The properties of protein ridges that have evolved in response to the physiological context of living cells are referred to as protein quinary structures. Quinary structure, in addition to differences, secondary, tertiary, and quaternary structures, is the fifth level of protein complexity. Unlike the previous four levels of protein structure, which apply to isolated proteins in dilute settings, quinary structure originates from the cellular context’s crowding, where transient contacts between macromolecules arise regularly.
Quinary Structure of Protein
Proteins frequently need to identify a specific counterpart to whom they will bind in a somewhat long encounter to accomplish their tasks. Such a search becomes difficult in a crowded cytosol, where proteins engage in a vast and intricate network of attracting and repelling interactions because it requires sampling a large number of potential partners, of which only a few will be effective. To solve this problem, proteins must spend as little time as possible on each encounter so that they can investigate a greater number of surfaces, while still making this engagement as intimate as conceivable so that they do not miss out on the correct partner.
The quinary structure is the consequence of a series of modifications seen on protein surfaces that enable proteins to negotiate the cellular environment’s complexities.
- Long sequences of amino acids fold into complicated three-dimensional structures to form proteins.
- Proteins come in an almost infinite variety of shapes and sizes, each one working as a specialised molecular machine capable of performing a specific microscopic task.
- The basic structure of a protein polypeptide chain is the precise arrangement of amino acids. A protein chain can contain up to 20 various types of amino acids, each with its own set of characteristics (hydrophobic, hydrophilic, positive, negative, and cysteine).
- The alpha helices and beta-pleated sheets that make up a folded protein’s structure are known as secondary structures.
- The final position of an amino acid chain is called tertiary structure. The function of the protein is directly tied to its form.
- If more than one amino acid chain joins to make a protein complex, it is called a quaternary structure.
Quinary structure modulates protein stability in cells
Quinary interactions between proteins organise the cellular interior and metabolism. Even though the connections that stabilise secondary, tertiary, and quaternary protein structures are well understood, information on the protein–matrix contacts that make up quinary structures is lacking. Because proteins function in a busy cellular environment, yet are generally investigated in simple buffering solutions, this gap persists. We measure quinary contacts between the B1 domain of protein G and the cytoplasm of E. coli using NMR-detected H/D exchange. We show that a surface mutation in this protein is 10-fold more disruptive in cells than in buffer, an unexpected finding that demonstrates the importance of quinary interactions. Surprisingly, the energy required for these interactions can be comparable to that required to stabilise certain protein complexes.
The arrangement of amino acids in a protein, or primary structure, determines tertiary and secondary structures. Primary, secondary, and tertiary structures are all present in all proteins.
Proteins with a quaternary structure are composed of several amino acid chains. The same forces that hold particular protein chains together (hydrophobic, hydrophilic, positive/negative, and cysteine connections) hold these multi-chain proteins together. In some cases, the numerous protein chains in a protein complex are identical, whereas, in others, they are all different.
Which protein has a quaternary structure?
X-ray crystallography is frequently used to identify the quaternary structure, as previously mentioned. When crystallographic data was difficult to obtain or impossible to obtain, electron microscopy gave some quaternary structure insights.
Experimenting on the multisubunit makeup of a protein is fairly simple. Size-exclusion chromatography can be used to determine the molecular weight of each component by destroying the connections that hold them together. The composition can be computed using basic mathematics if the technique is repeated without bond breakage. Ultracentrifugation of a protein specimen in a viscous glucose solution can decide molecular weights depending on sedimentation coefficients, and when combined with light scattering, which determines the molecular aspects of proteins in solution, some details about how the multi-subunit protein is organised can be ascertained.
Conclusion:
The function of a protein is determined by its structure. Some functions, on the other hand, can only be achieved by contiguous protein complexes that defy standard structural concepts. These are the fifth (or quinary) levels of protein structure (beyond primary, secondary, tertiary, and quaternary) that are best characterised.
Quinary assemblies organise cellular biochemistry, amplify signals, control the movement of macromolecules and genetic material, and store information about the cell’s past. Quinary assemblies can be as simple as disorganised dynamic liquid droplets or as complex as crystalline arrays of globular protein subunits. Each polypeptide in a quinary assembly interacts with others through a variety of low-affinity contacts. As a result, the quinary form can be extremely cooperative and especially sensitive to protein concentration.