৪৯তম বিসিএস ⎯ প্রাণ রসায়ন [৬০১]

The correct answer is D.
Monosaccharides (from Greek mono, "one," and sacchar, "sugar") are the fundamental building blocks of carbohydrates. Disaccharides, oligosaccharides, and polysaccharides are all made of two or more monosaccharide units linked together and can be broken down (hydrolyzed) into these simpler units.

Explanation: The correct answer is B. Glucose has an aldehyde functional group at C-1, making it an aldose. It also contains six carbon atoms in its backbone, making it a hexose. Therefore, it is an aldohexose.

Explanation: The correct answer is C. Enantiomers are stereoisomers that are perfect, non-superimposable mirror images, like a left hand and a right hand. In carbohydrates, this is represented by the D- and L- configurations. Epimers (A) and anomers (B) are types of diastereomers (D), which are stereoisomers that are not mirror images of each other.

Explanation: The correct answer is B. When a pure anomer (e.g., α-D-glucose) is dissolved in water, its specific optical rotation gradually changes until it reaches a stable equilibrium value. This change is due to the ring opening to the linear aldehyde form and then re-closing to form either the α or β anomer. This dynamic equilibrium between the anomers is called mutarotation.

Explanation: The correct answer is C. Mild oxidizing agents like bromine water specifically oxidize the aldehyde group (at C-1) of an aldose to a carboxylic acid group. The resulting molecule is called an aldonic acid. For glucose, this product is gluconic acid. Stronger oxidation (e.g., with nitric acid) would oxidize both C-1 and C-6 to form glucaric acid (B).

Explanation: The correct answer is A. Maltose, also known as malt sugar, is a disaccharide formed from two units of α-D-glucose joined by a covalent α(1→4) glycosidic bond. This means the anomeric carbon (C-1) of the first glucose is linked to the hydroxyl group on C-4 of the second glucose.

Explanation: The correct answer is B. Amylose is an unbranched, helical polymer of glucose units linked by α(1→4) glycosidic bonds. Amylopectin is the branched component, having a backbone of α(1→4) linkages with branch points created by α(1→6) glycosidic bonds every 24-30 residues.

Explanation: The correct answer is C. Cellulose is a linear polymer of glucose units linked by β(1→4) glycosidic bonds. The human enzyme amylase is specific for α-linkages, like those found in starch. We lack the enzyme cellulase needed to break down the β-linkages in cellulose, which is why it passes through our digestive system as dietary fiber.

Explanation: The correct answer is B. Glycogen's structure is very similar to that of amylopectin. Both are highly branched polymers of glucose with α(1→4) linkages in the main chains and α(1→6) linkages at branch points. The key difference is that glycogen is even more highly branched than amylopectin, with branches occurring every 8-12 glucose units.

Explanation: The correct answer is B. Mucopolysaccharides, now more commonly called glycosaminoglycans (GAGs), are long, unbranched polysaccharides consisting of repeating disaccharide units. Each unit typically contains an amino sugar (like N-acetylglucosamine) and a uronic acid (like glucuronic acid). Their negative charges and ability to bind water make them excellent lubricants and shock absorbers in connective tissues.

Explanation: The correct answer is B. A glycosidic bond forms when the hydroxyl group on the anomeric carbon of one sugar reacts with a hydroxyl group on another molecule. This process eliminates a molecule of water (H2​O) and is therefore a condensation or dehydration reaction. Hydrolysis (A) is the reverse process, where water is used to break a glycosidic bond.

The correct answer is D.
Both glucose and fructose have the same molecular formula, C6​H12​O6​. However, they differ in the connectivity of their atoms: glucose has an aldehyde group, while fructose has a ketone group. Molecules with the same formula but different connectivity are constitutional isomers (or structural isomers).

Explanation: The correct answer is B. Hyaluronic acid is a prominent glycosaminoglycan (mucopolysaccharide), but it is unique because it is non-sulfated. Most other GAGs, like chondroitin sulfate and keratin sulfate, contain sulfate groups that contribute to their high negative charge.

Explanation: The correct answer is A. A reducing sugar must have a hemiacetal or hemiketal group, which can exist in equilibrium with the open-chain form containing a free aldehyde or α-hydroxyketone. This aldehyde/ketone group can be oxidized by the testing reagent (e.g., Cu2+ in Benedict's solution), while the reagent itself is reduced, causing a visible color change. This property is directly tied to a free anomeric carbon.

