Enzyme Catalysis

1
Enzyme Catalysis
2
Enzymes

• A catalyst increases the rate of a reaction
  without being consumed in the reaction
• Most biological catalysts are proteins called
  enzymes
• Reactants of enzyme catalysis are called
  substrates

3
Properties of Enzymes

• A catalyst enhances the rate of reaction
• Catalysts work by decreasing the activation
  energy for a reaction.
• The structure of the active site of the enzyme
  (shape and charge distribution) is used to
  optimally orient the substrate for reaction.

4
Activation Energy

• An enzyme lowers the activation energy but it
  does not change the standard free energy change
  (DG) nor the Keq.
• A catalyst cannot make an endergonic reaction
  exergonic or vice versa.
• Most enzymes are temperature/pH sensitive
  otherwise enzyme is denatured.

5
(No Transcript)
6
Classes of Enzymes

• Oxidoreductases (dehydrogenases)
• Catalyze oxidation-reduction reactions

7
Transferases

• Catalyze group transfer reactions

8
Hydrolases

• Catalyze hydrolysis reactions where water is the
  acceptor of the transferred group

9
Lyases

• Catalyze lysis of a substrate, generating a
  double bond in a nonhydrolytic, nonoxidative
  elimination (Synthases catalyze the addition to a
  double bond, the reverse reaction of a lyase)

10
Isomerases

• Catalyze isomerization reactions

11
Ligases (synthetases)

• Catalyze ligation, or joining of two substrates
• Require chemical energy (e.g. ATP)

12
Enzyme Kinetics

• Kinetics is the field of chemistry that studies
  the rate and mechanism of a reaction.
• Rates are usually measured in terms of how many
  moles of reactant or product are changed per time
  period.
• A mechanism is a detailed step-by-step
  description of how a reaction occurs at the
  molecular level.

13
Chemical Kinetics

• Experiments examine the amount of product (P)
  formed per unit of time (DP / Dt)
• Velocity (v) - the rate of a reaction (varies
  with reactant concentration)
• Rate constant (k) - indicates the speed or
  efficiency of a reaction

14
First Order Rate Equation

• Rate for nonenzymatic conversion of substrate (S)
  to product (P) in a first order reaction (k is
  expressed in reciprocal time units (s-1))

DP / Dt v kS
15
Second order reaction
16
Pseudo first order reaction

• If the concentration of one reactant is so high
  that it remains essentially constant, reaction
  becomes zero order with respect to that reactant
• Overall reaction is then pseudo first-order

v kS11S20 kS11
17
Enzyme Kinetics

• Enzyme-substrate complex (ES) - complex formed
  when specific substrates fit into the enzyme
  active site

• When S gtgt E, every enzyme binds a molecule of
  substrate (enzyme is saturated with substrate)
• Under these conditions the rate depends only upon
  E, and the reaction is pseudo-first order

18
Initial Velocity (vo)

• Velocity at the beginning of an enzyme-catalyzed
  reaction is vo (initial velocity)
• k1 and k-1 represent rapid noncovalent
  association /dissociation of substrate from
  enzyme active site
• k2 rate constant for formation of product from
  ES

19
Progress curve for enzyme-catalyzed reaction

• The initial velocity (vo) is the slope of the
  initial linear portion of the curve
• Rate of the reaction doubles when twice as much
  enzyme is used

20
The Michaelis-Menten Equation

• Maximum velocity (Vmax) is reached when enzyme is
  saturated with substrate (very high S)
• At high S the reaction rate is independent of
  S (zero order with respect to S)
• At low S reaction is first order with respect
  to S
• The shape of a vo versus S curve is indicative
  of saturation of the enzyme active site as S
  increases

21
Plots of initial velocity (vo) versus S
(a) Each vo vs S point is from one kinetic
run (b) Michaelis constant (Km) equals the
concentration of substrate needed for 1/2 maximum
velocity
22
The Michaelis-Menten equation

• Equation describes vo versus S plots
• Km is the Michaelis constant

VmaxS vo Km S
23
Practice problem

• The Km for the substrate of a particular enzyme
  is 2.0 x 10-5 M. If the initial velocity Vo is
  0.16 µmol/min for S 0.15M, what will be the
  initial velocity when S 5.0 x 10-4M?

24
The Meanings of Km

• Km S when vo 1/2 Vmax
• The lower the value of Km, the tighter the
  substrate binding
• Km can be a measure of the affinity of E for S
• Small value of Km means enzyme can achieve
  maximal catalytic efficiency at low S