Specificity is defined as the ability of an enzyme to choose an exact substrate from a group of the same chemical molecules. Actually, specificity is a molecular recognition mechanism that works through complementarity in conformation and structure between the enzyme and the substrate.
Types of Enzyme Specificity
Since the substrate should fit into the active site of the enzyme before catalysis can take place, only properly designed molecules may serve as substrates for a specific enzyme; in several cases, an enzyme will react with only one naturally taking place molecule. Two oxidoreductase enzymes will serve to describe the principle of enzyme specificity.
One (alcohol dehydrogenase) acts on the alcohol, the other (or the lactic dehydrogenase) on lactic acid; the two activities, even though both are oxidoreductase enzymes, they are not interchangeable - it means, alcohol dehydrogenase will not catalyze a reaction involving in the lactic acid or vice versa, because the structure of every substrate varies sufficiently to prevent its fitting into the active site of an alternative enzyme. Enzyme specificity is important because it distinguishes between the various metabolic pathways involving hundreds of enzymes.
Not all enzymes are highly specific. For example, digestive enzymes such as chymotrypsin and pepsin are able to act on almost any protein the specificity of enzyme action, as they should if they are to act upon the differential types of proteins consumed as food. Furthermore, since thrombin only interacts with the protein fibrinogen, it is a part of a very delicate blood-clotting process that can only react with one compound in order to keep the system working properly.
When the enzymes were first studied, it was thought that most were "absolutely specific"—that they would react with only a single compound. However, in most cases, a molecule other than the natural substrate may be synthesized in the laboratory; it is enough such as the natural substrate to react with enzymes. These synthetic substrates' use has been valuable in understanding the enzymatic action. However, it should be remembered that, in the living cell, several enzymes are absolutely specific for the compounds found.
As a result, all enzymes isolated from a long distance are unique for the chemical reaction form they catalyse - oxidoreductases do not catalyse hydrolysis reactions, and hydrolases do not catalyse reactions involving both reduction and oxidation. Therefore, an enzyme catalyzes a particular chemical reaction but can be able to do so on many similar compounds.
Mechanism of Enzymatic Action
An enzyme attracts substrates to its active site, then catalyses the chemical reaction that creates the products before dissociating the products (separate from the surface of the enzyme). The combination formed by an enzyme and its substrates is known as the enzyme–substrate complex. A ternary complex is made up of one enzyme and two substrates, whereas a binary complex is made up of one enzyme and one substrate. These substrates are attracted to the active site by hydrophobic and electrostatic forces that are known as noncovalent bonds because they are physical attractions but not chemical bonds.
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Assume two substrates (S1 and S2) bind to the enzyme's active site in step 1 and then react in step 2 to generate products (P1 and P2). In step 3, the products dissociate from the enzyme surface by releasing the enzyme. The enzyme, which is unchanged by the reaction, is capable of reacting with additional substrate molecules in this way several times per second to form the products. The phase in which the actual chemical transformation occurs is of great interest, and while much is known about it, it is still not fully understood. Generally, there are two types of enzymatic mechanisms, one is the so-called covalent intermediate forms, and the other is none forms.
In the mechanism, where a covalent intermediate—it means an intermediate is having a chemical bond between the enzyme and substrate—forms, for example, one substrate, B―X, reacts with the group N on the enzyme surface to produce an enzyme - B in the intermediate compound. The intermediate compound then reacts with the second substrate, Y, to create the BY and X products.
Several enzymes catalyze reactions by this mechanism type. Acetylcholinesterase can be used as a particular example in the sequence given here. The two substrates (S1 and S2) for the acetylcholinesterase are acetylcholine (it means B―X) and water (Y). After the acetylcholine (B―X) binds to an enzyme surface, a chemical bond is produced between the acetyl moiety (B) of acetylcholine and group N (which is part of amino acid serine) on the surface of the enzyme.
The formation result of this bond, known as an acyl–serine bond, is one product, choline (X), and enzyme-B intermediate compound (which is acetyl–enzyme complex). Then, the water molecule (Y) reacts with the acyl–serine bond to produce the second product, acetic acid (B―Y), that dissociates from the enzyme. Acetylcholinesterase is regenerated and can react with the other acetylcholine molecule once more. A double displacement reaction is a type of reaction that involves the formation of an intermediate compound on an enzyme surface.