MCQs

In alkaline conditions, monosaccharides undergo keto-enol tautomerism (Lobry de Bruyn–Alberta van Ekenstein transformation). For example, glucose can form an enediol intermediate that can then convert to either glucose, fructose, or mannose. This results in the epimerization of glucose to mannose at C-2.

Sugars are polar and partition between an organic mobile phase and a water stationary phase held by the paper. Slight differences in structure (e.g., number of -OH groups) cause them to partition differently and thus migrate at different rates, allowing for separation and identification.

The enzyme glucose (xylose) isomerase catalyzes the reversible isomerization of glucose to the sweeter fructose. This process is used to convert a portion of the glucose from corn starch into fructose, creating high-fructose corn syrup (HFCS).

Pancreatic α-amylase, like salivary amylase, is an endoglycosidase that hydrolyzes internal α-1,4 bonds. It cannot cleave α-1,6 bonds at branch points. Therefore, the products are the disaccharide maltose, the trisaccharide maltotriose, and oligosaccharides containing branch points called α-limit dextrins.

Chitin is the primary structural polysaccharide in the cell walls of fungi, the exoskeletons of arthropods, and the beaks of cephalopods. It provides rigidity and strength, analogous to the role of cellulose in plants.

The D/L nomenclature is a convention based on glyceraldehyde. A sugar is D if the -OH group on the chiral carbon farthest from the carbonyl group (the highest numbered chiral center) is drawn on the right side in a standard Fischer projection.

The vitrification (glass formation) hypothesis suggests that trehalose forms a stable glassy matrix. Its many -OH groups replace the hydrogen bonds normally provided by water to the polar head groups of membrane phospholipids and to the protein surface, maintaining their native conformation during dry periods.

Branching frequency is measured by the ratio of α-1,6 branch points to α-1,4 linkages in the linear chain. Glycogen has a higher ratio than amylopectin, with branches occurring roughly every 8-12 glucose units compared to every 24-30 in amylopectin.

The α-1,4 linkage in starch produces a helical chain that is flexible and digestible. The β-1,4 linkage in cellulose produces a straight, rigid chain that can form strong inter-chain hydrogen bonds, making it structural and indigestible to organisms lacking cellulase.

A five-membered ring (furanose) forms when the keto group on C-2 of a ketose reacts with the hydroxyl group on C-5. This is the predominant ring form for fructose in solution, creating a fructofuranose structure.

Inulin is a polysaccharide of D-fructose linked by β-2,1 glycosidic bonds. It is not metabolized and is freely filtered at the glomerulus without being reabsorbed or secreted, making it an ideal marker for measuring the glomerular filtration rate (GFR).

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.