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Classification of Enzymes
CH3 -C- COO-
O
CH3 -CH-COO-OH
lactate
dehydrogenase+ NADH + H + + NAD +
Pyruvate Lactate
1.Oxidoreductase
2.Transferase COO-
CH2
CH- NH3+
COO-
Aspartate
+
COO-
C= O
CH2
CH2
COO-
-Ketoglutarate
Aspartate amino
transfer ase
COO-
CH2
C= O
COO-
Oxalosuc cinate
+
COO-
C-NH3+
CH2
CH2
COO-
Glutamate
3.Hydrolase
CH3 -C-OCH2 CH2 N(CH3 ) 2
O
Acetylcholine
Acetylcholinesterease+ H2 OCH3 -C-O-
O
HOCH2 CH2 N( CH3 ) 2
Acetate Choline
4. Lyase
5. Isomerase
6. Ligase
COO-
CH2
C-COO-
CH
Aconitase
COO-
cis- Aconitate
+ H2 O
COO-
CH2
C-COO-
C-H
COO-
HO
Isocitrate
HO
CH2 OPO32 -
OH
HHO
CH2 OH
OHH
- D-Fructose-6-phosphate
O
HO
OH
OH
CH2 OPO32 -
OH
- D- Glucose-6-phosphate
Phosphohexose
isomerase
ATP + L -tyrosine + t-RNA
Ty rosine-tRNA
syntheta se L-tyro sy l-tDNA + AMP + PPi
Enzyme controlled reactions proceed 108 to 1011 times
faster than corresponding non-enzymic reactions.
Making reactions go faster
• Increasing the temperature make molecules
move faster,
• Biological systems are very sensitive to
temperature changes.
• Enzymes can increase the rate of reactions
without increasing the temperature.
• They do this by lowering the activation energy.
• They create a new reaction pathway “a short
cut”
Schematic of an Active Site
• Apoenzyme: the protein part of an
enzyme.
• Cofactor: a nonprotein portion of an
enzyme that is necessary for
catalytic function; examples are
metallic ions such as Zn2+ and Mg2+.
• Coenzyme: a nonprotein organic
molecule, frequently a B vitamin,
that acts as a cofactor.
• Substrate: the compound or
compounds whose reaction an
enzyme catalyzes.
• Active site: the specific portion of
the enzyme to which a substrate
binds during reaction.
Cofactors (simple vs. complex)
• An additional non-protein molecule that is needed by some enzymes to help the reaction
• Tightly bound cofactors are called prosthetic groups
• Cofactors that are bound and released easily are called coenzymes.
• Many vitamins are coenzymes.
Nitrogenase enzyme with Fe, Mo and ADP
cofactors
The shape and the chemical environment inside the
active site permits a chemical reaction to proceed more
easily.
Substrate of an
enzyme are the
reactants that are
activated by the
enzyme
The Lock and Key Hypothesis
• Fit between the substrate (key) and the active site of the enzyme (lock) is very precise
• Temporary structure called the enzyme-substrate complex formed
• Products have a different shape from the substrate
• Once formed, they are released from the active site
• Leaving it free to become attached to another substrate
Enzyme may
be used again Enzyme-
substrate
complex
E
S
P
E
E
P
Reaction coordinate
This explains enzyme
specificity and loss of
activity when enzymes
denature.
This explains the enzymes
that can react with a range of
substrates of similar types.
The Induced Fit Hypothesis
• Some proteins can change their shape (conformation)
• When a substrate combines with an enzyme, it induces a change in the enzyme’s conformation.
• The active site is then moulded into a precise conformation.
• Making the chemical environment suitable for the reaction
• The bonds of the substrate are stretched to make the reaction easier (lowers activation energy)
Factors Affecting Enzyme
Activity • substrate concentration
• pH
• temperature
• inhibitors
Substrate concentration: Non-enzymic
reactions
• The increase in velocity is proportional to the
substrate concentration
Reaction
velocity
Substrate concentration
Substrate concentration: Enzymic reactions
• Faster reaction but it reaches a saturation point when all the enzyme molecules are occupied.
