Factors Affecting the Rate of Enzyme Action: The functional specificity of every enzyme is the consequence of its specific chemistry and configuration. Any factor that can alter the chemistry and shape of an enzyme can affect its rate of catalysis. Some of the important factors that can affect the temperature, and pH of the medium.
The rate of reaction depends directly on the amount of enzyme present at a specific time at unlimited substrate concentration. If the amount of enzyme is increased by two fold the reaction rate is doubled.
By increasing the enzyme molecules an increase in the number of active sites takes place. More active sites will convert the substrate molecules into product(s), in the given period of time. After a certain limiting concentration, the rate of reaction will no longer depend upon this increase.
At low concentration of substrate the reaction rate is directly proportional to the substrate available.
If the enzyme concentration is kept constant and the amount of substrate is increased, a point is reached when a further increase in the substrate does not increase the rate of the reaction any more (Fig. 3.4). This is because at high substrate in the substrate does not increase the reaction rate.
Fig. 3.4:Effect of substrate concentration on the rate of an enzyme catalyzed reaction.
The rate of enzyme controlled reaction may increase with increase in temperature but up to a certain limit. All enzymes can work at their maximum rate at a specific temperature called as optimum temperature. For enzymes of human body 37°C is the optimum temperature (fig. 3.5)
Heat provides activation energy and therefore, chemical reactions are accelerated at high temperatures. Heat also supplies kinetic energy to the reacting molecules, causing them to move rapidly. Thus the reactants move more quickly and chances of their collision with each other are increased. However, further increase in heat energy also increases the vibrations of atoms which make up the enzyme molecule. If the vibrations become too violent, globular structure essential for enzyme activity is lost and the enzyme is said to be denatured.
Fig. 3.5: effect of temperature on the rate of an enzyme catalyzed reaction
Every enzyme function most effectively over a narrow range of pH known as the optimum pH as shown in Table 4.1.
A slight change in pH can change the ionization of the amino acids at the active site. Moreover, it may effect the ionization of the substrates. Under these changed conditions enzyme activity is either retarded or blocked completely. Extreme changes in pH cause the bonds in the enzyme to break, resulting in the enzyme denaturation.
Table 4.1 Optimum pH values for some enzymes
An inhibitor is a chemical substance which can react (in place of substrate) with the enzyme but is not transformed into product(s) and thus blocks the active site temporarily or permanently, for example poisons, like cyanide, antibiotics, anti-metabolites and some or permanently, for example poisons, like cyanide, antibiotics, anti-metabolites and some drugs. Inhibitors can be divided into two types: (i) Irreversible (ii) Reversible
They check the reaction rate by occupying the active sites or destroying the globular structure. They occupy the active sites by forming covalent bonds or they may physically block the active sites.
They form weak linkages with the enzyme. Their effect can be neutralized completely or partly by an increase in the concentration of the substrate.
They are further divided into two major types: A. competitive B. Non-competitive
- Competitive Inhibitors
Because of the structural similarity with the substrate they may be selected by the binding sites, but are not able to activate the catalytic sites. Thus product(s) are not formed (Fig.3.6).
- Non-competitive Inhibitors
They form enzyme inhibitor complex at a point than the active site. They alter the structure of the enzyme in such a way that even if genuine substrate binds the active site, catalysis fails to take place.
Fig. 3.6:mechanism of competitive inhibition. (a) Formation of enzyme-substrate complex resulting in the formation of product. (b) Inhibitor malonie acid does not fit the active site, hence no product is formed.