Introduction
Enzymes are organic and biological catalysts that participate and enhance chemical reactions without getting used up or consumed in the chemical reactions
Properties Of Enzymes
- They are majorly proteins except ribozymes which are nucleic acid enzymes
- They alter the rate but not the equilibrium of the reactions that they catalyze
- They are neither consumed nor produced during the course of a reaction
- They function within a moderate range of pH and temperature
- They are heat-labile
- They are water-soluble
- They are highly specific in their action
- They are also stereospecific
- They require non-protein organic molecules or metal ions that enhance their activities
- They are compartmentalized
Classification Of Enzymes
Enzymes are classified into 6 classes namely;
- Oxidoreductases
- Transferases
- Hydrolases
- Lyases
- Isomerases
- Ligases
Oxidoreductases
This group of enzymes will catalyze oxidation of one substrate with simultaneous reduction of another substrate or coenzyme
For Example
Alcohol + NAD+ → Aldehyde + NADH + H+
Enzyme = Alcohol dehydrogenase
Transferases
This class of enzymes transfers one group (other than hydrogen) from the substrate to another substrate
For Example
Hexose + ATP → Hexose-6-phosphate + ADP
Enzyme = Hexokinase
Hydrolases
This class of enzymes can hydrolyze ester, ether, peptide or glycosidic bonds by adding water and then breaking the bond
Acetylcholine + H2O → Choline + Acetate
Enzyme = Acetylcholine esterase
All digestive enzymes are hydrolases
LYASES
These enzymes can remove groups from substrates or break bonds by mechanisms other that hydrolysis
For Example
Fructose-1,6-bisphosphate → Glyceraldehyde-3-Phosphate + Dihydroxyacetone phosphate
Enzyme = Aldolase
Isomerases
These enzymes can produce optical, geometric or positional isomers of substrates. Racemases, epimerases, cis-trans isomers are examples
For Example
Glyceraldehyde-3-phosphate → Dihydroxyacetone phosphate
Enzyme = Triose phosphate isomerase
LIGASES
These enzymes link two substrates together, usually with the simultaneous hydrolysis of ATP
For Example
Acetyl-CoA + CO2 + ATP → Malonyl-CoA + ADP + Pi
Enzyme = Acetyl-CoA carboxylase
MODE OF ACTION OF ENZYMES
The following are theories explaining the mechanism of action of enzymes;
- Lowering of Activation Energy
- Enzymes create alternate energy pathways for reactions by lowering the activation energy of the
reactions
Acid Base Catalysis
Histidine residues 12 and 19 at the active site of ribonuclease function as acid and base in catalysis.
Histidine 12 acts as an acid and donates a proton. Histidine 119 accepts a proton and product is released
Substrate Strain
Binding of substrate to a preformed site on the enzyme can induce strain in the substrate. The energy level of the substrate is raised
Covalent Catalysis
In covalent catalysis, a nucleophilic (negatively charged) or electrophilic (positively charged) group of the enzyme attacks the substrate. This results in covalent bonding of the substrate to the enzyme before catalysis is effected
Entropy Effect
Enzymes enhance reaction rates by decreasing entropy. When correctly positioned and bound on the enzyme surface, the substrates are strained to the transition state before the product is formed
Product Substrate Orientation Theory
Enzymes have appropriate three-dimensional structures to keep the substrates in specific orientation, such that the reactive groups come to physical apposition, leading to speedy reactions
Factors Affecting Enzyme Activity
The following are the various factors that can affect enzyme activity;
- Enzyme concentration
- Substrate concentration
- Product concentration
- Temperature
- Hydrogen ion concentration (pH)
- Presence of activators
- Presence of inhibitors
- Presence of repressor or derepressor
- Covalent modification
Enzyme Concentration
Rate of a reaction is directly proportional to the enzyme concentration, when sufficient substrate is
present
Effect Of Substrate Concentration
As substrate concentration is increased, the velocity is also correspondingly increased until all the
enzyme molecules are saturated after which further increases in substrate concentration has no effect in the reaction velocity
Effect Of Concentration Of Products
When product concentration is increased, reaction is slowed, stopped or even reversed. This form of control limits the rate of formation of the product when the product is underused
Effect Of Temperature
The rate of an enzyme-catalyzed reaction usually increases with increasing temperature up to an
optimum point, and then it decreases because the enzymes are thermolabile
Effect Of Ph
Each enzyme has an optimum pH, on both sides of which the velocity will be drastically reduced. The
graph will show a bell shaped curve
Enzyme Inhibition
- Reversible inhibition
- Competitive inhibition
- Non-competitive inhibition
- Uncompetitive inhibition
- Irreversible inhibition
- Affinity labels
- Mechanism based or suicide inhibition
