Protein , association quaternary structure

Quaternary structure refers to the association of two or more peptide chains in the complete protein. Not all proteins have quaternary structure. The ones that do are those that associate together in their active form. For example, hemoglobin, the oxygen carrier in mammalian blood, consists of four peptide chains fitted together to form a globular protein. Figure 24-18 summarizes the four levels of protein structure. 

Finally, protein chains interact with each other as subunits associate to make a functional species. For example, hemoglobin, the mammalian oxygen carrier, contains two each of two different subunits. The ability of hemoglobin to deliver oxygen to the tissues is dependent on the association of these subunits. Interaction of proteins to form a multimer composed of several subunits is termed the protein s quaternary structure. Quaternary structure is often very important in determining the regulatory properties of a protein. 

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,... 

Protein molecules that have only one chain are called monomeric proteins. But a fairly large number of proteins have a quaternary structure, which consists of several identical polypeptide chains (subunits) that associate into a multimeric molecule in a specific way. These subunits can function either independently of each other or cooperatively so that the function of one subunit is dependent on the functional state of other subunits. Other protein molecules are assembled from several different subunits with different functions for example, RNA polymerase from E. coli contains five different polypeptide chains. 

The forces that stabilize quaternary structure have been evaluated for a few proteins. Typical dissociation constants for simple two-subunit associations... 

Some proteins are formed by a single chain and are called monomeric, but a large number are formed by several polypeptide chains that associate in a multimeric molecule. The relationships of the peptide chains in a multichain protein are known as the quaternary structure. These subunits may work either independently of each other or cooperatively, i.e. the function of one subunit depends on the functional state of the others11. 

Most proteins contain more than one polypeptide chain. The manner in which these chains associate determines quaternary structure. Binding involves the same types of noncovalent forces mentioned for tertiary structure van der Waals forces, hydrophobic and hydrophilic attractions, and hydrogen bonding. However, the interactions are now interchain rather than infrachain (tertiary structure determination). The quaternary structure of hemoglobin (four almost identical subunits) will be discussed in Chapter 4, that of superoxide dismutase (two identical subunits) will be discussed in Chapter 5, and that of nitrogenase (multiple dissimilar subunits) will be discussed in Chapter 6. 

Figure 8.10 The quaternary structure of proteins. The enzyme lactate dehydrogenase (EC 1.1.1.27) has a relative molecular mass of approximately 140 000 and occurs as a tetramer produced by the association of two different globular proteins (A and B), a characteristic that results in five different hybrid forms of the active enzyme. The A and B peptides are enzymically inactive and are often indicated by M (muscle) and H (heart). The A4 tetramer predominates in skeletal muscle while the B4 form predominates in heart muscle but all tissues show most types in varying amounts.

Primary structure of a protein is simply amino acids sequence of the peptide chain. The secondary structure is a result of the different conformations that the chain can take. The tertiary structure refers to the three dimensional shape that results from twisting, bending and folding of protein helix. The quaternary structure refers to the way in which these amino acid chains of a complex protein are associated with each other (oligomer, dimers, trimers, etc.). 

Quaternary Structure describes the association of the different chains in multichain proteins... 

Quaternary structure refers to the specific aggregation or association of separate protein chains to form a well-defined structure. Part D of Figure 2.4 compares the quaternary structure of a dimeric protein (two polypeptide chains) to the lower levels of protein structure. The separate protein chains are often referred to as subunits or monomers these subunits may be identical or may be of quite different sequence... 

Non-reserve polysaccharides seem to function in biological tissues through the part they play in cohesion, the retention of water and salts, the physical organization, and the elasticity and general texture. Polysaccharide conformation and association, as well as chemical structure, are obviously involved in the control of such properties. The polysaccharide-polysaccharide interactions considered in this Section can be regarded (in the nomenclature of protein biochemistry) as showing secondary, tertiary, and quaternary structure.4W>2W8 The... 

In many cases there are important interactions between protein molecules that may lead to highly organized structures such as the pleated sheet of silk fibroin (Figure 25-13) or the coiling of a helices, as found in a-keratins, the fibrous proteins of hair, horn, and muscles (Figure 25-17). This sort of organization of protein molecules is called quaternary structure and is an important feature of many proteins that associate into dimers, tetramers, and so on. The tetrameric structure of hemoglobin is an important example. 

Further association of domains results in the formation of the protein s tertiary structure—the overall folding of the polypeptide chain in three dimensions. Finally fully folded protein subunits can pack together to form quaternary structures. 

A number of functional studies have shown that the partially ligated species and valency hybrid Hbs exhibit slowly interconverting conformation states. Some of them have properties that are intermediate between those associated with the T- and R-quaternary conformations (Cassoly and Gibson, 1972 Samaja et al., 1987 Sharma, 1989 Berjis et al., 1990). Thus, one needs to be careful in making correlations between a given crystal structure of a specific Hb species and its functional properties. It should be kept in mind that crystallization is a selective procedure, i.e., it selects those proteins with specific structures that are crystallizable under a given set of crystallization conditions. The structures of these crystallized proteins may not be the dominant ones when function is measured under solution conditions different from those used in crystallizing the proteins. 

A schematic comparison of the levels of protein stmcture. Primary stmcture is the covalently bonded stmcture, including the amino acid sequence and any disulfide bridges. Secondary structure refers to the areas of a helix, pleated sheet, or random coil. Tertiary stmcture refers to the overall conformation of the molecule. Quaternary structure refers to the association of two or more peptide chains in the active protein. 

Many supramolecular structures are formed largely by the stepwise noncovalent association of macromolecules, such as proteins. The processes of assembly are governed by the same chemical and physical principles that govern protein folding and the formation of quaternary structures (see Chap. 4). The driving force for the assembly process generally depends on the formation of a multitude of relatively weak hydrophobic, hydrogen and ionic bonds that occur between complementary sites on subunits which are in van der Waals contact with each other. In addition, covalent... 

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