Oligomeric proteins and protomer interaction
Many proteins in the human body and other living organisms function not as single chains, but as complex complexes. Proteins composed of multiple polypeptide chains are called oligomeric. Each individual polypeptide chain in such a complex is called a protomer.
The number of protomers in a molecule can vary greatly. For example, a hemoglobin molecule contains 4 protomers, the enzyme aspartate aminotransferase consists of 12, and the tobacco mosaic virus protein includes as many as 2,120 subunits. Due to this structure, oligomeric proteins always have a large molecular weight.
Interaction between protomers occurs due to the strict complementarity of their contacting surfaces. They are connected to each other via numerous weak non-covalent bonds:
- Hydrophobic interactions;
- Ionic bonds;
- Hydrogen bonds.
In essence, the interaction of one protomer with others can be viewed as a classic special case of protein-ligand interaction, where each protomer acts as a specific ligand for the neighboring one. Depending on their composition, oligomers are divided into homodimers (consisting of absolutely identical protomers) and heterodimers (containing structurally different subunits).
Functional sites of heterodimers
If a protein consists of different protomers, binding sites with various spatial structures are formed on its surface. The classical scheme of a dimeric protein structure implies the presence of two main subunits, each performing its own task.
- The active site is located on one of the subunits. Its main task is to bind to a specific ligand. It is during this binding that the main, basic function of the given protein is manifested.
- The allosteric (regulatory) site is located on another protomer, spatially distant from the active site. It is designed to bind a specific substance — the allosteric ligand (effector). This site does not participate in performing the basic function, but strictly controls its intensity.
Allosteric regulation and cooperative effect
The ability to undergo allosteric regulation (i.e., regulation via the attachment of various ligands) is a key property of oligomeric proteins, as well as proteins with a pronounced domain structure. This very property cardinally distinguishes them from simple monomeric proteins (e.g., the difference between the oligomer hemoglobin and the monomer myoglobin).
The mechanism of allosteric regulation works as follows:
- The effector interacts with the allosteric site.
- Due to the high conformational lability of the protein, this binding causes immediate conformational changes throughout the complex structure of the oligomer.
- The spatial rearrangement affects the affinity of the active site for its specific ligand.
- As a result, the protein function is either enhanced or suppressed.
Substances that enhance protein function are called activators, and those that inhibit it are called inhibitors.
The most important consequence of the quaternary structure is the cooperative effect. This is a phenomenon where the interaction of an oligomeric protein with at least one ligand causes cooperative (joint) changes in the conformation and function of absolutely all protomers within the molecule.
Briefly: answers to topical questions
What is the main structural difference between hemoglobin and myoglobin?
The main structural difference lies in the presence of a quaternary structure in hemoglobin and its complete absence in myoglobin.
| Sign | Hemoglobin (Hb) | Myoglobin (Mb) |
|---|---|---|
| Level of organization | Oligomer (tetramer) | Monomer |
| Number of chains | 4 polypeptide chains | 1 polypeptide chain (one protomer) |
| Quaternary structure | Present | Absent |
In hemoglobin, polypeptide chains are interconnected by numerous non-covalent bonds, whereas myoglobin is incapable of forming a quaternary structure.
Do disulfide bonds participate in the formation of the quaternary protein structure?
Yes, but in the cited sources this is confirmed as a special case for immunoglobulins, whereas the general description of the quaternary structure indicates its stabilization by non-covalent bonds. In immunoglobulins, disulfide bonds are covalent bonds that connect individual chains within the monomer; in IgG, they bind heavy chains together in the hinge region. In polymeric immunoglobulin molecules, disulfide bonds connect monomers to each other; IgM has a pentameric form, and IgA has a dimeric form. Most intracellular proteins in the source are described as lacking disulfide bonds in their tertiary structure.
What specific substances act as allosteric effectors for hemoglobin?
In the cited sources for hemoglobin, the influence of CO2 and H+ concentration on Hb affinity for O2 is explicitly stated: this is the Bohr effect. An increase in H+ and CO2 in tissues decreases hemoglobin's affinity for O2 and promotes oxygen release. Regarding 2,3-BPG, the source only contains an incomplete mention ("The role of 2,3-BPG..."), so its action cannot be stated as a confirmed fact based on the provided excerpt.
What specific types of polypeptide chains does the adult human hemoglobin molecule consist of?
The main erythrocyte hemoglobin of an adult human (Hb A) has a tetrameric structure and consists of two types of polypeptide chains.
- Alpha chains ($\alpha$-chains) — the molecule always contains two such subunits, each consisting of 141 amino acid residues.
- Beta chains ($\beta$-chains) — the second pair of subunits is represented by two chains, each including 146 amino acid residues.
Together, this polypeptide chain composition ($2\alpha$ and $2\beta$) accounts for about 98% of all adult human hemoglobin.
To remember the difference between the sites, imagine a car: the active site is the engine (performs the main work), and the allosteric site is the gas and brake pedals (effector activators and inhibitors), which are located elsewhere but regulate the engine's operation.
Class and exam questions
What is the quaternary protein structure?
This is the method of connection and the number of individual polypeptide chains (protomers) within a single functional protein molecule (oligomer).
What types of bonds hold protomers together?
Protomers are connected exclusively through numerous weak non-covalent bonds: hydrophobic, ionic, and hydrogen bonds. They are formed due to the complementarity of contacting surfaces.
What is the function of the allosteric site?
The allosteric site is needed for binding to effectors (activators or inhibitors). This binding alters the conformation of the entire protein and regulates the affinity of the active site for the primary ligand.
What does the term "cooperative effect" mean?
This is a phenomenon where the attachment of a ligand to one protomer causes a synchronous change in the spatial conformation and functional activity of all other protomers in the protein molecule.
What else to study on this topic
- Functional features of hemoglobin as a classic oligomeric protein
- Comparison of myoglobin and hemoglobin properties
- Detailed mechanisms of conformational changes during allosteric regulation
- Specificity of protein-ligand interaction
- Structure and functions of proteins with domain organization
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