activity. Include examples. Use your monograph to help you.

2. Update your glossary.3. Past Paper Questions on catalysis:

2002 MC Q92003 MC Q6,10,112004 MC Q9,10,11 Q1A (ER)2005 MC Q7,9 Q8B (ER)2007 MC Q10,11

Due Monday

December 10th