Active site of a protein

Structure and the Principle of Complementarity

The active site forms when the polypeptide chain folds, acquiring its unique three-dimensional tertiary structure. In the linear primary protein structure, amino acid side chains can be located a vast distance apart from one another. However, during the folding process, they come into close proximity and form a specific depression or "pocket" on the surface of the functionally active molecule.

The interaction between the active site and a ligand (a molecule that binds to a protein) obeys the strict rule of complementarity—the exact spatial and chemical match of the interacting surfaces.

• Spatial matching means that the active site geometrically fits the incoming ligand perfectly, much like a key in a lock.
• Chemical matching implies that bonds must arise between the functional groups of the protein side chains and the ligand molecule.

Retention of the ligand in the pocket is ensured by weak interactions: ionic and hydrogen bonds, as well as hydrophobic interactions. It is precisely the binding of a specific ligand that allows the protein to execute its biological function.

Arrangement of Binding Sites

In domain proteins, ligand-binding sites are very often localized between domains—in characteristic grooves or deep clefts.

An excellent example is trypsin, a vital proteolytic enzyme produced by the exocrine pancreas. Its main biological task is the digestion of dietary proteins via the hydrolysis of their peptide bonds. The trypsin molecule has a pronounced two-domain structure. The active site of the enzyme is hidden in a groove between these two structural blocks. It is in this zone that optimal microconditions are created for the efficient binding of a specific region of the dietary protein (in this case acting as a ligand) and its subsequent rapid cleavage.

There are also multifunctional proteins. In such molecules, individual domains are capable of performing entirely independent functions by binding to various types of ligands in their own active sites.

Cofactors and Conjugated Proteins

To perform their functions, many proteins require cofactors—specialized ligands that attach to the active site and play a key auxiliary role.

By composition, proteins are divided into two large groups:

• Simple proteins—built exclusively from the protein moiety (amino acid residues).
• Conjugated proteins—contain an additional non-protein moiety.

If the non-protein moiety is tightly and irreversibly bound to the molecule, it is called a prosthetic group. A classic example is the heme, which contains an iron ion. Conjugated proteins containing heme are called hemoproteins (these include hemoglobin, myoglobin, and cytochromes).

A wide variety of substances that interact with the three-dimensional structure of the peptide chain can act as ligands for proteins:

1. Low-molecular-weight molecules (organic and inorganic compounds).
2. Macromolecules, such as DNA (in the case of DNA-binding proteins), RNA, polysaccharides, and other protein molecules.

A striking example of working with low-molecular-weight ligands is demonstrated by albumin, a crucial blood plasma protein. Its main function is transport. The mechanism of action involves the attachment of various hydrophobic ligands to the active site: fatty acids, bilirubin, and many pharmaceutical substances.

Drugs as Ligands

The protein-ligand interaction is characterized by high specificity, yet the conformational lability (structural mobility) of the molecule allows for the selection of other substances capable of binding to the active site. The action of many medications is based on this principle.

Drugs often act as structural analogs of ligands (non-natural ligands)—chemical substances that mimic the structure of the protein's natural partner. They are capable of interacting with the active site and modulating the molecule's function:

• Agonist—binds to the site and enhances protein function.
• Antagonist—reduces or completely blocks the function.

If a natural ligand and its synthetic analog compete for the exact same binding site, they are called competitive modulators. Most drugs and even dangerous poisons work as protein inhibitors. They are often obtained through targeted chemical modification of the structure of natural ligands.

Clinical example: the neurotransmitter acetylcholine, responsible for nerve impulse transmission across cholinergic synapses. It is released into the synaptic cleft and binds to the receptor protein of the postsynaptic membrane. There are two types of receptors:

1. M-receptors (muscarinic): localized in smooth muscles. Specifically interact with acetylcholine and muscarine (fly agaric toxin), causing smooth muscle contraction.
2. N-receptors (nicotinic): found in the synapses of striated skeletal muscles. Selectively bind to acetylcholine and nicotine.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Formation of the protein tertiary structure
• Mechanism of action of the allosteric site
• Mechanisms of enzymatic catalysis (using trypsin as an example)
• Transport function and ligands of albumin
• Detailed classification of conjugated proteins

Other topics in the section "Structure, Properties, and Functions of Proteins"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.