Protein Diversity and Determinants of Function
Proteins mediate the absolute majority of vital processes in cells and the organism as a whole. Their biological role is remarkably diverse, yet the specific function of each individual molecule is strictly determined by three key factors:
- Features of the primary structure (amino acid sequence) and the final spatial conformation.
- The unique structure of the active site.
- The ability to selectively interact with specific ligands.
Interestingly, out of the colossal number of theoretically possible amino acid combinations in peptide chains, only a minuscule fraction can fold into a stable three-dimensional structure. Most random sequences cannot form a stable structural framework.
Folding Thermodynamics and Stability
From a thermodynamic standpoint, any polypeptide chain can theoretically adopt a vast multitude of various conformations. Many of these states possess approximately identical Gibbs free energy values, but entirely different physicochemical properties.
However, biological evolution acted as a strict filter. Natural selection preserved only those primary structures that guarantee exceptional stability for one single conformation. It is this unique and thermodynamically favorable shape that determines the specific function the protein will perform.
What Are Protein Families
In the course of evolution, point mutations occurred, leading to the substitution of individual amino acid residues in polypeptide chains. This mechanism caused the emergence of entire groups of related macromolecules within a single species, which we call protein families.
All members of a single family share a set of common characteristics:
- Presence of homologous regions. Their primary structures show a high degree of similarity.
- Similar spatial conformation. Such proteins have identical or very similar numbers and mutual arrangements of secondary structure elements ($\alpha$-helices and $\beta$-sheets), as well as spatial bends and turns of the polypeptide chain.
- Related functions. Despite certain structural differences, the overall architecture of the molecule drives them to perform similar biological tasks in the organism.
Examples of Protein Families
Classic examples illustrating the grouping of proteins into families based on their structural and functional relatedness include:
- Myoglobin family (proteins specialized in oxygen binding and transport).
- Immunoglobulin family (mediate humoral immunity through the specific binding of antigens).
- Serine proteinases (a group of enzymes with a common catalytic mechanism that cleave peptide bonds).