Quaternary structures in proteins

C. The stability of quaternary structure in proteins is mainly a result of covalent bonds among the sub units. 

Ch 23 Synthesis and Characterization of a Coordination Compound Ch 24 Quaternary Structure in Proteins... 

Hydrophobic forces are responsible for self-organization in many molecular systems [53-55]. These forces are recognized as important determinants of tertiary and quaternary structure in proteins, and are responsible for the assembly of biological membranes and other supramolecular systems. In pre-... 

Role of the Amino Acid Sequence in Protein Structure Secondary Structure in Protein.s Protein Folding and Tertiary Structure Subunit Interaction.s and Quaternary Structure... 

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. 

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... 

Quaternary structure. Due to non-covalent interactions, many proteins assemble to form symmetrical complexes (oligomers). The individual components of oligomeric proteins (usually 2-12) are termed subunits or monomers. Insulin also forms quaternary structures. In the blood, it is partly present as a dimer. In addition, there are also hexamers stabilized by Zn ions (light blue) (3), which represent the form in which insulin is stored in the pancreas (see p.l60). 

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. 

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). 

Quaternary structure. In a protein, the way in which the different folded subunits interact to form the multisubunit protein. 

The polypeptide chain folds up to form a specific shape (conformation) in the protein. This conformation is the three-dimensional arrangement of atoms in the structure and is determined by the amino acid sequence. There are four levels of structure in proteins primary, secondary, tertiary and, sometimes but not always, quaternary. 

Fig. 8. The four levels of structure in proteins, (a) Primary structure (amino acid sequence), (b) secondary structure (a-helix), (c) tertiary structure, (d) quaternary structure.

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). 

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