Enzymes are globular proteins that can act as a biological catalyst to speed up any particular chemical reaction, by lowering the activation energy, without altering themselves.
Enzymes are known as biological catalysts due to the following reasons:
They are known as biological because they are protein in nature and can only be made by living cells.
They are known as catalysts as they speed up a particular chemical reaction.
The lock and key hypothesis
As all enzymes are globular proteins they have a precise 3-D shape and have a "dent" on them known as the active site.
The substrate molecules have a shape which is complementary to the active site.
The substrates can easily fit into it like a lock (enzyme) and key (substrate).
An enzyme-substrate complex is formed and the enzyme catalyses the reaction.
Induced fit hypothesis
The shape of the substrate is not exactly complementary to the active site shape.
The active site is partially flexible and changes its shape slightly when the substrate binds.
Enzyme substrate complex forms.
It lowers the activation energy so the products form.
Intracellular enzymes: Enzymes that are used in the cell after being produced inside the cell. Extracellular enzymes: Enzymes that are outside in the cell after being produced inside the cell.
Effect of temperature on enzyme-catalyzed reaction
Increasing temperature increases the rate of reaction.
At higher temperatures, the kinetic energy of the enzyme and substrate will be higher.
The frequency of collision between enzyme and substrate will increase.
More enzyme-substrate complexes are formed.
All enzymes work best at a particular temperature which is known as optimum temperature.
If the temperature increases beyond this the hydrogen bonds that hold the tertiary structure of the enzyme break.
The active site gets deformed and the enzyme becomes denatured.
Substrate molecules cannot fit so, no enzyme-substrate complex form.
Effect of pH on enzyme catalysed reaction
All enzymes work best at a particular pH known as optimum pH.
If pH becomes too low or too high the solution will contain more H+ or OH- ions.
The presence of these enzymes will alter and destroy the ionic bonds that hold up the tertiary structure.
The active site gets deformed and the enzyme becomes denatured.
Substrate molecules cannot fit so, no enzyme-substrate complex form.
Effect of substrate concentration on enzyme catalysed reaction
As substrate concentration increases the initial rate of reaction increases up to a certain limit.
At this point all the substrate molecules are occupying enzymes active site, the enzymes are in excess.
Fixed amount of substrate is limiting the rate.
Enzyme Inhibitors
It is a substance that reduces the rate of enzyme-controlled reaction by combing with the enzyme.
Competitive Inhibitor :
A molecule that has a shape similar to the substrate.
It fits the active site instead of the substrate.
Enzyme inhibitor-complex is formed.
The substrate molecules can no longer fit the active site.
Fewer enzyme-substrate complex forms so fewer products form.
Non-Competitive inhibitor:
A molecule that attaches to the enzymes allosteric site.
The tertiary structure of the enzyme gets distorted.
The shape of the active site changes.
Substrate molecules are no longer complementary.
They do this by inducing polarity.
The Michaelis–Menten constant Km.
It is the concentration of the substrate to reach half of the Vmax . Km shows the affinity of enzyme for its substrate. A higher value of Km indicates a lower affinity for its substrate.
To calculate the Km of any enzyme first find the Vmax.
Then find half of the Vmax.
Read the substrate concentration from the x-axis to reach half the Vmax.
Enzyme A has the lowest Km so it has the highest affinity for its substrate.
Enzyme B has the lowest affinity as it has the highest (x-axis) Km value.
Inhibitor A is a competitive inhibitor.
It does not alter the Km of the reaction.
Some substrates can always bind to form a product so the reaction does reach Vmax.
But more substrate is required to do this.
Therefore the Km value of the enzyme increases.
Increasing the substrate concentration decreases the effect of competitive inhibitors.
Inhibitor B is a non-competitive inhibitor.
It binds to the allosteric site so it does not alter the affinity, therefore Km value remains the same.
It does decrease the Vmax of the reaction as all the enzymes are not active in the presence of a non-competitive inhibitor.
Immobilising Enzymes
Process:
The enzyme to be immobilised is mixed with a solution of sodium alginate.
This mixture is dripped through a syringe into a solution of calcium chloride.
The calcium ions displace sodium ions forming NaCl & Calcium Alginate beads.
Calcium Alginate is hard and insoluble with the enzyme trapped in it.
The beads are left to harden further.
Advantages:
The product formed will have no enzymes so no purification steps are needed.
The enzymes can be reused.
Immobilised enzymes are more tolerant to temperature & pH changes
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