Practice Questions

The synthesis of sucrose involves UDP-glucose as the glucosyl donor. The reaction is: UDP-glucose + Fructose-6-phosphate → Sucrose-6-phosphate + UDP. ADP-glucose is the activated monomer for starch synthesis.

Starch and glycogen are composed of α-D-glucose units, where the -OH on the anomeric carbon (C1) is below the plane of the ring. Cellulose consists of β-D-glucose, where the anomeric -OH is above the plane of the ring. This simple difference makes starch digestible and cellulose indigestible to humans.

The number of possible stereoisomers for a molecule with 'n' chiral centers is 2ⁿ. A sugar with 4 chiral centers has 2⁴ = 16 possible stereoisomers. These are divided into 8 D-sugars and their 8 L-enantiomeric counterparts.

Endoglycosidases cleave glycosidic bonds internally within a polysaccharide chain, producing shorter oligosaccharides (dextrins). In contrast, exoglycosidases cleave sugar residues one at a time from the non-reducing end of the chain.

Hyaluronic acid is a linear, unbranched polymer with repeating disaccharide units containing glucuronic acid (negatively charged). The polymer chains are long and rigid, and they trap a large volume of water, forming a viscous solution that acts as an excellent shock absorber and lubricant.

The anomeric carbon is derived from the carbonyl carbon of the open-chain form. In glucose, this is C-1. The nucleophilic attack by the C-5 hydroxyl group on this planar carbonyl carbon can occur from either face, creating the two possible anomeric configurations: α (-OH down) or β (-OH up).

The enzyme lactase (a β-galactosidase) is specific for the β-configuration of the galactose residue and the 1,4 linkage to glucose. Sucrase acts on the α,β-1,2 linkage of sucrose, and maltase acts on the α-1,4 linkage of maltose.

The "sweetness" is a function of binding affinity to the receptor. Fructose exists in solution in several forms, including a significant proportion of a sweet furanose form, which interacts more optimally with the receptor's binding site than the pyranose forms of glucose.

Peptidoglycan (murein) is unique to bacteria. Its carbohydrate backbone is a linear chain of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds. Lysozyme hydrolyzes this specific bond.

Fructose is phosphorylated to fructose-1-phosphate, which is cleaved to glyceraldehyde and dihydroxyacetone phosphate, entering glycolysis downstream of PFK-1. This bypasses a critical regulatory checkpoint, allowing a rapid, unregulated influx of carbon that can overload the TCA cycle and lead to increased de novo lipogenesis.