Enzymes Definition and Function

Explain Enzymes Definition and Functions

Enzymes are protein molecules that function as biocatalysts for almost all cellular reactions. Enzyme activity regulates various metabolic pathways in response to different cellular needs. The branch of science that deals with enzymes is called enzymology.

Properties of Enzymes

All enzymes, except the ribozymes, are proteins. But not all proteins are enzymes.
They participate in cellular metabolic processes
They enhance the rate of reaction between biomolecules.
Some enzymes can reverse a reaction from the direction it would normally take. This is done by reducing the activation energy (Ea) to the extent that the reaction favours the reverse direction.
Similarly, enzymes can catalyze reactions that might not otherwise occur by lowering the Ea to a more “affordable” level for the cell.
It is their folded conformation that creates an area called the active site.
Depending on the nature and arrangement of amino acids in the active site, the enzymes are specific for particular substrates.
In the presence of different substrates, only those enzymes that have a specific shape complementary to the active site can bind with the active site.
The protein conformation and its integrity are crucial for the enzyme activity. Losing its conformation could lead to the loss of its activity.
Enzymes are thermostable as dried enzymes can sustain higher temperatures, even at 100-120 °C.
However, most enzymes are inactivated or destroyed at 60 °C.
Every enzyme has its specific temperature for its optimal activity.

Composition of Enzymes

Enzymes have molecular weights between 10,000 and over 1 million.
Some enzymes are not proteins, but have some catalytic RNA molecules.
Most often, enzymes are protein complexes made up of individual protein subunits.
These proteins may be of a single type repeatedly bound together or made of different proteins.
They may be composed of simple proteins or contain a non-protein group as well.
Enzymes that contain both protein and non-protein groups are called conjugated proteins. The non-protein part here is called a cofactor.
Cofactor binds to the protein through a covalent bond.
Depending on the type of the non-protein group, enzymes can be of two types.
- Metalloenzyme has an inorganic cofactor
- A coenzyme has an organic cofactor
- The tightly bound cofactor is called a prosthetic group.
At times, an enzyme requires a metal group and an organic group for its activity. Eg., Cytochromes have a prosthetic group, a heme, and a metal ion, such as Fe³⁺, bound to the protein molecule.
An active or functional conjugated protein is called a holoenzyme, and its protein part is called an apoenzyme.
While a cofactor can work with many enzymes, an apoenzyme is specific to an enzyme system.
Sometimes, enzymes have some loosely bound coenzymes that must be regenerated through independent reactions. Such coenzymes are called co-substrates, such as NAD⁺, NADP⁺, etc.

Enzyme Naming

Enzymes are named by adding a suffix-ase to the name of their substrate, the reaction it catalyses, or combining both. Eg, Lipases, maltases, etc aee based on the substrate. Hydrolases, isomerase, etc are based on the reactions. Succinic acid dehydrogenase shows the substrate and the reaction ti catalyses.

The IUB system identifies each enzyme by an EC numerical code with four digits, separated by decimal points.
The second and third digits of the EC code number represent the subclass and sub-subclass of the enzyme.
The final number is the serial number specific to each enzyme in a sub-subclass.

Enzyme Classification

Enzymes are classified according to the reactions they catalyze, classified under six classes:

Oxidoreductases: Catalyze the oxidation-reduction reaction between two substrates.
Transferases: Catalyze the transfer of a non-hydrogen group from one substrate to another.
Hydrolases: Catalyze the hydrolysis of various bonds.
Lyases: Catalyze the removal of groups from substrates without hydrolysis. The product contains double bonds.
Isomerases: Catalyze the interconversion of geometric, optical, or positional isomers.
Ligases: Catalyze the joining of two substrate molecules, coupled with the breaking of the pyrophosphate bond in adenosine triphosphate (ATP) or a similar compound.

Functions of Enzymes

Enzymes lower the activation energy.
They do not change the equilibrium of the reactions.
The enzyme binds to the substrate, forming an unstable complex, which breaks up into products.
At the end of the reaction, enzymes detach from the substrate.

Mode of Action of Enzymes

The lock and key theory and induced fit theory explain the enzyme-substrate complex formation.
Enzymes are highly specific in their action. Three types of enzyme specificity are well recognized: They are
- Stereo specificity
- Reaction specificity
- Substrate specificity
There are different mechanisms to explain enzyme catalysis – acid-base catalysis, covalent catalysis, metal ion catalysis, and transition state stabilization.

Role of Inhibitors on Enzymes

Compounds that bind to enzymes that will later inhibit their activity are called inhibitors. Inhibition could cause temporary or permanent changes or damage to the enzymes. The temporary changes are reversible, and such inhibitions that are reversible are called reversible inhibition, and the inhibitors are reversible inhibitors.

Reversible Inhibition

Reversible inhibition can be of two types.

Competitive inhibition: The presence of competitive inhibitors induces this type of inhibition. This is reversible as the concentration of substrate is increased. The increase in substrate concentration increases the possibility of binding of enzymes to active sites.
- Eg, Malonate inhibits succinic acid dehydrogenase that catalyses succinate into fumarate.
- Rubisco is inhibited by CO₂ and O₂ in plants.
Uncompetitive inhibition happens when the inhibitor binds to the enzyme-substrate complex, rather than free enzymes. This is increased with increased substrate concentration.

Irreversible Inhibition

Irreversible inhibition is the phenomenon where the inhibitors and enzyme protein are bound by a covalent bond. The inhibitor does not dissociate to produce the free enzyme. This inactivates the enzymes. Sometimes, even in the absence of covalent bonds, the binding of the inhibitor to the active site is strong enough to have irreversible inhibition. Eg, DIFP inhibits acetylcholine esterase.
Another type of irreversible inhibitor is the suicide inactivation. These inhibitors that bind to the enzymes allow a few reactions to happen normally. However, the conversion to the final product is inhibited. Instead, some active reactive molecules are formed. They combine with the enzymes to inhibit their activity.

References

S. C Bhatla, M. A. Lal, Plant Physiology, Development and Metabolism, https://doi.org/10.1007/978-981-13-2023-1_1
Dr. M. GhouseBasha, MSc., Botany, Major Paper- VII – Plant Physiology, Biochemistry and Biophysics. Centre for Distance Education Bharathidasan University
https://infinitabiotech.com/blog/properties-of-enzymes/

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