Explanation: The correct answer is B. Enzymes like glycogen phosphorylase release glucose units from glycogen by acting on its ends. The numerous branches create a multitude of non-reducing ends. This allows for the simultaneous release of many glucose molecules, providing a rapid burst of energy when the body needs it, such as during exercise. While it also contributes to compactness (A), the primary metabolic advantage is the rapid mobilization of glucose.

Explanation: The correct answer is B. Hemoglobin, found in red blood cells, is responsible for transporting oxygen from the lungs to the tissues. Collagen (A) is a structural protein, insulin (C) is a regulatory hormone, and amylase (D) is a catalytic enzyme.

Explanation: The correct answer is C. Lysine has a side chain containing a primary amino group, which accepts a proton and is positively charged at physiological pH (around 7.4), making it basic. Aspartate (A) is acidic, Leucine (B) is nonpolar (hydrophobic), and Serine (D) is polar, uncharged.

Explanation: The correct answer is D. In Proline, the side chain forms a covalent bond with the backbone α-amino group, creating a rigid five-membered ring. This makes the backbone nitrogen a secondary amine (R2​NH) and introduces a characteristic kink in polypeptide chains.

Explanation: The correct answer is A. The primary (1°) structure is the unique linear sequence of amino acids joined by peptide bonds. This sequence dictates all higher levels of protein structure and, ultimately, its function.

Explanation: The correct answer is C. These secondary (2°) structures are formed and stabilized by hydrogen bonds between the carbonyl oxygen atom of one peptide bond and the amide hydrogen atom of another peptide bond within the polypeptide backbone. Side chains are not directly involved in this stabilization.

Explanation: The correct answer is C. The hydrophobic effect is the major driving force. In an aqueous environment, the protein folds to bury its nonpolar (hydrophobic) amino acid side chains in the core, away from water, while exposing its polar (hydrophilic) side chains on the surface.

Explanation: The correct answer is B. The human body cannot synthesize essential amino acids (e.g., Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Leucine, Lysine) de novo. Therefore, they are an essential component of the diet.`

Explanation: The correct answer is B. Ninhydrin is a chemical reagent that reacts with the primary amino group of an amino acid to produce a deep purple product called Ruhemann's purple. It is a common method for detecting and quantifying amino acids.

Explanation: The correct answer is C. The Edman degradation procedure sequentially cleaves, removes, and identifies the N-terminal amino acid of a peptide without disrupting the rest of the chain. By repeating this cycle, the entire sequence can be determined.

Explanation: The correct answer is C. Quaternary (4°) structure only applies to proteins composed of two or more polypeptide chains (subunits). It describes how these individual subunits are arranged and interact to form the final, functional protein complex (e.g., hemoglobin, which has four subunits).

Explanation: The correct answer is C. The isoelectric point (pI) is the specific pH at which an amino acid has no net electrical charge. At this point, the amino group is protonated (−NH₃⁺​) and the carboxyl group is deprotonated (−COO−), forming a dipolar ion known as a zwitterion.

Explanation: The correct answer is B. Fibrous proteins are typically long, water-insoluble, and serve structural roles. Keratin (in hair and nails) and collagen (in skin and bones) are classic examples, providing strength and support.

Explanation: The correct answer is C. Enzymes are proteins (and some RNA molecules) that function as biological catalysts. They dramatically increase the rate of biochemical reactions without being consumed in the process.

Explanation: The correct answer is C. A disulfide bond (-S-S-) is a covalent linkage formed from the oxidation of the sulfhydryl or thiol groups (-SH) on the side chains of two cysteine residues. This bond is crucial for stabilizing the tertiary structure of many secreted proteins.

Explanation: The correct answer is B. Tyrosine has a side chain consisting of a benzene ring with a hydroxyl group attached (a phenol group), making it both aromatic and polar. Phenylalanine (A) is aromatic but nonpolar, and Tryptophan (C) has a bulkier indole ring.

Explanation: The correct answer is C. Before the individual polypeptide chains can be separated and sequenced, the covalent disulfide bonds that link them must be broken. This is typically achieved using a reducing agent like β-mercaptoethanol or DTT.

Explanation: The correct answer is B. The polypeptide backbone in a β-sheet is nearly fully extended. To minimize steric interference, the side chains of adjacent amino acids point in opposite directions, alternating above and below the sheet's plane.

Explanation: The correct answer is B. This phenomenon is called cooperative binding, an example of allostery. The binding of oxygen to one subunit induces a conformational change in the entire hemoglobin tetramer, increasing the affinity of the remaining subunits for oxygen.

Explanation: The correct answer is D. Phenylalanine is an essential amino acid, meaning it must be obtained from the diet, and it contains an aromatic benzene ring in its side chain. Tyrosine (C) is aromatic but can be synthesized from phenylalanine, making it conditionally non-essential.

