Classifications of Enzymes – Characteristics and Structure of Enzymes

classification of enzyme

Every living body consists of numerous essential participants that play a pivotal role in bringing it up. Such a participant is an enzyme. Now, what are Enzymes? This group of biomolecules synthesizes in the living cells and act as catalysts throughout their entire life. All enzymes tend to accelerate a chemical reaction. We further classify them into six primary groups that will be discussed later on in this article. Before that, let’s talk about the basics of enzymes.

enzyme structure

What Does An Enzyme Do?

The living bodies perform almost all of their functions with the aid of enzymes. The renowned of all duties are:

  • It helps in increasing the rate of reaction
  • Moreover, enzymes do not participate in the chemical reaction but only accelerates them

Where Are Enzymes Made?

Scientists called enzymes biocatalysts as nature manufactures them in living cells. Every existing cell has ribosomes in them, either scattered or fixed to the endoplasmic reticulum. The primary function of these organelles is to prepare proteins, which are then called enzymes. It means all enzymes are proteins, but not all proteins are enzymes. Soon after they mature, these enzymes undergo post-translation modification, proceeding to another organelle called Golgi bodies.

Characteristics of Enzymes

Protein in Nature

Enzymes are the natural proteins found everywhere in a body. Furthermore, these molecules vary with their entire protein weight so, specific for certain reactions.

An Enzyme Acts as a Catalyst

An Enzyme Acts as a Catalyst

Enzymes generally act on a substance to convert it to a product. And almost all the living organisms’ phenomenon occurs with enzymes being a catalyst and converting the substrates to the relevant products.

An Enzyme Accelerates the Reaction

Moreover, these enzymes speed up the reaction in the biological processes by reducing activation energy. It is the minimum energy required to kick start a chemical reaction. This activation energy can also help a single process run multiple times and regulate it.

Lock And Key Model

Emil Fischer, in 1894, proposed a “lock and key model.” According to this model, an enzyme is a rigid structure, only precise to substrates matching its active site. It is similar to lock and key, as a particular key can only open a specific lock. In a word, enzymes are specific in nature.

Specific in Functioning

Most people widely use the term “One Enzyme One Substrate.” It means an enzyme works specifically for a substrate and is not available for many. So, at a time, a single enzyme catalyzes a single reaction.

Reverse Function

Enzymes work only to accelerate a reaction but never determines its directions or pathways. An enzyme will continue performing its function until the reaction reaches an equilibrium state. Thus, these molecules are highly explicit.

Minute Amount of Enzymes Required

Its fundamental concern is to speed up a reaction. Therefore a chemical reaction requires a minute amount of enzymes only.

How Do You Name Enzymes?

The general method of naming an enzyme first comprehends the substrate it is about to act on. Then, after observing the name of the substrate, a suffix “ase” adds at its end. For example, a catalyst oxidoreductase speeds up the oxidation-reduction reaction. It is one of the primary catalysts, falling in enzyme classification. Similarly, an enzyme lipase is responsible for hydrolyzing lipid molecules.

What is a Co-Enzyme?

Enzymes can be simple or conjugated proteins. Sometimes an enzyme needs an external aid to work efficiently, for which it uses a co-enzyme or a co-substrate. This structure makes a bond with the enzyme molecule and leaves it at the end of the chemical reaction soon after the process completes. A coenzyme is named so because it makes an early bond with the enzyme than the substrate itself.

It is usually an organic, non-protein molecule that binds to the protein part of an enzyme. Furthermore, these activated vitamins make complexes for running metabolic pathways and transform nutrients into vital energy. In conclusion, a co-enzyme generates the biomolecules along with enzymes that are sufficient to process bioreactions.

Differentiate Between an Apoenzyme and a Holoenzyme?

Apoenzymes are the intact protein part of an enzyme, lying in their inactive form. On the contrary, a holoenzyme is a complex constituted from an apoenzyme and a co-factor. An apoenzyme triggers only when attached to a co-factor (non-protein factor). In contrast, a holoenzyme always stays in its active form to accelerate a biochemical reaction. Examples of apoenzymes include carbonic anhydrase, pepsin, etc. Similarly, some of the holoenzymes are DNA & RNA polymerases.

What is a Zymogen?


We can also pronounce zymogens as proenzymes. This group of enzymes usually do not remain in their active form but can be converted if needed. A zymogen activates when another enzyme from within the organism triggers it up. Zymogens function to stop excessive protein degradation.

Classification of Enzymes

types of enzyme

International Union of Biochemists classified enzymes into six primary types, depending upon where they act to catalyze a chemical reaction. Let’s briefly discuss them one by one:


The enzyme which is involved in the oxidation and reduction reaction of its substrate is called oxidoreductase. For instance, xanthine oxidase, lactate dehydrogenase gG-6-P dehydrogenase, etc.


Transferase molecules activate when a particular functional group transports from a donor to an acceptor molecule. For example, phosphoglucomutase, hexokinase, etc.


We call them hydrolases as these associate with breaking a bond by adding water (through hydrolysis). For example, pepsin glucose-6- phosphatase and glycoside hydrolases.


These are the group of enzymes that catalyzes a chemical reaction by forming double bonds. Lyases never tend to utilize oxidation or hydrolysis to accelerate a biochemical process. For instance, histidine decarboxylase, fumarase, etc.


This Enzyme is responsible for isomerizing a substrate, thus altering its structure. Some examples include UDP- glucose, epimerase, retinal isomerase, etc.


Another class of enzymes is Ligases. These involve the joining of two substrates with each other, such as glutamine synthase, RNA synthase. Besides, DNA ligases are also a prominent example, connecting DNA’s two fragments.

Proposed Models of Enzymes

Last but not least, there were two fundamental models of enzymes, so let’s briefly discuss them side by side:

Lock and Key Model

Emil Fischer first proposed this model in the late 1890s. He presented that the active site of an enzyme stands in proper confirmation. In other words, the active site of a catalyst provides a fixed and rigid shape where a substrate sets. The German chemist called it the lock and key model as only a definite key can open a specific lock. This model confirms that enzymes are distinct which can only act on a single substrate.

Induced Fit Model


Koshland, in 1958, presented Induced Fit Theory. This model completely contradicts the lock and key model, declaring that an enzyme is not a rigid structure and can be modified according to situations. In a particular chemical reaction involving enzymes, its active site can endure conformational alterations for augmented binding.

Nowadays, biologists prefer this model over the lock and key model due to its following advantages:

  • The Induced Fit model offers a broad-spectrum enzyme specificity, e.g., a lipase enzyme can accelerate various lipid molecule reactions.
  • The conformational variations can increase enzyme activity.

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