Factors Affecting Enzyme Activity- Introduction, General Features and Mechanism

Factors Affecting Enzyme Activity: Enzymes are core to every biochemical procedure. They catalyse the stepwise reactions involving degradation of nutrient molecules, conservation and transformation of chemical energy, and the synthesis of biological macromolecules from simple precursors. Enzymes are capable of catalysing reactions of natural origin and are synthesised by living cells only, and are also called biocatalysts.

Enzymes (E) bind to the substrate (S) and create a transient ES complex. Enzymes lessen the activation energy to attain the transition site. Read this article to learn more about enzymes, structure, and various factors affecting reaction rates.

History of Enzymes

W. Kuhne first reported the presence of enzymes in 1878. Edward Buchner put forward the term enzyme in 1897, who was the first to isolate the enzyme from yeast cells and gave the term Zymase.

Features of Enzymes

1. Most enzymes are proteins except for a small group of catalytic RNA molecules. The catalytic activity of enzymes depends on the integrity of their protein structure. If an enzyme gets denatured or separated, then its catalytic activity is lost.
2. Enzymes are highly specific. Their actions are specific to the substrates.
3. Enzymes exhibit enormous catalytic power. It increases the rate of a reaction by lowering the activation energy of a chemical reaction.

The activity of enzymes: Amounts of enzymes can either be expressed as molar amounts or measured in terms of activity. Enzymes are usually present in very small quantities. So, a convenient method of enzyme quantification is a measurement of catalytic activity. The activity of enzymes is then related to their concentration. There are two standard units to express enzyme activity.
1. Enzyme unit (U): One enzyme unit is defined as the amount of enzyme causing the transformation of 1μmole of substrate per minute at 25˚C under optimal conditions of measurement. (1 enzyme unit=1μmole min ⁻¹)
2. Katal (Kat): The katal is the SI unit of enzyme activity. One katal is defined as the amount of enzyme utilised to catalyse the transformation of 1 mole of substrate per second. (1 Katal= 1mol sec⁻¹) 1U= 16.67 nano kat

Mechanism of Action of Enzymes

Mechanism of action of enzymes can be studied under two steps:
1. Lowering of Activation energy
2. Formation of enzymes – substrate complex.

1. Lowering Activation Energy: Most chemical reactions have an energy barrier that separates the reactants and products. Therefore, an external supply of energy is needed to start a chemical reaction. This energy is called activation energy. It is quite high for the non-catalysed reactions and is lowered by the enzyme. The activation energy increases the kinetic energy of the system and brings about forceful collisions between the reactants. Enzymes lower the activation energy of a reaction by taking the reactants out of the solution-state (desolvation), establishing weak bonds between reactants and enzymes, and bringing reactant molecules close to one another in the region of active sites.

Fig: Graph showing lowering of activation energy

2. Formation of enzymes – substrate complex: All enzymes have a specific three-dimensional structure, and a part of their structure is known as active sites. An active site is a site into which a substrate fits to proceed with chemical reactions. The point where the substrate is bound on the active site is known as the substrate-binding site. The first step in an enzymatic reaction is that the enzyme forms a temporary association with its substrate called the enzyme-substrate complex (E-S complex). The following two models have been proposed to explain the formation of the E-S complex.

Factors Affecting Enzyme Activity

The factors affecting enzyme activity are:

1. Temperature: An enzyme activity is maximum within a narrow range of temperature. The temperature at which an enzyme shows its maximum activity is called optimum temperature. The optimum temperature for most of the enzymes is between 25-35°C.  Temperature above and below this range affects the enzyme activity. High temperature above 50°C results in the destruction of enzymes by causing their denaturation, and very low temperature preserves the enzymes in their inactive state.

Fig: Graph showing bell-shaped curve

2. Enzyme Concentration: The rate of enzymatic reaction increases with increased enzyme concentration up to a point called saturation point. Above this limit, there is little effect on enzyme activity.

Fig: Graph showing effect of enzyme concentration

3. pH: Enzymes work at their optimum pH. A rise or fall in pH reduces the activity of enzymes. Most of the intracellular enzymes function near-neutral pH except for several digestive enzymes that are active either at acidic or alkaline pH ranges. Change in pH causes alteration in the structure of the enzyme, including its active site. Under extreme pH, denaturation of enzymes occurs.

Fig. Effect of pH on enzyme activity

4. Substrate concentration: Initially, the rate of enzymatic reaction increases with the increase in substrate concentration. In the beginning, the velocity of the reaction is high, but later it does not increase progressively with the increase in substrate concentration. It happens because enzyme molecules get fully saturated, and no more active sites are left to bind additional substrates.

Fig: Graph showing effect of substrate concentration on enzyme activity

In the graph, Km is a constant known as the Michaelis-Menten constant. It is defined as the concentration of substrate (expressed in moles/lit) to produce half-maximum velocity in a reaction that is catalysed by an enzyme. It indicates that half of the enzymes molecules are bound with the substrate molecules when the substrate concentration is the same as the Km value.

