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Protein structure

The Complexity of proteins structure depends upon its molecular size and shape. For levels of organization have been described and these are :Primary, secondary, and quaternary structure.

Primary structure

Our knowledge of the primary structure of proteins stems from the work of F.Sanger and his associates in 1955, who elucidated the structure of insulin molecule, a polypeptide. The work resulted in the award of a Nobel prize in chemistry in 1957. The primary structure of a proteins molecule refers to the linear arrangement of amino acid residues in a given polypeptide chain linked through covalent peptide bonds. the precise sequence of residues is generated by the genetic information contained in DNA. It is the number and disturbance in this sequence would create a different protein. One of the examples that has been studied to explain this change is haemoglobin.

A single replacement of glutamic acid with valine in position six of haemoglobin molecule causes a disease, called sickle-cell anaemia.
           Native proteins do not occur as straight chains of amino acids, but the latter may be derived by suitable treatment to establish their sequence. Determination of primary structure is of paramount importance because it essentially describes the number of peptide chains, sites of linkage of peptide chains, sequence determination and the number of residues in a protein. The method of analyzing the primary structure will be described in an appropriate section.

Secondary Structure

The secondary structure of proteins refers to a three dimensional arrangement of various atoms of the molecule, generally known as protein conformation. Linus pauling and Robert Corey are accredited with the elucidation of the secondary structure. They demonstrated that the conformation of polypeptide chain is dependent on the bond lengths and bond angles formed along the backbone of the chain. The polypeptide is folded in a certain way, which is not random process. The peptide bonds are planar and have bond lengths and angles identical to those found in the crystals of secondary amides. The peptide bond has a partial double bond character which is not free to rotate. Thus there can be only two configuration, cis and trans with respect to α – carbons on each side of the peptide bond. Secondary conformation are generated due to binding forces between different segments of the peptide chain. Bonding between different segments of the peptide chain is brought about by various secondary bonds.

Hydrogen bonds The most important secondary forces that maintain protein conformation are hydrogen bonds that are formed due to interaction between a C = O group and the proton of an NH or OH when they come nearer than a distance of 2.8 A. In some cases bonds of this type link a number of peptide chains, while in other cases a single peptide chain is held in coils.
Disulphide bonds  An Important bond between the side chains of amino acids is the disulphide bond that is formed between the thiol groups of two cysteine molecules. Such bonds are found in the insulin molecule.

Hydrophobic bonds  Sometimes a secondary valence bond is formed when the hydrocarbon side- chains approach closer repelling water molecules. Water molecules are held tightly among themselves through hydrogen bonds forcing the hydrocarbon out of the water phase. Such bonds are known as hydrophobic bonds which are relatively strong and exist in the interior of protein molecules.

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