ENZYMES A catalyst Is a chemical agent that speeds up a reaction without being consumed by the reaction An enzyme is an organic catalyst Enzymes are proteins

1. An Enzyme Enzyme Active Site Substrate (Reactants) Inhibitor Allosteric Site

2. Enzyme Specificity Enzymes can only work with certain substrates The shape of the enzyme must match the shape of its substrate - The root of the enzyme’s name typically indicates the substrate which it acts upon e.g. ATPase, Amylase, Sucrase

3. Induced Fit Model Figure 8.16 Substate Active site Enzyme (a) Figure 8.16 (b) Enzyme- substrate complex When the substrate binds to the active site, the enzyme changes conformation (shape) to make a better fit. Interactions between chemical groups on substrate and those of the amino acids as well as the shape of the active site cause the induced fit

Sucrose Glucose + Fructose Sucrase Sucrose + Sucrase  Glucose + Fructose + Sucrase

Progress of the reaction Activation Energy The initial amount of energy needed to start a chemical reaction (i.e. break the bonds) Free energy Progress of the reaction EA Figure 8.14 A B C D Reactants Transition state Products activation energy  EA

Progress of the reaction What do enzymes do? Enzymes lower the activation energy How? Orienting substrates correctly Putting stress on substrate bonds Providing a favorable environment This increases the rate of the reaction. Progress of the reaction Products Course of reaction without enzyme Reactants with enzyme EA EA with is lower Free energy Figure 8.15

5. Increasing the Rate of Reaction Increase the number of substrate molecules in solution (increase conc’n) Increase the number of enzymes in solution (increase conc’n) Increase the temperature of the solution (up to a certain point)

6. Saturation A reaction is said to be “saturated” when 100% of enzymes have their active sites filled with substrate. Vmax is maximum velocity (speed) of rxn

Question 7. If there are left-over reactants (substrates), then you could add more enzymes. If there are no more left-over reactants (substrates), then adding more enzymes will not increase the rate.

8. Stopping a Reaction E.g. Add H2SO4(aq) (Sulfuric Acid) Change the pH so it is above or below its optimal value. This changes the enzyme’s conformation (shape) making it lose it dysfunctional e.g. add H2SO4(aq) – Sulfuric Acid E.g. Add H2SO4(aq) (Sulfuric Acid)

9. Effect of Increasing Temperature Above a certain temperature, enzymes’ activity starts to decline because the enzyme begins to denature (unravel)

Metabolic Pathways Reactions occur in a sequence and specific enzymes catalyze each step

10. Cyclic Metabolic Pathways Z U Y V W X Z + Y  U U  V  W W  X + Y (b) Initial Reactant: Z End product: X (desired product)

11. Competitive vs. Non-competitive Inhibition Figure 8.19 (b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding If the inhibitor attaches by weak bonds, it is usually reversible. Figure 8.19 A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor (c) Noncompetitive inhibition If the inhibitor attaches by covalent bonds, it is usually irreversible (usually (c) is)

12. Allosteric Regulation: Activation and Inhibition Stabilized inactive form Allosteric activater stabilizes active from Allosteric enyzme with four subunits Active site (one of four) Regulatory site (one of four) Active form Activator Stabilized active form Allosteric activater stabilizes active form Inhibitor Inactive form Non- functional active site (a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again. Oscillation Figure 8.20 When one activator or inhibitor bind to an allosteric site, and have an effect on all four subunits of an enzyme Bonds are non-covalent, so they are reversible

Feedback Inhibition Active site available Isoleucine used up by cell Feedback inhibition Isoleucine binds to allosteric site Active site of enzyme 1 no longer binds threonine; pathway is switched off Initial substrate (threonine) Threonine in active site Enzyme 1 (threonine deaminase) Intermediate A Intermediate B Intermediate C Intermediate D Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 End product (isoleucine) Figure 8.21 The end product of a metabolic pathway shuts down the pathway Prevents the cell from wasting chemical resources