• If you alter the concentration of the enzyme then Vmax will change too.
Reaction
velocity
Substrate concentration
Vmax
At constant substrate
concentration, increasing the
enzyme concentration,
increases the rate linearly.
(In practically all enzyme
reactions, the molar conc. of
enzyme is always much lower
than that of substrate.)
A saturation curve
A linear curve
At constant enzyme
concentration, increasing the
substrate concentration does
not increases the rate
continuously. A saturation
point is achieved.
The effect of pH
• Extreme pH levels will produce denaturation
• The structure of the enzyme is changed
• The active site is distorted and the substrate
molecules will no longer fit in it
• At pH values slightly different from the enzyme’s
optimum value, small changes in the charges of
the enzyme and it’s substrate molecules will
occur
• This change in ionisation will affect the binding
of the substrate with the active site.
The effect of temperature Q10 (the temperature coefficient) = the increase in
reaction rate with a 10°C rise in temperature.
For chemical reactions the Q10 = 2 to 3 (the rate of the reaction doubles or triples with every 10°C rise in temperature)
Enzyme-controlled reactions follow this rule as they are chemical reactions
BUT at high temperatures proteins denature
The optimum temperature for an enzyme controlled reaction will be a balance between the Q10 and denaturation.
The effect of temperature
Temperature / °C
Enzyme
activity
0 10 20 30 40 50
Q10 Denaturation
The effect of temperature
• For most enzymes the optimum temperature is
about 30°C
• Many are a lot lower,
cold water fish will die at 30°C because their
enzymes denature
• A few bacteria have enzymes that can withstand
very high temperatures up to 100°C
• Most enzymes however are fully denatured at
70°C
Organize the tiles!
When you organize these tiles, you will find a phrase describing a feature
of biological catalysts, that in fact is common to all catalysts:
• Allosterism: enzyme regulation based on an
event occurring at a place other than the active
site but that creates a change in the active site.
– An enzyme regulated by this mechanism is called
an allosteric enzyme.
– Allosteric enzymes often have multiple polypeptide
chains.
– Negative modulation: inhibition of an allosteric
enzyme.
– Positive modulation: stimulation of an allosteric
enzyme.
– Regulator: a substance that binds to an allosteric
enzyme.
• Feedback control: an enzyme-regulation process
where the product of a series of enzyme-
catalyzed reactions inhibits an earlier reaction in
the sequence.
• The inhibition may be competitive or noncompetitive.
A B C DE1 E2 E3
feedback inhibition
Inhibitors
Inhibitors are chemicals that reduce the
rate of enzymic reactions.
The are usually specific and they work at
low concentrations.
They block the enzyme but they do not
usually destroy it.
Many drugs and poisons are inhibitors of
enzymes in the nervous system.
nerve gases and pesticides, containing
organophosphorus, combine with serine
residues in the enzyme acetylcholine esterase.
Two categories of reversible inhibition:
1.) Competitive: These compete with the substrate
molecules for the active site. The inhibitor’s action is
proportional to its concentration. Resembles the
substrate’s structure closely.
Enzyme inhibitor
complex Reversible
reaction
E + I EI
2.) Non-competitive: The inhibitor
binds itself to a site other than
the active site (allosterism),
thereby changing the
conformation of the active site.
The substrate still binds but
there is no catalysis.
Examples
Cyanide combines with the Iron
in the enzymes cytochrome
oxidase.
Heavy metals, Ag or Hg,
combine with –SH groups.
These can be removed by
using a chelating agent such as
EDTA.
• Enzyme kinetics in the presence and absence of
inhibitors.
Maximum reaction rate
is the same without an
inhibitor and in the
presence of a
competitive inhibitor
(CI).
Maximum rate is
obtained at high
substrate concentration
for CI but low with no
inhibitor.
If the inhibitor is non
competitive, the
maximum rate of
reaction is lower.