BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 7. ENZYMOLOGY

7.6. Inhibition of Enzymatic Activity

7.6.2. Irreversible Inhibition

Irreversible inhibition occurs when stable covalent bonds are formed between the inhibitor molecule and the enzyme. Most commonly, the Active Site of the enzyme undergoes modification, rendering it unable to perform its catalytic function.

Irreversible inhibitors include heavy Metal Ions such as mercury (Hg²+), silver (Ag²⁺), and arsenic (As³+), which at low concentrations block The sulfhydryl groups of the active site. As a result, the substrate cannot undergo chemical transformation (Fig. 7.26). In the presence of reactivators, the Enzymatic Function is restored. At high concentrations, heavy metal ions cause Denaturation of the enzyme's protein molecule, leading to its complete inactivation.

Specific and non-specific inhibitors. The Use of irreversible inhibitors is of great interest for elucidating the MECHANISMS OF ENZYME action. For this purpose, substances that block specific groups within the active site of an enzyme are employed; such inhibitors are called specific. A number of compounds readily react with specific chemical groups. If these groups participate in catalysis, Enzyme inactivation ensues.

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Fig. 7.26. MECHANISM OF ACTION of mercury ions as an irreversible inhibitor:

at low concentrations, mercury ions block the sulfhydryl groups of the active site, leading to a decrease in The rate of the enzymatic reaction

The Role of Serine hydroxyl groups in the catalytic mechanism is investigated using fluorophosphates, such as diisopropyl fluorophosphate (DFP), which specifically reacts with only one of the many serine residues in the active site of the enzyme. The Ser residue capable of reacting with DFP has an identical or very similar amino acid environment (Table 7.2). Its high reactivity compared to other Ser residues is due to surrounding amino acid residues that are also part of the enzyme's active site.

Table 7.2

Investigation of The amino acid residue sequence surrounding the reactive serine residue that interacts with DFP

DFP belongs to the specific irreversible inhibitors of "serine" Proteolytic Enzymes because it forms a covalent bond with the hydroxyl group of serine located in the active site, which plays a key role in the catalysis process (Fig. 7.27).

Fig. 7.27. Inhibition of chymotrypsin activity by diisopropyl fluorophosphate

Monoiodoacetic acid and p-chloromercuribenzoate readily react with the SH groups of protein Cysteine residues (Fig. 7.28). These inhibitors are classified as non-specific because they react with any free SH groups in Proteins. If the SH groups directly participate in catalysis, these inhibitors can be used to determine the role of the enzyme's SH groups in the catalytic process.

Fig. 7.28. Inhibition of enzyme activity due to Covalent Modification of cysteine residues

An example of a drug whose action is based on irreversible Enzyme Inhibition is the widely used medication aspirin. The non-steroidal anti-inflammatory drug aspirin exerts its pharmacological effect by inhibiting cyclooxygenase, the enzyme that catalyzes The formation of Prostaglandins from arachidonic acid. As a result of this chemical reaction, the acetyl group of aspirin attaches to the free terminal OH group of the cyclooxygenase serine:

This leads to a reduction in The production of prostaglandin reaction products, which have a wide spectrum of biological Functions, including acting as mediators of inflammation.