We all heard about enzymes, but it is unlikely that each of us thoroughly knows exactly how these substances are arranged and why they are needed. This article will help to understand the structure and functions of enzymes (enzymes) in general and their active centers in particular.
Research history
In 1833, the French chemist Anselm Payen discovered and described the properties of the amylase enzyme.
A few years later, Louis Pasteur, studying the conversion of sugar to alcohol with the participation of yeast, suggested that this process occurs due to the chemicals that make up the yeast.
At the end of the 19th century, the physiologist Willy Kune introduced the term enzyme for the first time.
The German Eduard Buchner in 1897 isolated and described zymase - an enzymatic complex that catalyzes the reaction of the conversion of sucrose into ethyl alcohol. In nature, zymase is found in large quantities in yeast.
It is not known exactly when and who discovered the active center of the enzyme. This discovery is attributed to the Nobel Prize winner chemist Edaurd Buchner, American biologist James Sumner and other famous scientists who worked on the study of enzymatic catalysis.
Enzyme Overview
Recall that enzymes are substances of a protein nature that perform the functions of catalysts for chemical reactions in living organisms. In the enzyme there are sites that do not directly take part in this; the course of the reaction provides the active center of the enzyme.
Here are some properties of enzymes:
1) Efficiency. A small amount of catalyst is enough to accelerate the chemical reaction by 10 6 times.
2) Specificity. One enzyme is not a universal catalyst for any reaction in the cell. The specificity of the action is expressed for enzymes: each enzyme catalyzes only one or several reactions with similar substrates (initial reagents), but for reagents of a different chemical nature the same enzyme can be useless. Interaction with suitable substrates and further acceleration of the reaction provides the active center of the enzyme.
3) Variable activity. The activity of enzymes in the cell is constantly changing from low to high.
4) The concentration of some enzymes in the cell is not constant and may vary depending on external conditions. Such enzymes are called inducible in biology.
Enzyme classification
In their structure, enzymes are usually divided into simple and complex. The simple ones consist exclusively of amino acid residues, the complex ones have a non-protein group in the composition of the substance. Complex are called coenzymes.
According to the type of catalyzed reactions, enzymes are divided into:
1) Oxidoreductases (catalyze redox reactions).
2) Transferases (carry separate groups of atoms).
3) Liases (cleave chemical bonds).
4) Lipases (form bonds in reactions due to ATP energy).
5) Isomerase (participate in the reactions of the mutual transformation of isomers).
6) Hydrolases (catalyze chemical reactions with hydrolytic cleavage of bonds).
Enzyme structure
An enzyme is a complex three-dimensional structure, which mainly consists of amino acid residues. There is also a prosthetic group - a component of non-protein nature associated with amino acid residues.
Enzymes are mainly globular proteins that can combine into complex complexes. Like other substances of protein nature, enzymes denature with increasing temperature or under the influence of certain chemical reagents. During denaturation, the tertiary structure of the enzyme and, accordingly, the properties of the active center of the enzymes change. As a result, enzyme activity decreases dramatically.
The catalyzed substrate is usually much smaller than the enzyme itself. The simplest enzyme consists of sixty amino acid residues, and its active center is only two.
There are enzymes whose catalytic site is not represented by amino acids, but by a prosthetic group of organic or (more often) inorganic origin - a cofactor.
The concept of an active center
Only a small portion of the enzyme is directly involved in chemical reactions. This part of the enzyme is called the active center. The active center of an enzyme is a lipid, several amino acid residues, or a prosthetic group that binds to a substrate and catalyzes a reaction. Amino acid residues of the active center can belong to any amino acids - polar, non-polar, charged, aromatic, uncharged.
The active center of the enzyme (a lipid, amino acids or other substances that can interact with reagents) is the most important part of the enzyme, without it these substances would be useless.
Typically, an enzyme molecule has only one active site that binds to one or more similar reagents. Amino acid residues of the active center form hydrogen, hydrophobic, or covalent bonds, forming an enzyme-substrate complex.
Active Center Structure
The active center of simple and complex enzymes is a pocket or gap. This structure of the active center of the enzyme must electrostatically and geometrically correspond to the substrate, since a change in the tertiary structure of the enzyme can change the active center.
The binding and catalytic centers are sites of the active center of the enzyme. Obviously, the binding center "checks" the substrate for compatibility and binds to it, and the catalytic center is directly involved in the reaction.
The binding of the active center to the substrate
In order to explain how the active center of the enzyme binds to a particular reagent, several theories have been proposed. The most popular of them is Fisher’s theory, it’s the theory of “lock and key”. Fisher suggested that there is an enzyme that is ideally suited to each substrate in its physicochemical properties. After the formation of the enzyme-substrate complex, no modifications occur.
Another American scientist, Daniel Koshland, supplemented Fisher's theory with the assumption that the active center of the enzyme can change its conformation until it fits a specific substrate.
Kinetics of enzymatic reactions
Features of the course of enzymatic reactions are studied by a separate branch of biochemistry - enzymatic kinetics. This science studies the characteristics of reactions at various concentrations of enzymes and substrates, the dependence of the reaction rate on the temperature inside the cell, as well as the properties of the active center of enzymes depending on changes in the physical and chemical parameters of the medium.
Enzymatic kinetics operates with such concepts as reaction rate, activation energy, activation barrier, molecular activity, specific activity, etc. Let us consider some of these concepts.
For a biological reaction to occur, some energy must be transferred to the reagents. This energy is called activation energy.
Adding the enzyme to the reagents reduces the activation energy. Some substances do not react without enzymes, since the activation energy is too high. The balance of the reaction when the enzyme is added does not shift.
Reaction rate - the amount of reaction product that appears or disappears per unit of time.
The dependence of the reaction rate on the concentration of the substrate is characterized by a dimensionless physical quantity — the Michaelis constant.
Molecular activity is the number of substrate molecules converted by one enzyme molecule per unit time.