Polypeptides quaternary structure

Arts. The primary structure is the particular amino acid sequence in each of the a- and /3-polypeptide chains. There are two types of secondary structure—a-helical and bend conformations. Tertiary structure is the spatial relationship between the different a-helical and bend portions of the molecules and the placement of heme and the Fe - ion relative to the polypeptide. Quaternary structure relates to the spatial relationship between the four polypeptide chains. 

Figure 1.1 The amino acid sequence of a protein s polypeptide chain is called Its primary structure. Different regions of the sequence form local regular secondary structures, such as alpha (a) helices or beta (P) strands. The tertiary structure is formed by packing such structural elements into one or several compact globular units called domains. The final protein may contain several polypeptide chains arranged in a quaternary structure. By formation of such tertiary and quaternary structure amino acids far apart In the sequence are brought close together in three dimensions to form a functional region, an active site.

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. 

Blake, C.C.F., et al. Structure of human plasma prealbumin at 2.5 A resolution. A preliminary report on the polypeptide chain conformation, quaternary structure and thyroxine binding. J. Mol. Biol. 88 1-12, 1974. 

Rather than existing as a single polypeptide chain, some proteins aie assemblies of two or more chains. The manner in which these subunits aie organized is called the quaternary structure of the protein. 

Many proteins consist of two or more interacting polypeptide chains of characteristic tertiary structure, each of which is commonly referred to as a subunit of the protein. Subunit organization constitutes another level in the hierarchy of protein structure, defined as the protein s quaternary (4°) structure (Figure 5.10). Questions of quaternary structure address the various kinds of subunits within a protein molecule, the number of each, and the ways in which they interact with one another. 

FIGURE 19.21 These structures show how a protein first forms a helices and p sheets and then how the coils and sheets fold together to form the shape of a protein. Finally, if the protein has a quaternary structure, the protein subunits stack together, (a) Newly formed polypeptide (b) intermediate (c) subunit ... 

Proteins may also Have a quaternary structure, in which neighboring polypeptide units stack together in a specific arrangement. The hemoglobin molecule, for example, has a quaternary structure of four polypeptide units, one of which is shown in Fig. 19.20. 

Proteins are polymers made of amino acid units. The primary structure of a polypeptide is the sequence of amino acid residues secondary structure is the formation of helices and sheets tertiary structure is the folding into a compact unit quaternary structure is the packing of individual protein units together. 

Disulfide bonds between and within polypeptides stabilize tertiary and quaternary structure. However, disulfide bond formation is nonspecific. Under oxidizing conditions, a given cysteine can form a disulfide bond with the —SH of any accessible cysteinyl residue. By catalyzing disulfide exchange, the rupture of an S— bond and its reformation with a different partner cysteine, protein disulfide isomerase facilitates the formation of disulfide bonds that stabilize their native conformation. 

Tertiary structure concerns the relationships between secondary structural domains. Quaternary structure of proteins with two or more polypeptides (oligomeric proteins) is a feature based on the spatial relationships between various types of polypeptides. 

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. 

Quaternary structure refers to the overall spatial arrangement of polypeptide subunits within a... 

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. 

To the best of our knowledge, there is one host which conforms to the structure of an Archimedean dual. Harrison was the first to point out that the quaternary structure of ferritin, a major iron storage protein in animals, bacteria, and plants, corresponds to the structure of a rhombic dodecahedron. [45] This protein, which is approximately 12.5 nm in diameter, consists of 24 identical polypeptide subunits (Fig. 9.18), and holds up to 4500 iron atoms in the form of hydrated ferric oxide with... 

All proteins have at least three levels of structure primary, secondary, and tertiary. Proteins with more than one polypeptide chain— hemoglobin and nitrogenase are examples—also possess quaternary structure. The primary. 

Quaternary structure the protein structure that results from the assembly of two or more independent polypeptide chains. 

D. Quaternary structure occurs in proteins that have multiple polypeptide chains, called subunits. 

Most antibodies have a complex quaternary structure, being composed of four individual polypeptide chains, two heavy (H) chains and two light (L) chains. 

Quaternary structure exists when two or more polypeptide chains are linked only by weak forces of attraction among R groups at the surface of the chains. Such proteins are called oligomers (dimers, trimers, and so on). 

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... 

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