Explanation: The correct answer is C. The peptide bond (C−N) has about 40% double-bond character due to resonance of electrons between the carbonyl oxygen and the amide nitrogen. This restricts free rotation around the bond, forcing the atoms of the peptide group into a rigid, planar conformation, which is fundamental to protein folding.

Explanation: The correct answer is B. Lactose is composed of one molecule of D-galactose joined to one molecule of D-glucose by a β(1→4) glycosidic bond. Its systematic name is β-D-galactopyranosyl-(1→4)-D-glucopyranose.

Explanation: The correct answer is C.
Chitin is a long-chain polymer of N-acetylglucosamine, a derivative of glucose. It is structurally analogous to cellulose, with β(1→4) linkages that form strong fibers, but with an acetylated amino group on the C-2 carbon of each monomer.

Explanation: The correct answer is A. A sugar is "reducing" if its anomeric carbon is free to form an open-chain aldehyde. In sucrose, the glycosidic bond links the anomeric carbon of glucose (C-1) and the anomeric carbon of fructose (C-2). Since both anomeric carbons are locked in the bond, the rings cannot open, and the sugar is non-reducing.

Explanation: The correct answer is C. Reducing agents convert the aldehyde group of an aldose into a primary alcohol. The reduction of D-glucose produces the corresponding sugar alcohol (alditol), which is named D-Glucitol, more commonly known as sorbitol.

Explanation: The correct answer is C. Heparin is a glycosaminoglycan (or mucopolysaccharide) that is extremely rich in sulfate groups, giving it the highest negative charge density of any known biomolecule. This high charge is crucial for its function in binding and activating antithrombin III to prevent blood clotting.

Explanation: The correct answer is B. The α(1→4) glycosidic bonds in amylose cause it to coil into a left-handed helix. The interior of this helix is perfectly sized to accommodate iodine molecules (I2​ and I3−​), forming a charge-transfer complex that absorbs light and produces the characteristic deep blue-black color of the starch-iodine test.

Explanation: The correct answer is A. A proteoglycan has a specific "bottle-brush" structure consisting of a core protein to which one or more long, linear glycosaminoglycan (GAG) chains are covalently attached. These molecules are extremely hydrated and are critical for the cushioning and lubrication of connective tissue.

Explanation: The correct answer is C. The aromatic side chains of Tryptophan and Tyrosine contain conjugated ring systems that strongly absorb UV light around 280 nm. Tryptophan has a much higher absorbance than tyrosine. This property is widely used to quickly estimate the concentration of a protein solution.

Explanation: The correct answer is B. Cooperativity is an allosteric effect that requires communication between multiple subunits. Hemoglobin is a tetramer (four polypeptide chains), allowing the binding of oxygen to one subunit to induce a conformational change that increases the affinity of the other subunits. Myoglobin, being a single chain (monomer), cannot exhibit this behavior.

Explanation: The correct answer is B. Salt bridges are electrostatic attractions between oppositely charged groups. At pH ~7.4, the acidic side chain of Aspartate is deprotonated (−COO−), carrying a negative charge, while the basic side chain of Lysine is protonated (−NH3+​), carrying a positive charge.

Explanation: The correct answer is C. Denaturation unfolds a protein by breaking the weak, non-covalent interactions (hydrogen bonds, hydrophobic interactions, ionic bonds) that stabilize the secondary, tertiary, and quaternary structures. The primary structure—the sequence of amino acids held together by strong, covalent peptide bonds—is not broken during this process.

Explanation: The correct answer is B. In SDS-PAGE, the detergent SDS denatures proteins and coats them with a uniform negative charge. This masks their intrinsic charge, causing them to migrate through the gel matrix based only on their size. Smaller proteins navigate the gel pores more easily and travel farther, allowing for separation by molecular mass.

Explanation: The correct answer is C. With a side-chain pKa of ~6.0, Histidine is unique because it can exist in both its protonated (charged) and deprotonated (neutral) forms at physiological pH (~7.4). This ability to mediate proton transfer makes it a vital catalytic residue in many enzymes, including chymotrypsin and carbonic anhydrase.

Explanation: The correct answer is C. Immunoglobulins, or antibodies, are the cornerstone of the adaptive immune system. Their function is to recognize and bind with high specificity to foreign molecules called antigens, thereby targeting them for elimination.

Explanation: The correct answer is D. A protein domain is a conserved part of a protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. In large proteins, the polypeptide chain often folds into several of these compact, stable domains, each associated with a particular function (e.g., a DNA-binding domain).