The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary structure of the protein.
These Factors Affect Enzyme Activity:
1. Effect of Temperature
The optimum temperature for enzyme activity is 30oC – 40oC in animals and 20oC – 30oC in plants.
High temperature (above 45oC) denatures enzymes due to the degradation of linkages in its polypeptide chain whereas low temperature inactivates enzymes due to the reduction in speed of molecular movement.
2. Effect of pH
Most intracellular enzymes function near neutral pH with the exception of several digestive enzymes which work either in an acidic range of pH or alkaline.
3. Effect of Substrate Concentration
With the increase in substrate concentration, the velocity of the enzymatic reaction rises at first.
- The reaction ultimately reaches a maximum velocity (Vmax) which is not exceeded by any further rise in the concentration of the substrate.
- This is because the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there is no free enzyme molecule to bind with the additional substrate molecule.
Leonor Michaelis and Maud Menten proposed a mathematical model in 1913, to determine the effect of substrate concentration in enzymatic reaction and they derived a relationship that is mathematically expressed as:
Km = Michaelis – Menten constant i.e., the substrate concentration to produce half maximum velocity.
V = The velocity of the reaction
Vmax = maximum velocity
[S] = substrate concentration
Inhibition of Enzymatic Action
The activity of an enzyme is also sensitive to the presence of specific chemicals that bind to the enzyme.
When the binding of the chemical shuts of enzyme activity, the process is called inhibition and the chemical is called an inhibitor.
Types of inhibition:
- Reversible inhibition: – It is temporary inhibition, and is overcome by increased concentration of substrate, dilution, and dialysis.
- Irreversible inhibition: – This inhibition is of permanent nature, in which the inhibitor combines with a specific functional group of the enzyme through a covalent bond.
It does not overcome by increased concentration of substrate, dilution or dialysis.
On the basis of competitiveness enzyme inhibition can be divided into the following types:
- Competitive inhibition
- Non-competitive inhibition
- Allosteric inhibition
- Competitive inhibition: This inhibition occurs due to substrate or increase enzyme analogue.
- In competitive inhibition, an inhibitor that resembles the normal substrate binds to the enzyme, usually at the active site, and prevents the substrate from binding to the active site.
- During competitive inhibition, the inhibitor and substrate compete for the active site. The active site is a region on an enzyme where a particular substrate can bind. In competitive inhibition, the inhibitor resembles the substrate, therefore, taking its place and binding to the active site of an enzyme.
- Increasing the substrate concentration would diminish the “competition” for the substrate to properly bind to the active site and allow a reaction to occur.
- Non-competitive Inhibition: In this inhibition, the inhibitor forms a complex with the enzyme at a site other than the active site.
- Allosteric Inhibition: In this inhibition, the Inhibitor binds to the enzyme at a specific site other than the active site and change the structure of the active site to affect substrate binding.
- Allosteric modulation or feedback inhibition is an enzyme regulatory mechanism where a product or intermediates of a reaction functions as a temporary allosteric inhibitor, (which combines with a regulatory site) if its concentration crosses the threshold value.
The End Product Inhibitor functions as a negative modulator and the enzyme inactivated are called an allosteric enzyme.
Michaelis-Menten or Km constant is not applicable in allosteric enzymes.