Question

2. Sugars are essential components of nucleic acids.

a) Provide the precise chemical description (anomer, isomer and either pyranose or furanose form) of the circular stereoisomers that are formed by ribose in cells.

b) Which circular stereoisomer (anomer, isomer and either pyranose or furanose form) is found in DNA?

c) Using the language of the unit, give an explanation whether the sugar components of DNA are reducing sugars? . (no more than 50 words; use key words).

Answer 1

Answer

2 a) Isomers are the chemical compounds which have same molecular formula, but differ in their chemical structures. Eg: glucose and fructose. Glucose is an aldose sugar and fructose is a keto-sugar. Both have the same molecular formula, C₆H₁₂O₆.

If these chemical structures differ in the spatial arrangement of the atoms, they are said to be stereo-isomers. An epimer differs in the spatial arrangement of the atoms at the asymmetric carbon atom, that is carbon attached to four different groups in its valency.

An anomer is also an epimer which differs in the arrangement at its hemiacetal or acetal carbon atom. They exist as alpha and beta forms. Mostly observed in cyclic carbohydrates.

b) Most of the carbohydrates exist in equilibrium among their open chain and ring structures.

These ring structures may be a five-membered ring or six-membered ring. The five-membered ring is the furanose form of ring structure, whereas the six-membered ring is the pyranose form of ring structure. Furan ring contains four carbons and one oxygen atom. Pyran ring contains five carbons and one oxygen atom.

Sugars or carbohydrates containing 6 carbons exhibits pyranose form of ring structure and sugars with 5 carbons exhibits furanose form of the ring structure.

Ribose sugar is a five-carbon sugar, with the molecular formula C₅H₁₀O₅ that exhibits furanose and anomeric form in the cells. It is found mostly in the ribonucleic acid (RNA).

Deoxyribose nucleic acid (DNA) is a nucleic acid containing a five-carbon sugar called deoxyribose sugar. It also exists in furanose form.

Deoxy-ribose sugar and ribose sugar differ in their 2-carbon. Ribose sugar have hydroxyl group and hydrogen atom at the second carbon atom, whereas deoxyribose sugar has two hydrogen atoms at its second carbon atom.

c) The molecules which are capable of donating hydrogen atoms to other molecule in a reaction are called reducing compounds. The sugar in RNA is a ribose sugar with a functional reducing hydroxyl group at its second carbon.

The sugar in DNA is deoxyribose sugar, do not have a functional reducing group such as hydroxyl-group at its second carbon. So, it is a not a reducing sugar.