Quaternary structures of proteins

Quaternary structure is the hnal level of protein structure and pertains to proteins that consist of more than one polypeptide chain. Each chain is called a subunit. The number of chains can range from two to more than a dozen, and the chains may be identical or different. Commonly occurring examples are dimers, trimers, and tetramers, consisting of two, three, and four polypeptide chains, respectively. (The generic term for such a molecule, made up of a small number of subunits, is oligomer.) The chains interact with one another noncovalently via electrostatic attractions, hydrogen bonds, and hydrophobic interactions. 

As a result of these noncovalent interactions, subtle changes in structure at one site on a protein molecule may cause drastic changes in properties at a distant site. Proteins that exhibit this property are called allosteric. Not all multisubunit proteins exhibit allosteric effects, but many do. 

Some proteins can be complex, when they contain multiple subunits of polypeptide structural entities. The way in which three-dimensional subunits interact to form the complete functional protein is called the quaternary structure of a protein. This level of hierarchy is possible only if the protein has multiple units. An example is hemoglobin. 

Generally, protein denaturation includes the complete or partial unfolding of the polypeptide chain, cleavage of disulfide linkages, and breakage of noncovalent interactions. Denaturation is sometimes reversible. The reversing process is called renaturation. 

All the following amino acids are optically active, EXCEPT  

At a particular pH, the amino acid glutamic acid has a net charge of zero. When this pH is achieved, it is said to be at glutamic acid s  

Which of following groups are not present in any of the common amino acids  

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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. 

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

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

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.

Secondary structure, as well as tertiary and quaternary structure of proteins, is intimately dependent on the primary sequence of amino acids in the chain. In fact, the manner in which proteins fold into their ultimate structures in biological species is a subject of much research and continued uncertainty even... 

As described in the beginning of this chapter, the peptide bond is rigid, polar, and prefers a planar structure with hydrogen of the amino group and oxygen of the carbonyl almost trans. It is easily understood that this conformational preference and rigidity has profound implications to the tertiary and quaternary structure of proteins and similarly on the binding of smaller peptides to receptors. 

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. 

Knowledge and understanding of protein structure and properties in the 1950 s was rapidly evolving. The unique secondary, tertiary, and even quaternary structures of proteins were becoming understood6 8) and the delicateness of protein three-dimensional conformation was recognized, including the possibility for denatura-tion at liquid/air and solid/liquid interfaces x 3). 

Tertiary and quaternary structure of proteins, glycoproteins, denaturation 21.9-21.11... 

Formaldehyde introduces both intramolecular and intermolecular crosslinks between proteins involving hydroxymethylene bridges, which change the three-dimensional structure of proteins. Such changes involve the tertiary and quaternary structures of proteins, whereas the primary and secondary structures are little affected. It has been shown that the secondary structure of purified protein molecules remains mostly unaltered during fixation with formaldehyde (Mason and O Leary, 1991). Even when the quaternary structure is changed by formaldehyde fixation, the secondary structure can remain intact. 

Figure II-4 Examples of the quaternary structure of proteins, (a) A drawing of glutamine synthetase of coli showing the orientation of the 12 identical subunits of the enzyme. (b) A drawing of aspartate transcarbamylase of coli showing the proposed orientation of the 6 catalytic subunits (labeled C, each MW = 33,000), and 6 regulatory subunits (labeled R, each MW =...

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. 

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... 

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