Amino acids can be classified as $\alpha-, \beta-, \gamma-, \delta$ - and so on depending upon the relative position of amino group with respect to carboxyl group. Which type of amino acids form polypeptide chain in proteins?
$\alpha$ amino acid forms polypeptide chain by elimination of water molecules.
$\alpha$-helix is a secondary structure of proteins formed by twisting of polypeptide chain into right handed screw like structure. Which type of interactions are responsible for making the $\alpha$-helix structure stable?
$\alpha$-helix is a secondary structure of proteins formed by twisting of polypeptide chain to right handed screw like structure. Hydrogen bonds formed between - NH- group of amino acids in one turn with the 
groups of amino acids belonging to adjacent turn is responsible for making the $\alpha$-helix structure stable.
Some enzymes are named after the reaction, where they are used. What name is given to the class of enzymes which catalyse the oxidation of one substrate with simultaneous reduction of another substrate?
Oxidoreductase enzymes A class of enzymes which catalyses the oxidation of one substrate with simultaneous reduction of another substrate is known as oxidoreductase enzymes.
During curdling of milk, what happens to sugar present in it?
Curdling of milk is caused due to formation of lactic acid by the bacteria present in milk. It is an example of denaturation of protein, i.e., when a protein is subjected is some physical or chemical chages. Hydrogen bond get disturbed. Globules unfold and helix uncoil and protein loss its biological acturty.
How do you explain the presence of five $-$OH groups in glucose molecule?
Glucose on reaction with acetic anhydride produces glucose pentaacetate.
This reaction explain presence of five $-$OH groups.