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
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
Substrate(Reactants)
Allosteric Site
Active Site
Inhibitor
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
• 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
Figure 8.16 (b)
Enzyme- substratecomplex
Figure 8.16
Substate
Active site
Enzyme
(a)
Sucrose Glucose + FructoseSucrase
Sucrose + Sucrase Glucose + Fructose + Sucrase
Activation Energy• The initial amount of energy needed to start a
chemical reaction (i.e. break the bonds)
Fre
e en
ergy
Progress of the reaction
EA
Figure 8.14
A B
C D
Reactants
A
C D
B
Transition state
A B
C D
Products
activation energy EA
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
Course of reaction with enzyme
EA
withoutenzyme EA with
enzymeis lower
Fre
e en
ergy
Figure 8.15
5. Increasing the Rate of Reaction
(1)Increase the number of substrate molecules in solution (increase conc’n)
(2)Increase the number of enzymes in solution (increase conc’n)
(3)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
• 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 PathwaysZ U
V
WX
Y
(a) Z + Y U U V W W X + Y
(b) Initial Reactant: Z End product: X (desired product)
11. Competitive vs. Non-competitive Inhibition
• If the inhibitor attaches by covalent bonds, it is usually irreversible (usually (c) is)
Figure 8.19 (b) Competitive inhibition
A competitiveinhibitor mimics the
substrate, competingfor the active site.
Competitiveinhibitor
A substrate canbind normally to the
active site of anenzyme.
Substrate
Active site
Enzyme
(a) Normal binding
Figure 8.19
A noncompetitiveinhibitor binds to the
enzyme away fromthe active site, altering
the conformation ofthe enzyme so that its
active site no longerfunctions.
Noncompetitive inhibitor
(c) Noncompetitive inhibition
• If the inhibitor attaches by weak bonds, it is usually reversible.
12. Allosteric Regulation:Activation and Inhibition
– 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
Stabilized inactiveform
Allosteric activaterstabilizes active fromAllosteric enyzme
with four subunitsActive site
(one of four)
Regulatorysite (oneof four)
Active formActivator
Stabilized active form
Allosteric activaterstabilizes active form
InhibitorInactive formNon-functionalactivesite
(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
Feedback Inhibition• The end product of a metabolic pathway shuts
down the pathway• Prevents the cell from wasting chemical resources
Active siteavailable
Isoleucineused up bycell
Feedbackinhibition
Isoleucine binds to allosteric site
Active site of enzyme 1 no longer binds threonine;pathway is switched off
Initial substrate(threonine)
Threoninein active site
Enzyme 1(threoninedeaminase)
Intermediate A
Intermediate B
Intermediate C
Intermediate D
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
End product(isoleucine)
Figure 8.21