ENZYME: HOW IT WORKS AND EXAMPLES - BIOLOGY - 2025

An enzyme, biological catalyst or biocatalyst is a molecule, generally of protein origin, that has the ability to accelerate the chemical reactions that occur inside living beings. The catalytic protein molecules are enzymes, and those of an RNA nature are ribozymes.

In the absence of enzymes, the enormous number of reactions that take place in the cell and that allow life could not occur. These are responsible for speeding up the process by orders of magnitude close to 10 ⁶ - and in some cases much higher.

Schematic diagram of a key-lock junction of an enzyme-substrate complex. Source: Competitive_inhibition_es.svg: * Competitive_inhibition.svg: Authored by Jerry Crimson Mann, modified by TimVickers, vectorized by Fvasconcellosderivative work: Retama (talk) derivative work: Bekerr

Catalysis

A catalyst is a molecule capable of altering the rate of a chemical reaction without being consumed in said reaction.

Chemical reactions involve energy: the initial molecules involved in the reaction or reactants start with one degree of energy. An additional amount of energy is absorbed to reach the "transition state". Subsequently, the energy is released with the products.

The energy difference between reactants and products is expressed as ∆G. If the energy levels of the products is higher than the reactants, the reaction is endergonic and is not spontaneous. In contrast, if the energy of the products is lower, the reaction is exergonic and spontaneous.

However, just because a reaction is spontaneous does not mean that it will occur at an appreciable rate. The speed of the reaction depends on the ∆G * (the asterisk refers to the activation energy).

The reader must keep these concepts in mind in order to understand how enzymes work.

Enzymes

What is an enzyme?

Enzymes are biological molecules of incredible complexity, made up mainly of proteins. Proteins, in turn, are long chains of amino acids.

One of the most prominent characteristics of enzymes is their specificity on the target molecule - this molecule is called a substrate.

Characteristics of enzymes

Enzymes exist in various forms. Some are composed entirely of proteins, while others have regions of a non-protein nature called cofactors (metals, ions, organic molecules, etc.).

Thus, an apoenzyme is an enzyme without its cofactor, and the combination of the apoenzyme and its cofactor is called a holoenzyme.

They are molecules of a considerably large size. However, only a small site on the enzyme is directly involved in the reaction with the substrate, and this region is the active site.

When the reaction begins, the enzyme engages its substrate like a key engages its lock (this model is a simplification of the actual biological process, but it serves to illustrate the process).

All the chemical reactions that occur in our body are catalyzed by enzymes. In fact, if these molecules did not exist, we would have to wait hundreds or thousands of years for the reactions to complete. Therefore, the regulation of enzyme activity must be controlled in a very specific way.

Nomenclature and classification of enzymes

When we see a molecule whose name ends in –ase, we can be sure that it is an enzyme (although there are exceptions to this rule, such as trypsin). This is the naming convention for enzymes.

There are six basic types of enzymes: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases; in charge of: redox reactions, atom transfer, hydrolysis, addition of double bonds, isomerization and union of molecules, respectively.

How do enzymes work?

In the catalysis section we mentioned that the speed of the reaction depends on the value of ∆G *. The higher this value, the slower the reaction. The enzyme is responsible for decreasing this parameter - thus increasing the speed of the reaction.

The difference between the products and the reactants remains identical (the enzyme does not affect it), as does their distribution. The enzyme facilitates the formation of the transition state.

Enzyme inhibitors

In the context of the study of enzymes, inhibitors are substances that manage to decrease the activity of the catalyst. They are classified into two types: competitive and non-competitive inhibitors. Those of the first type compete with the substrate and the others do not.

Generally the inhibition process is reversible, although some inhibitors can remain attached to the enzyme almost permanently.

Examples

There are vast amounts of enzymes in our cells - and in the cells of all living things. However, the best known are those that participate in metabolic pathways such as glycolysis, the Krebs cycle, and the electron transport chain, among others.

Succinate dehydrogenase is an enzyme of the oxidoreductase type that catalyzes the oxidation of succinate. In this case, the reaction involves the loss of two hydrogen atoms.

Difference between biological catalysts (enzymes) and chemical catalysts

There are chemical catalysts that, like biological ones, increase the speed of reactions. However, there are notable differences between the two types of molecules.

Enzyme-catalyzed reactions happen faster

First, enzymes are able to increase the speed of reactions by orders of magnitude close to 10 ⁶ to 10 ¹². Chemical catalysts also increase speed, but only by a few orders of magnitude.

Most enzymes work under physiological conditions

As biological reactions are carried out inside living beings, their optimal conditions surround the physiological values ​​of temperature and pH. Chemists, for their part, need drastic conditions of temperature, pressure and acidity.

Specificity

Enzymes are very specific in the reactions they catalyze. In most cases, they only work with one or a few substrates. Specificity also applies to the type of products they produce. The range of substrates for chemical catalysts is much broader.

The forces that determine the specificity of the interaction between the enzyme and its substrate are the same that dictate the conformation of the protein itself (Van der Waals interactions, electrostatic, hydrogen bonding, and hydrophobic).

Enzyme regulation is precise

Finally, enzymes have a greater regulatory capacity and their activity varies according to the concentration of different substances in the cell.

Regulatory mechanisms include allosteric control, covalent modification of enzymes, and variation in the amount of enzyme that is synthesized.

References

1. Berg, JM, Stryer, L., & Tymoczko, JL (2007). Biochemistry. I reversed.
2. Campbell, MK, & Farrell, SO (2011). Biochemistry. Sixth edition. Thomson. Brooks / Cole.
3. Devlin, TM (2011). Textbook of biochemistry. John Wiley & Sons.
4. Koolman, J., & Röhm, KH (2005). Biochemistry: text and atlas. Panamerican Medical Ed.
5. Mougios, V. (2006). Exercise biochemistry. Human Kinetics.
6. Müller-Esterl, W. (2008). Biochemistry. Fundamentals for medicine and life sciences. I reversed.
7. Poortmans, JR (2004). Principles of exercise biochemistry. Karger.
8. Voet, D., & Voet, JG (2006). Biochemistry. Panamerican Medical Ed.