Enzymes

1
Enzymes
Biological catalysts Increase rate of
reactions by lowering activation energy
(EA) Spontaneous reactions can take a long
time! Need enzymes to speed reactions for cell
survival
2
Activation Energy (EA)

• Needed to destabilize bonds of reactants

3
LE 8-14
A
B
C
D
Transition state
Could raise temp. to break bonds
EA
A
B
Free energy
C
D
Reactants
A
B
DG lt O
C
D
Products
Progress of the reaction
4
Why dont cells rely on increases in temperature
to break bonds?
Because proteins could be denatured causing cell
damage.
5
LE 8-15
Course of reaction without enzyme
EA without enzyme
EA with enzyme is lower
Reactants
Free energy
Course of reaction with enzyme
DG is unaffected by enzyme
Products
Progress of the reaction
6
LE 8-13
Example
Sucrose C12H22O11
Glucose C6H12O6
Fructose C6H12O6
7
Structure Function of Enzyme DRAW

• Enzymes bind substrate molecules (the reactant)
• Substrates bind to active site on enzyme
• Binding induces conformational change in
  enzyme--better fit for substrate
• Active sites are highly specific and
  discriminatory i.e. sucrase does not accept
  lactose

8
LE 8-16
Substrate
Active site
Enzyme-substrate complex
Enzyme
9
How does enzyme lower activation energy of
reaction?

• Orients substrates for optimal interaction

• Strains substrate bonds

• Provides a favorable microenvironment

-May covalently bond to the substrate
10
LE 8-17
Substrates enter active site enzyme changes
shape so its active site embraces the substrates
(induced fit).
Substrates held in active site by
weak interactions, such as hydrogen bonds
and ionic bonds.

• Active site (and R groups of
• its amino acids) can lower EA
• and speed up a reaction by
• acting as a template for
• substrate orientation,
• stressing the substrates
• and stabilizing the
• transition state,
• providing a favorable
• microenvironment,
• participating directly in the
• catalytic reaction.

Substrates
Enzyme-substrate complex
Active site is available for two
new substrate molecules.
Enzyme
Products are released.
Substrates are converted into products.
Products
11
Environmental Conditions Affect Enzyme Function
?
Temperature cold--gtdecreased chance of bumping
into substrate hot--gt good chance of substrate
interaction but chance of denaturation at some
point pH-gtchange in charge (H or OH-) can
denature proteins and substrate Examples of pH
sensitive enzymes?
12
LE 8-18
Optimal temperature for typical human enzyme
Optimal temperature for enzyme of thermophilic
(heat-tolerant
bacteria)
What is your normal body temp.?
Rate of reaction
0
20
40
60
80
100
Temperature (C)
Optimal temperature for two enzymes
Optimal pH for pepsin (stomach enzyme)
Optimal pH for trypsin (intestinal enzyme)
Rate of reaction
0
1
2
3
4
5
6
7
8
9
10
pH
Optimal pH for two enzymes
13
Cofactors

• Non-protein enzyme helpers (like metal, Fe)

• Coenzymes
• organic cofactors (con-enzyme A)
• Vitamins
• e.g. Vitamin K required for blood clotting
• Required in certain carboxylation reactions

14
Regulation of Enzymes Enzyme Inhibitors

• Competitive inhibitor
• binds to active site of enzyme
• blocks substrate binding by competition

• Noncompetitive inhibitor
• binds to another part of enzyme
• causes enzyme to change shape
• prevents active site from binding substrate
• Allosteric effect

DRAW
15
LE 8-19
Substrate
A substrate can bind normally to the active site
of an enzyme.
Active site
Enzyme
Normal binding
A competitive inhibitor mimics the substrate,
competing for the active site.
Competitive inhibitor
Competitive inhibition
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.
Example of allosteric effect
Noncompetitive inhibitor
Noncompetitive inhibition
16
Allosteric Regulation of Enzymes

• Where protein function at one site is affected by
  binding of a regulatory molecule at another site
• May inhibit or stimulate enzyme activity

17
Allosteric Activation and Inhibition

• Most allosterically regulated enzymes are made
  from polypeptide subunits
• active and inactive forms
• binding of activator stabilizes active form of
  enzyme
• binding of inhibitor stabilizes inactive form of
  enzyme

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LE 8-20a
Allosteric activator stabilizes active form.
Allosteric enzyme with four subunits
Active site (one of four)
Regulatory site (one of four)
Activator
Active form
Stabilized active form
Oscillation
Allosteric inhibitor stabilizes inactive form.
Non- functional active site
Inhibitor
Stabilized inactive form
Inactive form
Allosteric activators and inhibitors
19
LE 8-20b
Binding of one substrate molecule to active site
of one subunit locks all subunits in active
conformation.
Substrate
Stabilized active form
Inactive form
Cooperativity another type of allosteric
activation
20
Shift from regulation of one enzyme to
regulation of an enzymatic pathway
21
Feedback Inhibition

• End product of a metabolic pathway shuts down the
  pathway
• Prevents over-production of unneededmolecules

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LE 8-21
Initial substrate (threonine)
Active site available
Threonine in active site
Enzyme 1 (threonine deaminase)
Isoleucine used up by cell
Intermediate A
Feedback inhibition
Enzyme 2
Active site of enzyme 1 cant bind theonine pathwa
y off
Intermediate B
Enzyme 3
Intermediate C
Isoleucine binds to allosteric site
Enzyme 4
Intermediate D
Enzyme 5
End product (isoleucine)
23
Metabolic pathways are often localized in cell

• Cellular structures organize and concentrate
  components of enzymatic pathways
• e.g. organelles (mitochondria, chloroplast,
  lysosomes)
• Pathways respiration, photosynthesis, hydrolysis

24
LE 8-22
Mitochondria, sites of cellular respiration
1 µm
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LE 8-22
Its nice to get so much attention!