Chapter 6.3

How Enzymes Make Substances React

AP BiologyFall 2010

Bell Ringer

Objectives

• Analyze the way enzymes function• Describe the induced fit model • Understand the way enzymes lower activation

energy

How Enzymes Work

• Enzymes: catalysts working to speed up reactions by lowering activation energy

• Enzymes can be reused , reactions do not use them up or alter them

• Some enzymes can recognize both reactants and products in order to catalyze a reaction in both directions

How Enzymes Work

• Reactants cannot proceed until reactants have minimum amount of internal energy – Activation energy

• Enzymes lower activation energy • Compared with surrounding environment,

enzymes offer a stable microenvironment, more favorable for reaction

How Enzymes Work

• Substrates: reactants that bind to a specific enzyme

• Active sites: pockets or crevices where the substrates bind and where specific reactions can proceed rapidly and repeatedly

• Enzymes are very selective about the substrates to which they will bind and thereby bring about change

How Enzymes Work

• Part of a substrate is complementary in shape, size, solubility, and charge to the active site

• Transition state: occurs when reactions overcome activation energy and run spontaneously to product

How Enzymes Work

• Enzyme mediated reactions occur via 4 mechanisms:

1.Helping substrates get together– When at low concentrations, molecules of

substrates rarely react – Binding at an active site is as effective as a

localized boost in concentration, by as much as ten million fold

How Enzyme Work

2. Orienting substrates in positions favoring reaction – On their own, substrates collide from random

directions– By contrast, the weak but extensive bonds at an

active site put reactive groups close together

How Enzymes Work

3. Shutting out water molecules– Because of its capacity to form hydrogen bonds,

water can interfere with the breaking and formation of chemical bonds in reactions

– Certain active sites have an abundance of non-polar amino acids with hydrophobic groups, which repel water and keep it away from the reactions

How Enzymes Work

4. Inducing changes in enzyme shape (induced fit model)– Substrate is almost complementary to an active

site – Enzyme restrains substrate and stretches it into a

certain shape, often next to another molecule or reactive group

– By optimizing fit between them, moves substrate into transition state

Review

1. Describe the induced fit model.2. Which route is the one traveled by an

enzyme subsidized reaction?3. Where is the transition state located?

Answers

1. Only the proper substrate is capable of inducing the proper alignment of the active site that will enable the enzyme to perform its catalytic function. The active site continues to change until the substrate is completely bound to it, at which point the final shape and charge is determined. Then moves substrate into transition state.