Factors Affecting Enzyme Activity

In the human body, enzymes are found in all tissues and fluids. Intracellular enzymes carry out catalysis of all reactions taking place in metabolic pathways. Most of the critical life processes are established on the functions of enzymes. For example, the enzymes in the plasma membrane govern the catalysis in cells and enzymes in the circulatory system regulate blood clotting.

They are large proteins that can also be found in RNA form (catalytic RNAs or ribozymes). They aid in a variety of bodily functions, including growth and development, reproduction, and numerous life processes.

Important factors in enzyme structure

The catalytic process carried out by enzymes occurs at a specific location on the enzyme. This area is known as the active site, and it accounts for only a small portion of the enzyme’s overall size. The active site is made up of polypeptide chain segments, the active site has a three-dimensional structure.

The majority of enzymes are proteins with catalytic properties required to carry out various operations. Because of their importance in supporting life processes, enzyme regulation has long been a significant component in clinical diagnosis. They are used as biological markers (alteration in a measurable medium such as tissue fluid) to diagnose diseases such as cancer and infarction.

An enzyme’s catalytic cycle is explained below:

1. At first, the substrate binds to the enzyme’s active site, fitting into the active site. The enzyme’s shape changes as a result of the substrate’s binding, causing it to fit more tightly around the substrate (induced fit).

2. The enzyme’s active site, which is now close to the substrate, breaks the substrate’s chemical bonds, forming a new enzyme-product complex.

3. Lastly, the reaction products are released by the enzyme, and the free enzyme is ready to attach to another molecule of the substrate, and this catalytic cycle is repeated again.

Combining of the enzyme (E) and the substrate (S) so that a highly reactive enzyme-substrate complex (ES) is produced.

$\mathrm{E}+\mathrm{S}\to \mathrm{ES}$

The disintegration of the complex molecule to give the enzyme-product complex (EP).

$\mathrm{ES}\to \mathrm{EP}$

Thus, the whole catalyst action of enzymes is summarised as:

$\mathrm{E}+\mathrm{S}\to \left[\mathrm{ES}\right]\to \left[\mathrm{EP}\right]\to \mathrm{E}+\mathrm{P}$

Fischer’s Lock and Key model

Smaller keys, larger keys, or teeth on keys that are wrongly positioned (substrate molecules incorrectly formed or sized) do not fit into the lock (enzyme). A key with the correct shape can only open a specific lock.

Induced Fit Theory

For many years, scientists thought that enzyme-substrate binding took place in a simple “lock-and-key” fashion. Daniel Koshland suggested the induced fit model in 1958. It is the more accepted model for enzyme-substrate complex than the lock-and-key model.

This theorises that the initial connection between enzyme and substrate is weak, but these weak interactions cause rapid conformational changes in the enzyme, resulting in a stronger binding. In other words, as the enzyme and substrate contact, a minor alteration in the enzyme’s structure occurs, confirming a perfect binding arrangement between the enzyme and the substrate. This dynamic binding enhances the enzyme’s ability to catalyse its reaction.

Factors affecting enzyme activity

Enzyme concentration

The reaction will no longer speed up once all of the substrates have been bound to the enzyme, as there will be nothing for the new enzymes to bind to.

• An increase in enzyme concentration will increase the reaction rate until it reaches a certain point, after which it will remain constant. The number of active sites available increases as the enzyme concentration rises.

The rate of reaction is proportional to the amount of enzyme present. Therefore, we see a straight line in the graph, where the x-axis is enzyme concentration, and the y-axis is the rate of reaction.

Substrate concentration

An enzyme’s activity increases with the rise in substrate concentration. The enzyme activity rises until it reaches a maximum limit.

In other words, the enzyme molecules are completely saturated with the substrate. This means that all enzymes’ active sites are occupied. The surplus substrate molecules will not react until the substrate that has already been bound to the enzymes has reacted and has been released or released without reacting.

To help you visualise, the rate of reaction increases initially. However, the reaction rate reaches a plateau when all enzymes are occupied. For an enzyme-catalysed reaction, there is usually a hyperbolic relationship between the reaction rate and the substrate concentration.

Effect of pH on the rate of reaction

pH has an impact on enzyme activity. A bell-shaped curve emerges when enzyme activity is plotted versus pH. Each enzyme has a certain optimal pH at which the reaction rate is the fastest. The optimal pH is when a specific enzyme’s activity is at its peak. The enzyme activity is greatly reduced below and above the optimal pH, and at high pH, the enzyme becomes completely inactive.

Acidic carboxylic groups (COOH–) and basic amino groups (${\mathrm{NH}}_2$) are found in enzymes. As a result, changing the pH value affects the enzymes.

For most enzymes, the optimum pH for the enzymatic activity of ranges from 6 to 7.

However, there are a few exceptions.

Effects of temperature on the rate of reaction

An optimum temperature range is required to maximise the enzyme activity. Temperatures that are either higher or lower than the optimal temperature causes a decrease in the enzyme activity and can lead to denaturation (when the temperature is too high).

However, there are some exceptions, for example, in extremophiles. Few enzymes, such as Taq DNA polymerase found in thermophilic bacteria, are active at temperatures as high as 100°C.

Enzyme inhibitors and activators

Activators (also known as co-enzymes), such as metal ions, are required for optimal enzyme activity.

Enzyme inhibitors can be used as drugs in the treatment of various diseases. Some antimicrobial drugs are enzyme inhibitors that deactivate the enzymes that are needed for the survival of pathogens.

Depending on the inhibition kinetics, enzyme inhibition can be classified as reversible or irreversible inhibition.

Irreversible inhibitors bind to an enzyme in a strong covalent connection. These inhibitors might act on the active site, close to it, or far away. As a result, adding more substrate may not be enough to eliminate them. In either event, the enzyme’s basic structure is altered to the point that it no longer functions.

In simple words, the inhibitor binds very securely to the enzyme and does not dissociate from it.

Reversible inhibitors

In reversible inhibition, the inhibitor rapidly dissociates from the enzyme-inhibitor complex. There are three types of reversible inhibitions: competitive, non-competitive and uncompetitive (anti-competitive).

Competitive inhibition

When an inhibitor and a substrate bind to the enzyme in a competitive way, this is known as competitive inhibition. Any molecule with a chemical structure and molecular geometry similar to the substrate could be used as a competitive inhibitor. Contacts between the inhibitor and the enzyme in the active site may be strong. Still, no reaction will occur since the inhibitor does not have the same chemical reactivity as the enzyme, eventually lowering the reaction rate. Competitive inhibition is usually temporary and reversible.

Non-competitive inhibitors

Non-competitive inhibitors bind to the enzyme at a different location, causing the active site to change shape and prevent the substrate from binding to it. During non-competitive inhibition, the enzyme may form complexes with either the substrate, the inhibitor, or both. Allosteric inhibition is non-competitive inhibition because the inhibitor binds to the allosteric site rather than the active site. Binding to an allosteric site causes the enzyme’s 3-dimensional tertiary structure to be distorted, preventing it from catalysing the process.

Because they effectively denature the enzymes, some non-competitive inhibitors are irreversible and permanent.

Uncompetitive inhibition

Uncompetitive inhibition is a rare occurrence. An uncompetitive inhibitor binds to the enzyme and increases the substrate’s binding affinity, but the resulting enzyme-inhibitor-substrate complex reacts slowly to create the product. Note that before binding to the inhibitor, uncompetitive inhibition necessitates the creation of an enzyme-substrate complex.