proteins represents overall folding of the polypeptide chains i.e., further folding of the secondary structure. It gives rise to two major molecular shapes viz. fibrous and globular. The main forces which stabilise the ° and ° structures of proteins are hydrogen bonds, disulphide linkages, van der Waals and electrostatic forces of attraction.
(iv) Quaternary structure of proteins : Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure. Fig. .
: Diagrammatic representation of protein structure (two sub-units of two types in quaternary structure) A diagrammatic representation of all these four structures is given in Figure . where each coloured ball represents an amino acid. Fig. .
: Primary, secondary, tertiary and quaternary structures of haemoglobin Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of .
. Denaturation of Proteins protein. During denaturation secondary and tertiary structures are destroyed but primary structure remains intact. The coagulation of egg white on boiling is a common example of denaturation.
Another example is curdling of milk which is caused due to the formation of lactic acid by the bacteria present in milk. . The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain.
. Where does the water present in the egg go after boiling the egg? Life is possible due to the coordination of various chemical reactions in living organisms. An example is the digestion of food, absorption of appropriate molecules and ultimately production of energy.
This process involves a sequence of reactions and all these reactions occur in the body under very mild conditions. This occurs with the help of certain