Secondary, Tertiary, and Quaternary Structure of Proteins

PRIMARY, SECONDARY, TERTIARY AND QUATERNARY STRUCTURES OF PROTEINS... 

Primary, Secondary, Tertiary and Quaternary Structures of Proteins.47... 

With the exception of a small group of catalytic RNA molecules (Chapter 26), all enzymes are proteins. Their catalytic activity depends on the integrity of their native protein conformation. If an enzyme is denatured or dissociated into its subunits, catalytic activity is usually lost. If an enzyme is broken down into its component amino acids, its catalytic activity is always destroyed. Thus the primary, secondary, tertiary, and quaternary structures of protein enzymes are essential to their catalytic activity. 

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). 

The diversity in primary, secondary, tertiary, and quaternary structures of proteins means that few generalisations can be made concerning their chemical properties. Some fulfil structural roles, such as the collagens (found in bone) and keratin (found in claws and beaks), and are insoluble in all solvents. Others, such as albumins or globulins of plasma, are very soluble in water. Still others, which form part of membranes of cells, are partly hydrophilic ( water-loving , hence water-soluble) and partly lipophilic ( lipid-loving , hence fat-soluble). 

Now that you have learned some of the chemistry of amino acids, it s time to study proteins, the large polymers of amino acids that are responsible for so much of the structure and function of all living cells. We begin with a discussion of the primary, secondary, tertiary, and quaternary structure of proteins. 

Figure 28.15 The primary, secondary, tertiary, and quaternary structure of proteins...

Denaturation of protein - The comparatively weak forces responsible for maintaining secondary, tertiary and quaternary structure of proteins are readily disrupted with resulting loss of biological activity. This disruption of native structure is termed denaturation. 

Primary, Secondary, Tertiary, and Quaternary Structure of Proteins Secondary and Tertiary Structures of Nucleic Acids... 

Elevated pressures can induce functional and structural alterations of proteins. The effects of pressure are governed by Le Chatelier s principle. According to this principle, an increase in pressure favours processes which reduce the overall volume of the system, and conversely increases in pressure inhibit processes which increase the volume. The effects of pressure on proteins depend on the relative contribution of the intramolecular forces which determine their stability and functions. Ionic interactions and hydrophobic interactions are disrupted by pressure. On the other hand, stacking interactions between aromatic rings and charge-transfer interactions are reinforced by pressure. Hydrogen bonds are almost insensitive to pressure. Thus, pressure acts on the secondary, tertiary, and quaternary structure of proteins. The extent and the reversibility, or irreversibility, of pressure effects depend on the pressure range, the rate of compression, and the duration of exposure to increased pressures. These effects are also influenced by other environmental parameters, such as the temperature, the pH, the solvent, and the composition of the medium. 

Recognize the amino adds and understand how they form peptides and proteins via amide bond formation. (Section 24.7) Understand the differences among the primary, secondary, tertiary, and quaternary structures of proteins. (Section 24.7)... 

The peptide (amide) linkages of peptides and proteins can be hydrolyzed under appropriate conditions. This destroys the primary structure and produces smaller peptides or amino acids. The characteristic secondary, tertiary, and quaternary structures of proteins can also be disrupted by certain physical or chemical conditions such as extreme temperatures or pH values. The disruption of these structures is called denaturation and causes the protein to become nonfunctional and, in some cases, to precipitate. 

Fig. 20. Primary, secondary, tertiary, and quaternary structure of proteins. In the secondary structure, the small unlettered circles represent hydrogen atoms, R = sidechains of the amino acids. In the tertiary structure the black disc represents the hemin group. In the quaternary structure the two a-chains of hemoglobin are white, the two /3-chains black. One of them is easy to distinguish from the white background of the a-chains (modified from Sund as presented in Wieland and Pfieiderer 1969).

Armed with these new insights into primary, secondary, tertiary, and quaternary structure of proteins, let us now return to the question of the relation one gene-one polypetide. In doing so we will not concern ourselves with the enumeration of instances of one gene-one enzyme relationships. (There is a variety of obvious examples in higher plants.) Instead we want to verify whether in fact one gene does induce one polypeptide, which can then, possibly, combine with other polypeptides of the same or a slightly different kind to form a quaternary structure. To do this we must study isoenzymes. 

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