Reversible Inhibition
Competitive Inhibition
Inhibitors compete directly with substrate for binding to the active site (i.e. the catalytic site)
E + S ⇋ ES → E + P
E + I ⇋ EI
Km increases
Vmax remains the same
Increase in substrate concentration [S] can knock off the inhibitors that are bound to the active site
Non-Competitive Inhibition
Inhibitors bind both to the free enzyme and to the enzyme-substrate complex (ES) at a site different from the active site (i.e. the allosteric site)
E + I ⇋ EI
ES + I ⇋ ESI
Km remains the same
Vmax decreases
Increase in the substrate concentration generally does not relieve this inhibition
Uncompetitive Inhibition
Inhibitor does not have any affinity for free enzyme
It binds only to the ES complex at the allosteric site
ES + I ⇋ ESI
Km decreases
Vmax decreases
Irriversible Inhibition
Affinity Labels
Definition
These are substrate analogues that possess a highly reactive group that is not present on the natural substrate
Action
The active site is permanently blocked from the substrate because the group reacts covalently with an amino acid residue
The residue that is modified is not necessarily involved in catalysis
Mechanism-Based/Suicide Inhibition
Definition
These are substrate analogs that are transformed by the catalytic action of the enzymes
Action
Their structures are such that the product of this reaction is highly reactive and subsequentially combines covalently with an amino acid residue in the active site, thus inactivating the enzyme
Transition-State Analogs
Definition
These are substrate analogs whose structures closely resemble the transition state of the natural substrate
Action
Transition-state analogs do not covalently modify the enzymes but bind the active site so tightly that they irreversibly inactivate it
ENZYME REGULATION
Mechanisms of enzyme regulation include;
- Allosteric Regulation
- Feedback Inhibition
- Induction
- Repression
- Compartment
Action
The active site is permanently blocked from the substrate because the group reacts covalently with an amino acid residue. The residue that is modified is not necessarily involved in catalysis.
Mechanism-Based/Suicide Inhibition Definition
These are substrate analogs that are transformed by the catalytic action of the enzymes.
Action
Their structures are such that the product of this reaction is highly reactive and subsequentially combines covalently with an amino acid residue in the active site, thus inactivating the enzyme.
Transition-State Analogs Definition
These are substrate analogs whose structures closely resemble the transition state of the natural substrate.
Action
Transition-state analogs do not covalently modify the enzymes but bind the active site so tightly that they irreversibly inactivate it.
ENZYME REGULATION
Mechanisms of enzyme regulation include:
- Allosteric Regulation
- Feedback Inhibition
- Induction
- Repression
- Compartmentalization
- Hormonal Regulation
- Covalent Modification
Allosteric Regulation
Allosteric enzymes have one catalytic site where the substrate binds and another separate allosteric site where the modifier binds. Modifiers can be positive, enhancing enzyme activity (allosteric activation) or negative, inhibiting enzyme activity (allosteric inhibition).
For Example:
Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP
Enzyme = Phosphofructokinase
- AMP = positive modifier
- ADP = positive modifier
- ATP = negative modifier
Feedback Inhibition
This is a negative modulation of the committed step of a metabolic pathway by its end product. It prevents unnecessary production of an excess of the end product by shutting down the pathway until more is needed.
For Example:
→ → →
In this pathway, if D inhibits E1, it is called feedback inhibition.
Induction
This is effected through the process of derepression. The inducer will relieve the repression on the operator site and will remove the block on the biosynthesis of the enzyme molecules.
For Example:
- Lactose is induced by milk
- Glucokinase is induced by insulin
Repression
This is the inhibition of enzyme activity by shutting off the operator site where DNA transcription for the enzyme production occurs. This prevents the formation of more enzymes.
For Example:
- Heme – excess heme represses heme synthesis
Compartmentalization
This is the regulation of enzyme activity by localization of enzymes catalyzing different steps in a metabolic pathway in different locations. Certain enzymes of the pathway may be located in the mitochondria whereas certain other enzymes of the same pathway are cytoplasmic.
For Example:
- Heme synthesis
- Urea cycle
- gluconeogenesis
Hormonal Regulation
This is the regulation of enzyme activity by hormones.
For Example:
- Insulin stimulates glucokinase
Covalent Modification
This is the regulation of enzyme activity either by:
- Addition of a group to the enzyme protein by a covalent bond; or
- Removal of a group by cleaving a covalent bond
For Example:
- Phosphorylation