The low value of Km indicates a strong affinity between enzyme and substrate. On the other hand, a high value of Km shows a weak affinity between the enzyme and the substrate.

5. Product concentration: The accumulation of the products of reaction causes decreases in the activity of enzymes. When products accumulate, they combine with the active site of enzymes, thus forming a loose complex that inhibits the activity of enzymes.

6. Activators and Poisons: Some chemical substances or molecules increase the activity of enzymes such as co-factors, for example, potassium ion (K⁺), manganese ion (Mn²⁺), etc.  These chemicals which increase the activity of enzymes are known as activators. On the other hand, salts of heavy metals and compounds such as cyanides, azides, and iodoacetate destroy the tertiary structure of enzymes, thus affecting the activity of enzymes. These chemicals are known as poisons.

7. Water: Water is a medium in which enzymatic reactions take place. The presence of water increases the reaction rate, whereas the absence of water inhibits the enzyme activity.

8. Light: Enzymes are sensitive to light. The presence of light increases the reaction rate in some enzymes whereas the presence of harmful radiation such as UV rays and X-rays decreases enzymes’ catalytic activity.

Inhibition of Enzyme Activity

Enzyme inhibitors are substances that decrease the activity of enzymes when they bind to the active site present on the enzyme.

Types of enzyme inhibitors:
a) Based on specificity:
1. Co-enzyme inhibitors: They inhibit the activity of co-enzymes only.For example, hydroxylamine inhibits co-enzyme pyridoxal phosphate.
2. Ion-cofactor inhibitor: They inhibit the ion factor, for example,fluoride chelate Mg²⁺ ion of enolase enzyme.
3. Prosthetic group inhibitors: They inhibit the prosthetic group; for example, cyanide inhibits the Heme of cytochrome oxidase.
4. Apoenzyme inhibitors: They inhibit the apoenzymes, for example, antibiotics

b) Based on origin:
1. Natural enzyme inhibitor: For example, Aflatoxin
2. Artificial enzyme inhibitor (synthetic): For example,drugs

c) Based on whether the inhibition is reversible or irreversible
1. Reversible inhibition: It is a temporary inhibition that can be overcome by the withdrawal of inhibition. It occurs due to blocking of active sites or binding to linkages required for maintenance of active sites. It is characterised by the rapid dissociation of enzyme-inhibitor complexes.

Types of reversible inhibition:
(a) Competitive inhibition: In this type of inhibition, the inhibitor competes with the substrate and binds at the active sites of enzymes but does not undergo any catalysis.
(b) Non- Competitive Inhibition: In this type of inhibition, the inhibitor has no structural resemblance with the substrate. It does not interfere with the enzyme-substrate binding, but it prevents catalysis.
(c) Allosteric Inhibition: In this type of inhibition, the inhibitor is non-competitive and is usually a low molecular weight intermediate or product of a metabolic pathway. It is also called end product or feedback inhibition.

Fig: Graph showing type of inhibition

2. Irreversible or permanent Inhibition: This type of inhibition occurs due to change or destruction in the conformation of enzymes. It occurs due to the covalent binding of heavy metals or other inhibitors with the enzymes, resulting in their inactivation.

Summary

Enzymes are catalysts that are capable of catalysing reactions of biological origin. They are protein in nature and are highly specific. Enzymes have an active site into which the substrate gets fit to proceed with the chemical reaction. Enzymes combine with the substrate molecule to form an enzyme-substrate complex. Enzymes act by lowering the activation energy of the reaction and then combine with the substrate molecule to form an enzyme-substrate complex. This, in turn, results in the formation of products. Various factors affect the activity of enzymes like temperature, pH, the concentration of substrates, etc.

Frequently Asked Questions(FAQs) on Factors Affecting Enzyme Activity

Q.1. Define enzymes.
Ans: Enzymes are biocatalysts that are capable of catalysing reactions of biological origin.

Q.2. What are the characteristic features of enzymes?
Ans: Enzymes are proteins in nature and are highly specific. They exhibit enormous catalytic power.

Q.3. What are the factors affecting the action of enzymes?
Ans: Various factors affect the activity of enzymes like temperature, pH, the concentration of substrates, the concentration of enzymes, and activators and poisons.

Q.4. Define activation energy.
Ans: Most of the chemical reactions have an energy barrier that separates the reactants and products. Therefore, an external supply of energy is needed to start a chemical reaction. This energy is called activation energy.

Q.5. What is allosteric inhibition?
Ans: It is a type of inhibition in which the inhibitor is non-competitive and is usually a low molecular weight intermediate or product of a metabolic pathway. The inhibitor has no structural resemblance with the substrate, so it does not interfere with the enzyme-substrate binding but prevents catalysis. It is also called end product or feedback inhibition.

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