Polypeptide chain subunits

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. 

Allosteric enzymes have an oligomeric organization. They are composed of more than one polypeptide chain (subunit) and have more than one S-bind-ing site per enzyme molecule. 

On the basis of these data, the following model of the subunit structure is proposed the snail hemagglutinin consists of 6 identical, polypeptide chains (subunits), each containing one intrachain disulfide bond and a carbohydrate binding-site. Furthermore, two subunits are linked by an intrachain, disulfide bond to form subunit dimers of molecular weight 26,000, and three dimers (mol. wt. 26,000) are held together by noncovalent interactions.569... 

Many proteins are composed of more than one polypeptide chain (subunit). Peptide mapping procedures are often useful in determining how many subunits are present and whether the subunits are identical. Consider, for example, a hypothetical protein with 20 residues of arginine plus lysine per molecule as determined by amino acid analysis. (The molecular weight of the protein must be known.) If the molecule is composed of one subunit, treatment with trypsin would give 21 peptides observable by peptide mapping procedures. On the other hand, if the molecule is composed of four identical subunits, only six peptides would be obtained by tryptic digestion. 

All enzyme molecules possess the primary, secondary, and tertiary structural characteristics of proteins (see Chapter 20). In addition, most enzymes also exhibit the quaternary level of structure. The primary structure, the linear sequence of amino adds linked through their a-carboxyl and a-amino groups by peptide bonds, is specific for each type of enzyme molecule. Each polypeptide cham is coiled up into three-dimensional secondary and tertiary levels of structure. Secondary structure refers to the conformation of limited segments of the polypeptide chain, namely a-helices, P-pleated sheets, random coils, and p-turns. The arrangement of secondary structural elements and amino acid side chain interactions that define the three-dimensional structure of the folded protein is referred to as its tertiary structure. In many cases biological activity, such as the catalytic activity of enzymes, requires two or more folded polypeptide chains (subunits) to associate to form a functional molecule. The arrangement of these subunits defines the quaternary structure. The subunits may be copies of the... 

In some cases proteins are divided into two or more domains (Fig. 1), each of which is like a globular protein but connected covalently to other domain(s) by the continuous polypeptide chain. Other proteins are oligomeric in that they are composed of several unconnected polypeptide chains (subunits) that usually, but not always, fold up independently and assemble to form the complete protein. The arrangement of the subunits relative to each other is referred to as the quaternary structure. Hemoglobin (a202) (Fig. 6) and aspartate transcarbamoylase (a6/36), where a and j3 refer to different types of subunits, are well-studied cases where different quaternary structures occur with significantly altered properties. 

Quaternary (4°) structure, the most complex level, occurs in proteins made up of several polypeptide chains (subunits) and refers to the way the chains assemble into the overall protein. 

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. 

The number of distinct polypeptide chains (subunits) in the protein must first be determined. 

Lactate dehydrogenase (EC 1.1.1.27) catalyzes the reaction lactate-I-NAD pyruvate+NADH, which in the reverse direction represents the last reaction of anaerobic glycolysis. The enzyme consists of four identical polypeptide chains (subunits), each of molecular mass about 35 kDa. For a summary of this work, see Ref. [1]. 

Quaternary structure (1) The three-dimensional structure of a protein molecule formed by the association of two or more polypeptide chains. (2) For a protein, the level of structure that results when separate, folded polypeptide chains (subunits) associate in a specific way to produce a complete protein. Compare primary structure, secondary structure tertiary structure. 

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