Enzymes

Enzymes are the biological catalysts, required by living cells in a reaction. In this article, we will discuss enzymes in detail. Let us know more about its action, properties, classification, etc.

Enzymes

Enzymes are biocatalysts, that is, the catalysts of life. A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any change in the overall process. In this article, we will learn about the biological catalysts, i.e. enzymes in detail. We will discuss the classification of enzymes, their mechanism, properties, and more.

We can understand the function of catalysts through a very popular example of a catalyst that we all know- the student-teacher relationship. Students often find it difficult to learn from a textbook on their own. The teacher demonstrates the subject to the students and increases their understanding capability. With the help of a teacher, students absorb the difficult things quickly, which they might take days to understand. Thus, the teacher acts as a catalyst in improving the understanding ability of students. A good teacher is always a good catalyst in students’ life.

Author – Kshitij Dwivedi

What are enzymes?

Enzymes are biocatalysts synthesized by living cells. They are protein in nature (exception – RNA acting as ribozyme), colloidal and thermolabile in character, and specific in their actions. Thus, we define enzyme as:

“Enzymes are the biocatalysts produced by living cells that bring about a specific biochemical reaction.”

Classification of enzymes

Since 1964, the IUB system of classification of enzymes has been in force. Enzymes have six major classes.

Classification of enzymes is important in order to show the properties of substrates or reactants more accurately.

Furthermore, each class represents the general type of reaction brought about by the enzymes of that class. So, the different types of enzymes are:

  1. Oxidoreductases : Enzymes involved in oxidation-reduction reactions.
  2. Transferases :Enzymes that catalyze the transfer of functional groups.
  3. Hydrolases : Enzymes that bring about hydrolysis of various compounds.
  4. Lyases : Enzymes specialized in the addition or removal of water, ammonia, CO2 etc.
  5. Isomerases:Enzymesinvolved isomerization reactions.
  6. Ligases : Enzymes catalyzing the synthetic reactions (Greek : ligate-to bind) where two or more molecules join together, with the expense of ATP.

Enzyme example

Now that we know the different classification of enzymes, let us see the various examples of enzymes. They are:

  • Lipases: These are a group of enzymes that help in digestion of fats.
  • Amylase: These help in conversion of starches into sugars. It is present in saliva.
  • Trypsin: It is present in the small intestine. It breaks protein into amino acids.
  • Acetylcholinesterase: It breaks down the neurotransmitter acetylcholine in nerves and muscles.
  • Helicase – It is an important enzyme present during DNA replication, which unravels DNA.
  • DNA polymerase – It is responsible for synthesizing DNA from deoxyribonucleotides.

Active sites

Enzymes are big in size as compared to substrates, which are relatively smaller. Evidently, a small portion of the huge enzyme molecule is directly involved in substrate binding and catalysis. Thus,

“The active site of an enzyme represents the small region at which the substrate(s) binds and participates in the catalysis.”

Mechanism of Enzyme Action

Lock and key model or Fischer’s template theory-

Emil Fischer, a German biochemist, was the first to propose this theory. This is in fact the very first model that explains an enzyme-catalyzed reaction.

According to this model, the structure or conformation of the enzyme is rigid. The substrate fits in the active site, just as a key fits into the proper lock or a hand into the proper glove. Thus, the active site of an enzyme is a rigid and pre-shaped template where only a specific substrate can bind.

Factors affecting Enzyme Activity

  • Concentration of enzyme: As the concentration of the enzyme is increases, the velocity of the reaction proportionately increase.
  • Concentration of substrate: Increasein the substrate concentration gradually increases the velocity of enzyme reaction within the limited range of substrate levels. A rectangular hyperbola is obtained when velocity is plotted against the substrate concentration.
  • Effect of temperature: Velocity of an enzyme reaction increases with increase in temperature up to a maximum and then declines. A bell-shaped curve is usually observed.
  • Effect of pH: Increase in the hydrogen ion concentration(pH) considerably influences the enzyme activity and a bell-shaped curve is normally obtained. Each enzyme has an optimum pH at which the velocity is maximum. Below and above this pH, the enzyme activity is much lower and at extreme pH, the enzyme becomes totally inactive.

