Structure and the Principle of Complementarity
The active site is formed when the polypeptide chain folds, acquiring its unique three-dimensional tertiary structure. In the linear primary structure of a protein, amino acid side chains can be located far apart from one another. However, during folding, they are brought together to 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—exact spatial and chemical matching of the interacting surfaces.
- Spatial correspondence means that the active site geometrically fits the incoming ligand perfectly, much like a lock and key.
- Chemical correspondence implies that bonds must form between the functional groups of the protein side chains and the ligand molecule.
The retention of the ligand within the pocket is ensured by weak interactions: ionic and hydrogen bonds, as well as hydrophobic interactions. It is precisely the attachment of a specific ligand that allows a protein to realize its biological function.
Location of Binding Sites
In multidomain proteins, ligand-binding sites are very often localized between domains—in characteristic grooves or deep clefts.
A prime example is trypsin, a crucial proteolytic enzyme produced by the exocrine pancreas. Its main biological task is digesting dietary proteins by hydrolyzing their peptide bonds. The trypsin molecule has a pronounced two-domain structure. The enzyme's active site is hidden in a groove between these two structural blocks. It is within this zone that optimal microconditions are created for the efficient binding of a specific region of a dietary protein (acting here as a ligand) and its subsequent rapid cleavage.
There are also multifunctional proteins. In such molecules, individual domains can perform completely independent functions by binding to various types of ligands in their own respective 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.
According to their composition, proteins are divided into two large groups:
- Simple proteins — composed exclusively of a 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 protein molecule, it is called a prosthetic group. A classic example is heme, which contains an iron ion. Conjugated proteins containing heme are called hemoproteins (these include hemoglobin, myoglobin, and cytochromes).
Ligands for proteins can be a wide variety of substances interacting with the three-dimensional structure of the peptide chain:
- Low-molecular-weight molecules (organic and inorganic compounds).
- 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 vital 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 agents.
Pharmaceutical Agents as Ligands
Protein-ligand interactions are characterized by high specificity, yet the conformational lability (structural mobility) of the molecule allows other substances to bind 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 the protein's function.
- Antagonist — reduces or completely blocks the function.
If a natural ligand and its synthetic analog compete for the same binding site, they are called competitive modulators. Most medications and even dangerous poisons work as protein inhibitors. They are often obtained through targeted chemical modification of the structure of natural ligands.
A clinical example: the neurotransmitter acetylcholine, responsible for transmitting nerve impulses across cholinergic synapses. It is released into the synaptic cleft and binds to a receptor protein on the postsynaptic membrane. There are two types of receptors:
- M-receptors (muscarinic): localized in smooth muscles. Specifically interact with acetylcholine and muscarine (fly agaric toxin), causing smooth muscle contraction.
- N-receptors (nicotinic): located in the synapses of striated skeletal muscles. Selectively bind to acetylcholine and nicotine.