Proteins are the basic building blocks of life, constituting 60% of dry cells in the body and serving as the foundation for life’s structure and functions. Eggs, lentils, fish, meat, milk, and other milk products are excellent sources of protein, which aid in body growth and maintenance. The word “protein” comes from this chapter is part of unit 14 Biomolecules, which has an overall weightage of 6 to 7 marks and 8 periods. Biomolecular complexes of proteins with nucleic acids and other cofactors are also known as protein quaternary structures.
Quaternary Structure of Protein
The fourth (and highest) classification level of protein structure is quaternary structure. The structure of proteins that are made up of two or smaller protein chains is known as quaternary structure (also referred to as subunits). The quantity and order of numerous folded protein subunits in a multi-subunit complex are described by protein quaternary structure. It encompasses structures ranging from simple dimers to huge homo-oligomers, as well as complexes with fixed or variable component counts. Because certain proteins act as single units, unlike the first various layers of protein structure, not all proteins will have a quaternary structure.
Examples and descriptions
Many proteins are made up of several polypeptide chains. The quantity and arrangement of protein subunits about one another are referred to as quaternary structures. Haemoglobin, DNA polymerase, ribosomes, antibodies, and ion channels are examples of proteins possessing quaternary structure. Holoenzymes are enzymes made up of subunits with different functions, with certain components referred to as regulatory components and the operational core referred to as the catalytic subunit Multiprotein complexes, for example, have a quaternary structure as well.
Nucleosomes and microtubules are two examples. Individual subunit conformational changes or reorientation of the members relative to each other might cause changes in quaternary structure. Many proteins are regulated and perform their physiological functions as a result of such modifications, which underpin cooperativity and allostery in “multimeric” enzymes. The following concept is based on a traditional approach to biochemistry, which was developed at a time when distinguishing between a protein and a functional, proteinaceous unit was difficult. Protein-protein interaction is now commonly used when describing the quaternary structure of proteins, and all protein assemblies are referred to as protein complexes.
Structures of the protein
The primary structure of the protein
Primary structure, the most basic level of protein structure, is just the sequence of amino acids in a polypeptide chain. Insulin, for example, has two polypeptide chains, A and B, as seen in the picture below. (Though the structure of the insulin molecule illustrated here is similar to that of human insulin, it is cow insulin.) Each chain has its own set of amino acids, which are put together in a certain order. For example, the A chain’s sequence differs from the B chains in that it starts with glycine at the N-terminus and finishes with asparagine at the C-terminus.
The secondary structure of the protein
Secondary structure, the next level of protein structure, refers to local folded structures that arise within a polypeptide as a result of interactions between backbone atoms. The two most common secondary structures are the helix and the beta-sheet. Between the carbonyl O of one amino acid and the amino H of another, hydrogen bonds form, binding both structures together.
Quaternary Structure of the protein
Many proteins are built up of a single polypeptide chain with only three levels of structure (the ones we just spoke about). Some proteins, on the other hand, are made up of several polypeptide chains known as subunits. The quaternary structure of the protein is formed when these subunits join together.
A range of experimental procedures that need a sample of protein in a variety of experimental settings can be used to determine the quaternary structure of proteins. The experiments frequently offer an estimate of the native protein’s mass, which, when combined with knowledge of the subunit masses and/or stoichiometry, allows the quaternary system to be predicted with a certain degree of precision. For a variety of reasons, it is not always possible to acquire an exact determination of the subunit composition. The number of subunits in a protein complex is frequently estimated by measuring the intact complex’s hydrodynamic atomic volume or mass, which necessitates native solution conditions. The partial quantity is 0.73 ml/g can be used to derive the mass of folded proteins from their volume. However, because unfolded proteins appear to have a significantly larger volume than folded proteins, volume measurements are less reliable than mass measurements; additional studies are required to confirm whether a protein is unfolded or has formed an oligomer.
Conclusion
Quaternary structure is the fourth (and highest) classification level of protein structure. Quaternary structure refers to the structure of proteins that are made up of two or smaller protein chains (also referred to as subunits). Protein quaternary structure describes the quantity and order of many folded protein subunits in a multi-subunit complex. It includes complexes with fixed or variable component counts, as well as configurations ranging from simple dimers to massive homo-oligomers. Not all proteins will have a quaternary structure since certain proteins work as single units, unlike the first several levels of protein structure.