Km or the Michaelis-Menten constant

Km or the Michaelis-Menten constant is the substrate concentration (in moles/l) to produce half-maximum velocity in an enzyme-catalyzed reaction.

Furthermore, it indicates that half of the enzyme molecules (i.e.50%) are bound with the substrate molecules when the substrate concentration equals the Km value.

Km value is a constant and a characteristic feature of a given enzyme. It is a representative for measuring the strength of the Enzyme-substrate complex. A low Km value indicates a strong affinity between enzyme and substrate, whereas a high Km value reflects a weak affinity between them.

Enzyme Inhibition

“An enzyme inhibitor is a substance that binds with the enzyme and brings about a decrease in the catalytic activity of that enzyme.”

Moreover, the inhibitor may be organic or inorganic in nature. It is of two types-

1. Competitive Inhibition

During competitive inhibition, the inhibitor closely resembles the real substrate and is regarded as a substrate analog. The inhibitor competes with the substrate and binds at the active site of the enzyme but does not undergo any catalysis.

In addition, as long as the competitive inhibitor holds the active site, the enzyme is not available for the substrate to bind, thus rendering the enzyme inactive. A high substrate concentration overcomes this inhibition.

Furthermore, in competitive inhibition, the Km value increases whereas Vmax remains unchanged.

2. Non-competitive Inhibition

Here, the inhibitor binds at a site other than the active site on the enzyme surface. This binding impairs the enzyme function. The inhibitor has no structural resemblance with the substrate. However, there usually exists a strong affinity for the inhibitor to bind at the second site.

Furthermore, the inhibitor does not interfere with the enzyme-substrate binding. However, catalysis does not occur possibly due to a distortion in the enzyme conformation.

Also, for non-competitive inhibition, the Km value does not change while Vmax gets low.

Coenzymes

The protein part of the enzyme is not always enough to bring about the catalytic activity. Many enzymes require certain non-protein small additional factors, collectively referred to as cofactors for catalysis. These cofactors may be organic or inorganic. Thus, we define coenzyme as:

“Coenzymes are organic, non-proteinaceous, dialyzable substances with low molecular weight, that help in enzyme function.”

The functional enzyme is referred to as a holoenzyme which is made up of a protein part called the apoenzyme and a non-protein part called a coenzyme.

When the non-protein molecules bind tightly to the enzyme, in such a way that it is not easily separable by dialysis, it forms a prosthetic group.

Further, the term activator refers to the inorganic cofactor (like Ca2+, Mg2+, Mn2+, etc.) that are necessary to enhance enzyme activity.

However, some authors make no distinction between the terms cofactor, coenzyme, and prosthetic group and use them interchangeably.

Properties of Enzymes

Let us now see different enzyme properties.

  1. Any reaction in the presences of enzymes, usually take place in mild conditions i.e. temperatures below 100oC, at atmospheric pressure and neutral pH.
  2. Enzymes are highly specific in nature and action.
  3. Enzymes can not start a reaction. However, they help in accelerating a reaction.
  4. They affect the rate of a biochemical reaction and not the direction of the reaction.
  5. The rate of enzyme activity increases with an increase in substrate concentration.

Important points for NEET-UG Exam:

  • Enzymes are specific and one enzyme catalyzes only one type of reaction.
  • High temperature leads to denaturation of enzymes i.e. the enzyme loses it’s functional ability.
  • Low temperature causes temporary inactivation of the enzyme.
  • All enzymes are proteins but all proteins are not enzymes. ( Exception:  Ribozyme– it is not protein).
  • Coenzymes do not determine enzyme specificity.
  • Enzymes lowers the activation energy of the reaction.

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