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Enzyme
 biological catalyst that changes the rate of a reaction
 not consumed by reaction, reusable
 some enzymes are globular proteins, others are ribose nucleic acids.
(Active site + R groups) 1. act as template for substrate orientation, 2. Stresses
substrates and stabilizes transition state, 3. Provides favorable
microenvironment, 4. Participating directly in catalytic reaction.
Induced Fit Model of enzyme-substrate interaction
-
-
energy is needed to contort reactant molecules so that bonds can change
initial investment of energy = free energy of activation/ activation energy
reactants break when molecules absorb enough energy to become unstable
o absorption of thermal energy increases speed of reactant molecules
they collide more often, more forcefully
thermal agitation of the atoms in molecules (bonds more likely to break)
reactant molecules that have absorbed sufficient free energy to react are
unstable = transition state
Enzymes lower activation energy barrier, enabling reactant molecules to
reach transition state at moderate temperatures.
An enzyme cannot change the free energy change required for reaction.
Protein Structure of an Enzyme
HIV Protease
2 subunits (polypeptide chains) & flaps
1. to guard entrance to active site
2. exclude incoming water molecules
3. flexible; allow a substrate to access active site by opening, closing
Model of Enzyme action: Lock and Key Hypothesis
Enzymes have specific surface configurations produced by 3D folding of
polypeptide chain
substrate= key whose shape is complementary to enzyme (lock)
when enzyme and substrate collide in correct orientation, substrate
attached to active site of enzyme
short lived enzyme-substrate complex formed (held by weak interactions
e.g. H bonds, ionic bonds)
products no longer fit into active site, escapes into surrounding medium
active site free to receive further substrate molecules
1.
2.
3.
specificity
enzymes lower activation energy barrier
heating, pH curtails enzyme activity (denaturation)
-
Both enzyme and substrate change shape straining bonds during reaction.
Strained form of the substrate (transition state) exist for short while
Molecular Basis of enzyme action (lowering activation energy)
1. Proximity Effects
- Temporary binding of reactants next to each other on an enzyme increases chance
of reaction
- uncatalysed reactions depend on random collisions between substrate molecules
2. Strain Effect
- Slight distortion of reactants as they bind to enzyme strains bonds which are to be
broken
- Chance of breakage is increased
3. Orientation Effects
– Reactants are held by the enzyme in such a way bonds are exposed to attack
4. Microenvironment effects
- Hydrophobic amino acids create a water-free zone in which non-polar reactants
may react more easily
5. Acid-base catalysis
- Acidic and basic amino acids in enzyme facilitate transfer of reactants to and from
reactants.
Rate of reaction
1.
2.
measure formation of product
measure disappearance of substrate
[Note graphs and expt e.g. on p.12]
Temperature Coefficient = Rate of reaction at (x+10) degrees / (x degrees)
At lower temperatures, enzyme is in active state, rate of reaction depends on kinetic
energy of molecules.
At optimum temp, enzyme is in active state
At higher temp, enzyme is irreversibly altered in shape and flexibility of the enzyme
molecule.
(Thermal agitation breaks hydrogen, ionic bonds and other weak interactions that
stabilize the active configuration.)
Substrate Concentration
Michealis constant = conc. of substrate required to make reaction rate maximum
Low Km – HIGH affinity of enzyme for substrate (low substrate conc. needed to
Attain maximum velocity)
High Km – LOW affinity of enzyme for substrate (high substrate conc. needed)
Enzyme Inhibitors
1.
2.
Competitive Inhibitors
Non-competitive inhibitors
a. Reversible
b. Irreversible
Substrate
Binding
Conc.
Effect of
inhibition
Rate of
rxn
Competitive Inhibition
Similar in structure and charge to
normal substrate
Temporarily bound to enzyme at
active site
Degree of inhibition depends on
relative concentration of inhibitor
and substrate
reversible
Still reaches maximum at higher
substrate concentration but slower.
Non-competitive inhibition
Not similar in structure and
charge
Temporarily bound to enzyme
at allosteric site (reversibly or
not)
Degree of inhibition depends
on concentration of inhibitor
Reversible or irreversible
Rate of reaction reaches lower
maximum level.
For non-competitive inhibition, enzyme-inhibitor complex at point on enzyme
other than active site. Globular structure of enzyme rendering active site
unreceptive to substrate. Substrate may still be able to bind to active site but
catalysis cannot take place. Effects of inhibitors cannot be overcome by high
substrate conc. Rate of reaction continues decreasing with increasing substrate
concentration till inhibitor saturation is reached (rate of reaction nil).
Allosteric Regulation
Cooperativity
Allosteric enzymes consist of 2 or morepolypeptide chains or subunits. Each subunit
has its own active site. Binding of an allosteric inhibitor stabilizes inactive form of
the enzyme.
e.g. feedback inhibition
Allosteric activator causes induced fit in one subunit – triggering favorable change
in conformation in all subunits.