MCQs

Specificity is the ability of an enzyme to choose exactly one substrate from a pool of similar molecules. This is due to the exact complementary fit and specific chemical interactions (ionic, H-bonding, hydrophobic) between the substrate and the R-groups lining the active site.

The nitrogenous bases in double-stranded DNA are stacked and have a lower absorbance. When the double helix is denatured into two random, single-stranded coils, the bases become unstacked. This unstacking increases their absorbance of UV light at 260 nm, a phenomenon known as the hyperchromic effect.

With 20 different amino acids as monomers, the number of possible sequences and lengths for a polypeptide is astronomically large. Furthermore, the diverse chemical properties of the 20 R-groups (charged, polar, non-polar, etc.) enable a protein to fold into an immense variety of complex 3D shapes.

Bile salts are amphipathic cholesterol derivatives secreted from the liver. Their hydrophobic side associates with lipid droplets, and their hydrophilic side faces the aqueous intestinal fluid. This coating breaks large globules into smaller ones (micelles), vastly increasing the surface area for lipase action.

Many enzymes require metal ions (like Mg²⁺, Ca²⁺) as activators or cofactors. EDTA chelates (binds tightly to) these divalent cations, making them unavailable to the enzyme. This effectively inhibits the enzyme's activity. Adding back an excess of the metal ion reverses the inhibition.

The 2'-OH group in the ribose sugar of RNA is reactive. It can act as a nucleophile and attack the adjacent phosphodiester bond under alkaline conditions, leading to the self-hydrolysis (cleavage) of the RNA strand. DNA, lacking this 2'-OH, is far more chemically stable.

Metabolism is the sum of all cellular reactions. Catabolism is the breakdown of complex molecules into simpler ones, releasing energy. Anabolism is the synthesis of complex molecules from simpler ones, consuming energy. Their regulation is central to life.

When [S] is saturating (>> Km), all enzyme active sites are occupied, and the reaction rate is solely a function of how fast the enzyme can process substrate (Vmax). Vmax is proportional to the total enzyme concentration, so doubling the enzyme doubles V0 under these conditions.

tRNA acts as an adaptor, carrying a specific amino acid at its 3' end and recognizing a specific three-nucleotide codon on the mRNA through its complementary anticodon loop. This bridges the genetic code and the amino acid sequence of a protein.

On a double-reciprocal plot, a non-competitive inhibitor produces a line that intersects the control line at the x-axis (Km is unchanged), but has a steeper slope and a higher y-intercept (Vmax is decreased). It reduces the number of functional enzyme molecules.

When glucose forms a ring, carbon 1 becomes an asymmetric carbon (the anomeric carbon). In the α-anomer, the -OH on C1 is below the plane of the ring (trans to the CH2OH at C5). In the β-anomer, the -OH is above the plane of the ring (cis to the CH2OH).

Saponification is the base-catalyzed hydrolysis of the ester bonds in a fat or oil. This reaction cleaves the triglyceride, producing glycerol and the salts of the fatty acids (soaps). Lipases perform an analogous enzymatic hydrolysis.

Buffers are aqueous systems that resist changes in pH. The bicarbonate system (H2CO3/HCO3⁻) neutralizes small amounts of added acid or base, keeping the blood pH within the narrow physiological range (7.35-7.45) essential for enzyme function and protein stability.

Condensation (dehydration synthesis) is the universal anabolic reaction for building all major biological macromolecules: monosaccharides to polysaccharides, amino acids to proteins, and nucleotides to nucleic acids. Water is the byproduct of each new bond formed.

Protein folding is a cooperative process. The breaking of a few weak interactions in one region of the protein during heating can destabilize neighboring interactions. This leads to a rapid, domino-like collapse of the entire tertiary structure over a very small temperature range.

Heavy metals have high affinity for sulfur. They react with the thiol (-SH) groups of cysteine residues, forming mercaptides. This can block essential catalytic groups, disrupt disulfide bonds (if present), and severely distort the protein's tertiary and quaternary structure, leading to irreversible denaturation.

Water's dipole nature (O is δ-, H is δ+) allows it to form hydrogen bonds with and dissolve other polar and charged molecules. It forms hydration shells around these molecules (like sugars and amino acids), effectively separating them from their crystal lattice and bringing them into solution.

The enzymes responsible for the synthesis of phospholipids (the major membrane lipid) and steroids (including cholesterol and steroid hormones) are located primarily in the membrane of the smooth ER. It is therefore a major site of lipid biosynthesis.

The conformational change in the induced fit model positions the essential catalytic amino acid side chains in the precise orientation needed to perform chemistry on the substrate. This is in addition to the strain and proximity effects also associated with the model.