MCQs

Lysozyme specifically targets the β-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in the peptidoglycan layer of bacterial cell walls. This bond cleavage weakens the cell wall and causes bacterial lysis.

The 20 common amino acids all share a common backbone (amino group, α-carbon, carboxyl group) but differ only in their side chain, the R-group. The size, shape, charge, hydrophobicity, and chemical reactivity of the R-group confer the unique properties to each amino acid.

A longer saturated hydrocarbon chain has a greater surface area for van der Waals interactions with neighboring chains, requiring more thermal energy (higher temperature) to disrupt these interactions and melt. Unsaturation, conversely, introduces kinks that lower the melting point.

Except for methionine and tryptophan, all 18 other amino acids are encoded by 2 to 6 synonymous codons. This property is called degeneracy and provides a buffer against the harmful effects of point mutations.

The conformational change of the enzyme upon substrate binding physically distorts the substrate molecule. This "strain" on specific bonds makes them less stable and closer to the transition state, thereby reducing the energy required to break them (the activation energy).

Histones are basic proteins that associate with and neutralize the negative charge of the DNA phosphate backbone. The DNA wraps around an octamer of histone proteins to form a nucleosome, the fundamental unit of chromatin packaging, allowing the long DNA molecule to be compacted.

Zymogens (like pepsinogen to pepsin) are activated by the irreversible hydrolytic cleavage of a portion of their polypeptide chain. This proteolytic cut induces a conformational change that forms the functional active site. This mechanism prevents premature activity in the cell of origin.

This self-assembly is an entropically driven process. When phospholipids are mixed with water, they spontaneously arrange into bilayers or micelles to bury their hydrophobic fatty acid tails away from water, while the polar head groups interface with the aqueous environment.

An enzyme's catalytic function is entirely dependent on the specific 3D shape of its active site. Denaturation by heat, pH, or chemicals disrupts the weak bonds maintaining the tertiary structure, causing the active site to lose its precise conformation, preventing substrate binding.

The extensive branching of glycogen creates many non-reducing ends. Multiple glycogen phosphorylase enzymes can act simultaneously on these ends, allowing for a very rapid release of glucose monomers to meet high metabolic demands, particularly in muscles.

Coenzymes are organic carrier molecules that participate directly in the reaction. NAD+, for example, accepts a hydride ion (H⁻) to become NADH, effectively acting as an electron carrier. It is chemically changed during the reaction and must be regenerated. An enzyme cannot alter the reaction's equilibrium constant.

Cis-double bonds in unsaturated fatty acids create kinks that prevent the molecules from packing closely together. This reduces the intermolecular van der Waals forces, resulting in a lower melting point and a liquid state (oil) at room temperature.

While hydrogen bonds provide specificity, the planar, non-polar nitrogenous bases "stack" together via hydrophobic interactions to minimize their exposure to water. This base stacking is a major thermodynamic driving force for the stabilization of the DNA double helix.

A prosthetic group is a non-protein component that is covalently or very tightly, permanently bound to an enzyme. A coenzyme is an organic cofactor (often a vitamin derivative) that binds loosely and transiently. A zymogen is an inactive enzyme precursor.

During protein folding, hydrophobic R-groups tend to cluster in the protein's interior to avoid contact with the aqueous cellular environment (hydrophobic effect). Conversely, hydrophilic and charged R-groups are typically positioned on the surface where they can interact with water.

Human amylases are specific for the α-1,4 glycosidic bonds found in starch and glycogen. Cellulose consists of glucose monomers linked by β-1,4 glycosidic bonds, which requires the enzyme cellulase, an enzyme humans do not produce.

ATP is a modified nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and three phosphate groups. The anhydride bonds between the phosphates are "high-energy" bonds, making ATP the primary energy currency of the cell.

Fibrous proteins (e.g., collagen, keratin) have long, chain-like, repetitive secondary structures that form strong, water-insoluble fibers. Their primary role is structural support, contrasting with the soluble, dynamic, roughly spherical nature of globular proteins like enzymes and antibodies.

Complementarity is the specific pairing dictated by hydrogen bonding potential: adenine pairs only with thymine (or uracil), and guanine pairs only with cytosine. This ensures a purine always pairs with a pyrimidine, maintaining a consistent double helix structure.