Quaternary structure The

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 ... 

The quaternary structure - the spatial relationship between the protein chains in a multimeric protein. 

Quaternary structure the four separate chains Of hemoglobin assembled S3 into an oligomeric protein... 

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 tour separate chains of hemoglobin assembled into an oligomeric proteir ... 

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

The diversity among catalases, evident in the variety of subunit sizes, the number of quaternary structures, the different heme prosthetic groups, and the variety of sequence groups, enables them to be organized in four main groups the classic monofunctional enzymes (type A), the catalase-peroxidases (type B), the nonheme catalases (type C), and miscellaneous proteins with minor catalatic activities (type D). 

In addition to the primary structure, proteins also exhibit secondary, tertiary, and quaternary structure. The overall structure of proteins is related to several factors. Primary among these factors is the electrostatic nature of amino acids. The structures displayed in Figure 16.10 do not show the charge distribution displayed by amino acids. In neutral solutions, the carboxyl group tends to donate a proton (hydrogen ion) to the amino group. The transfer of a proton means the amino end of the molecule... 

It is common practice to include under the umbrella of quaternary structure other kinds of complexes between biopolymers. For example, the complex of DNA with histones to form nucleosomes may be said to have quaternary structure, the DNA also being regarded as a component subunit. 

The breakdown of glycogen in skeletal muscles and the liver is regulated by variations in the ratio of the two forms of glycogen phosphorylase. The a and b forms differ in their secondary, tertiary, and quaternary structures the active site undergoes changes in structure and, consequently, changes in catalytic activity as the two forms are interconverted. 

Proteins consisting of more than one polypeptide chain have quaternary structure. The... 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each halfreaction is an a cleavage which leads to a thiamin- bound enamine (center, Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step b followed by a in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an a4 tetramer and one with an ( P)2 quaternary structure. The isolation of ohydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate52 provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase126 in the biosynthesis of the plant hormone indoIe-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25).1243/127... 

The structure of proteins, as with the structure of carbohydrates, has various levels—primary, secondary, tertiary and quaternary structure. The tertiary and quaternary structures and their subtleties are most important in the biological function of the molecule. Consider an enzyme (a protein-based catalyst)—its structure allows the binding of specific molecules which then react catalytically to give products. Conversely, enzymes are very susceptible to environmental conditions which alter their tertiary structure. 

The enzyme contains six Zn2+ per molecule, two per R subunit. The zinc is not required for catalytic activity, but is essential for the maintenance of the quaternary structure. The structure has been determined to a resolution of 2.8 A in the presence and absence of CTP.528 The zinc-binding site is located in the C-terminal region of the R chains, and involves four cysteinyl residues, with tetrahedral geometry. The zinc domain represents the major site of interaction between the R and C chains, explaining the importance of zinc for the association of the subunits and the dissociative effect of mercurial reagents. When E. coli is grown in a zinc-deficient medium, some 70% of the enzyme is found to be dissociated into subunits.529... 

The overall relative orientation of the secondary structures of an enzyme determines its three-dimensional shape, or tertiary structure. Some enzymes require multiple copies of the same enzyme to function. The individual enzymes cluster into groups of two or more (called dimers, trimers, etc.) and are held together by intermolecular forces. The relative positioning of the separate enzymes in the cluster determines the overall structure, or quaternary structure, of the supramolecular complex. While all enzymes have tertiary structure, only clusters of multiple enzyme subunits have quaternary structure. The overall folded conformation of a protein in its active, catalytic form is called the active or native conformation. 

The order of amino acids in protein molecules, and the resulting three-dimensional structures that form, provide an enormous variety of possibilities for protein structure. This is what makes life so diverse. Proteins have primary, secondary, tertiary, and quaternary structures. The structures of protein molecules determine the behavior of proteins in crucial areas such as the processes by which the body s immune system recognizes substances that are foreign to the body. Proteinaceous enzymes depend on their structures for the very specific functions of the enzymes. 

Proteins that have more than one polypeptide chain require a higher level of organization. In the quaternary structure, the different chains are packed together to form the overall three-dimensional structure of the protein. The individual polypeptide chains can be arranged in a variety of shapes as part of the quaternary structure. 

Quaternary structure The structure formed by the association of two or more peptide chains. 

Know basic language to describe and/or classify proteins primary, secondary, tertiary, and quaternary structures, the different types of conjugated proteins, and acidic and basic proteins. 

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. 

Quaternary structure the interaction between different polypeptide chains to produce an oligomeric structure, stabilized by noncovalent bonds only. 

Hemoglobin, the oxygen-carrying protein of vertebrate blood, is similar in structure to myoglobin. However, hemoglobin is composed of four chains i.e., it has a quaternary structure. The four chains... 

The conformation of a protein in a particular environment affects its functional properties. Conformation is governed by the amino acid composition and their sequence as influenced by the immediate environment. The secondary, tertiary and quaternary structures of proteins are mostly due to non-covalent interactions between the side chains of contiguous amino acid residues. Covalent disulfide bonds may be important in the maintenance of tertiary and quaternary structure. The non-covalent forces are hydrogen bonding, electrostatic interactions, Van der Waals interactions and hydrophobic associations. The possible importance of these in relation to protein structure and function was discussed by Ryan (13). 

There are a number of protein interface sites that have the properties of an enzyme structural zinc-binding site in that they are composed of four protein ligands and no water molecules. These sites may be involved in local protein conformation or be involved in stabilizing quaternary structure. The metal ion acting in this manner is not directly involved in the function of the protein but may still play an indirect role in aligning critical components of the protein for its intended function. 

Figure 3.48. Quaternary Structure. The Cro protein of bacteriophage A, is a dimer of identical subunits.

Figure 3.50. Complex Quaternary Structure. The coat of rhinovirus comprises 60 copies of each of four subunits. (A) A schematic view depicting the three types of subunits (shown in red, blue, and green) visible from outside the virus. (B) An electron micrograph showing rhinovirus particles. [Courtesy of Norm Olson, Dept, of Biological Sciences, Purdue University.]... 

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