Question

i) Write the chemical names of water soluble vitamins and the diseases caused by their deficiency.
ii) Explain the cyclic structure of glucose. [2+3]
OR
i) Explain the primary, secondary, tertiary and quaternary structures of protein.
ii) Write a note on denaturation of protein. [4+1]

Hint:

Water soluble vitamins are the B-complex and vitamin C (e.g. C deficiency causes scurvy). Glucose forms a six-membered pyranose ring via a C-5 OH to C-1 aldehyde hemiacetal, giving the anomeric carbon. For proteins, recall primary sequence, secondary helix/sheet, tertiary 3-D fold, quaternary sub-units, and that denaturation destroys secondary and tertiary structure but not the peptide bonds.

Answer 1

Option 1

i) Water soluble vitamins: These are vitamins that dissolve in water and are therefore not stored in body fat; the body flushes out the excess, so a daily supply is needed. They comprise the vitamin-B complex together with vitamin C.
Step 1: Thiamine, \(B_1\) — its lack leads to Beri-beri.
Step 2: Riboflavin, \(B_2\) — its lack leads to cheilosis and glossitis.
Step 3: Niacin (nicotinic acid), \(B_3\) — its lack leads to Pellagra.
Step 4: Pyridoxine, \(B_6\) — its lack leads to anaemia and convulsions.
Step 5: Cobalamin, \(B_{12}\) — its lack leads to pernicious anaemia.
Step 6: Ascorbic acid, vitamin C — its lack leads to Scurvy.

ii) Cyclic (ring) structure of glucose:
Step 1: Pure glucose can exist as an open chain aldohexose, but many of its reactions cannot be explained by that structure alone; hence a ring form is proposed.
Step 2: The oxygen of the C-5 hydroxyl bridges across to the C-1 carbonyl carbon. This addition of an alcohol to an aldehyde produces a hemiacetal, and the resulting six-membered oxygen-containing ring is named the pyranose ring (drawn as a Haworth structure).
Step 3: The former carbonyl carbon C-1 now carries four different groups, so it is a new stereocentre known as the anomeric carbon. Depending on whether its –OH points below or above the ring plane we get the \( \alpha \) or the \( \beta \) anomer of D-glucose.
Step 4: Evidence for the ring: glucose fails to give the Schiff aldehyde test, does not form a hydrogen-sulphite addition product, and slowly changes its specific rotation in solution (mutarotation) as the two anomers equilibrate through the trace open-chain form.

Option 2

i) Levels of protein structure:
Primary: Fixes the identity of the protein — it is simply the linear order of amino acid residues joined head to tail by –CONH– (peptide) links.
Secondary: Describes how short stretches of that chain twist into regular local patterns held by backbone hydrogen bonds; the right-handed \( \alpha \)-helix and the \( \beta \)-pleated sheet are the two standard patterns.
Tertiary: Describes how the complete chain packs into a compact 3-D shape, locked by disulphide links, salt bridges, hydrogen bonds and hydrophobic contacts; this decides whether the protein is fibrous or globular.
Quaternary: Applies only to proteins made of more than one chain, and describes how these separate sub-units fit together into the working molecule, as in haemoglobin (four chains).

ii) Denaturation: Denaturation is the loss of the natural (native) shape of a protein when it meets heat, radiation, strong acids or bases, or heavy-metal ions. These agents snap the weak hydrogen bonds and other side-chain interactions, so the coiled and folded chain opens out. The secondary and tertiary arrangements collapse while the primary chain of peptide bonds survives unchanged, and the protein can no longer perform its function. Boiling an egg (soluble albumin turning into solid white) and the setting of milk into curd are everyday examples.