Quaternary structure of a protein

The quaternary structure of a protein is the aggregation of independent protein subunits to produce a functional molecule. 

Description of subcellular location of mature protein Description of quaternary structure of a protein Description of tissue specificity of a protein... 

Primary, Secondary, Tertiary and Quaternary Structure of a Protein. 

The quaternary structure of a protein can be largely determined by careful molecular weight determination of the native protein, followed by dissociation of the protein into its constituent polypeptide chains with denaturing agents. If nonidentical subunits are found, they must be separated from each other. Then the molecular weight of the isolated subunits must be determined, usually under dena-... 

Secondary, tertiary, and quaternary structures of a protein are mainly stabilized by hydrophobic interactions, van der Waals forces, and by hydrogen bonds. 

Figure 2-8. Schematic diagram of the primary, secondary, tertiary, and quaternary structure of a protein.

For many proteins the functional form is not composed of a single peptide but is rather an aggregate of smaller globular peptides. For instance, the protein hemoglobin is composed of four individual globular peptide subunits two identical a-subunits and two identical fj-subimits. Only when the four peptides are bound to one another is the protein molecule functional. The association of several polypeptides to produce a functional protein defines the quaternary structure of a protein. 

The forces that hold the quaternary structure of a protein are the same as those that hold the tertiary structure. These include hydrogen bonds between polar amino acids, ionic bridges between oppositely charged amino acids, van der Waals forces between nonpolar amino acids, and disulfide bridges. 

Studies discussed below have provided much insight into factors that determine the quaternary structure of a protein. However, in view of the fact that rat GDH, which is kinetically and structurally similar to the bovine enzyme, does not polymerize, it seems unlikely that the phenomenon of polymerization plays any major physiological role. 

Protein molecules are described by several levels of structure. The primary structure of a protein is the sequence of amino acids in the chain and the location of all the disulfide bridges. The secondary structure describes the regular conformation assumed by segments of the protein s backbone. In other words, the secondary structure describes how local regions of the backbone fold. The tertiary structure describes the three-dimensional structure of the entire polypeptide. If a protein has more than one polypeptide chain, it has quaternary structure. The quaternary structure of a protein is the way the individual protein chains are arranged with respect to each other. 

The quaternary structure of a protein refers to the association of individual polypeptide chain subunits in a geometrically and stoichiometrically specific manner. Many proteins function in the cell as dimers, tetramers, or oligomers, proteins in which two, four, or more subunits, respectively, have combined to make one functional protein. The subunits of a particular protein always combine in the same number and in the same way, because the binding between the subunits is dictated by the tertiary structure, which is dictated by the primary structure, which is determined by the genetic code. 

Ans. Different secondary, tertiary, or quaternary structures of a protein are different conformational isomers. There is a negligible difference in stability for the many conformational isomers of molecules such as dodecane. Dodecane molecules are continuously undergoing conversion from one conformational isomer to another. The situation is dramatically different for polypeptides. For the typical polypeptide, one of the many conformations is much more stable than the others and the polypeptide is present exclusively in that conformation with its specific three-dimensional shape. A complete description of this conformation or shape is what we refer to as the secondary, tertiary, and quaternary structures of a protein. Conformational shape directly affects the physiological function of a protein. 

Figure 1 Primary, secondary, tertiary and quaternary structure of a protein. The enzyme o-xylose isomerase is given as an example. The primary structure is the amino acid sequence, the secondary structure is the folding to a helices, p sheets and turns, the tertiary structure is the domain structure and the quaternary structure is the overall assemblage of the entire protein, often composed of several subunits...

It is, of course, important to keep the subunits well cemented together. The subunits are cemented together through hydrophobic patches on their surfaces. They join with other such patches and the resulting interactions of subunits with other subunits give the total protein [189]. This, however, has to be done in a controlled way. Each subunit is folded into an apparently independent globular structure which then interacts with other similarly folded subunits. The resulting quaternary structure of a protein is often vital to its function. 

Some proteins can be complex, when they contain multiple subunits of polypeptide structural entities. The way in which three-dimensional subunits interact to form the complete functional protein is called the quaternary structure of a protein. This level of hierarchy is possible only if the protein has multiple units. An example is hemoglobin. 

Denaturation is the loss of secondary, tertiary, and quaternary structure of a protein by a chemical or physical agent and the resulting loss of function.The previous two problems revealed that heat can cause denaturation. As a group, discuss other physical or chemical agents that could cause denaturation and explain the processes that would effect denaturation. 

In 1936 Mirsky and Pauling recognized the role of hydrogen bonds as important determinants of the secondary and tertiary structure of the proteins, in addition to intra-and interchain S—S linkages and ionic forces. More than a decade later hydrophobic interactions between certain amino acids were added as important conformational factors. That non-covalent linkages between proteins and non-protein compounds can determine the quaternary structure of a protein was shown in 1957 by the reconstitution of the tobacco mosaic virus from its isolated RNA and protein parts by Fraenkel-Conrat. On the other hand, Anfinsen< > could show that when the tertiary structure and activity of the bovine pancreas RNase are destroyed by reduction of its S—S linkages, re-oxidation of the SH groups finally leads to complete restitution of the enzymatic activity. Further structural studies of the reconstituted active enzyme support the assumption that in this case initial tertiary structure of the protein was restored. This supports the assumption that the tertiary structure of a protein is defined by its amino acid sequence. 

Some proteins have more than one polypeptide chain. The individual chains are called subunits. A protein with a single subunit is called a monomer one with two subunits is called a dimer one with three subunits is called a trimer and one with four subunits is called a tetramer. The quaternary structure of a protein describes the way the subunits are arranged with respect to each other. 

Many protein molecules do not consist of just one but of several identical or different polypeptide chains that form a complex structure, which is known as the quaternary structure of a protein. The quaternary structure is the arrangement of protein subunits in space and the association of its inter-subunit contacts and interactions,... 

There are various levels of structural organization of proteins primary, secondary, tertiary and quaternary. The primary structure has been defined as the sequential order of amino acid residues linked by covalent peptide bonds. The secondary structure refers to the molecular geometry located in the polypeptide chains within ordered structures, such as a-helix, (3-sheet and random coil (unordered). The tertiary structure contains the information on how the elements of the secondary structure are folded. Finally, the quaternary structure of a protein with more than one polypeptide chain shows how the different principal chains are associated and oriented with one another. The structure of proteins is stabilized by different types of interactions covalent and hydrogen bonds, hydrophobic interactions, electrostatic and van der Waals forces [3,4]. 

Denaturation of a protein occurs when high temperatures, acids or bases, organic compounds, metal ions, or agitation destroy the secondary, tertiary, or quaternary structures of a protein with a loss of biological activity. 

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