Biochemistry - MCQ Practice Questions

Leucine has a nonpolar, hydrophobic isopropyl side chain. Serine and asparagine are polar, while lysine is positively charged.

Hemoglobin has quaternary structure consisting of 2 α-globin and 2 β-globin subunits held together by non-covalent interactions.

Proteases are endopeptidases and exopeptidases that hydrolyze peptide bonds in proteins. Amylase acts on carbohydrates, lipase on fats, and nuclease on nucleic acids.

The isoelectric point is the pH at which the net charge on the protein is zero, resulting in minimum solubility and maximum precipitation.

Cytochrome c oxidase contains heme a, heme a3, and copper centers (CuA and CuB) essential for electron transfer and oxygen reduction.

Cooperative binding (positive cooperativity) occurs when binding of one substrate molecule enhances the affinity of the enzyme for additional substrate molecules, as seen in hemoglobin and phosphofructokinase.

Chaperone proteins like HSP70 and HSP90 facilitate proper protein folding, prevent aggregation, and assist in maintaining protein stability, though some also work with degradation pathways.

The Lineweaver-Burk plot is a double reciprocal plot where 1/v is plotted against 1/[S], allowing easy determination of Km (x-intercept) and Vmax (y-intercept) from linear regression.

Km is the substrate concentration at which the reaction velocity is half of Vmax (V = Vmax/2), and it represents the affinity of enzyme for substrate. Lower Km indicates higher affinity.

In competitive inhibition, the inhibitor competes with substrate for the active site, so more substrate is needed to achieve half-maximal velocity (increased apparent Km), but Vmax remains unchanged.

In α-helix, hydrogen bonds form between C=O and N-H groups within the same chain. In β-sheet, hydrogen bonds form between adjacent polypeptide chains running parallel or antiparallel to each other.

Gaucher disease results from deficiency of β-glucosidase (glucocerebrosidase), leading to accumulation of glucocerebroside in lysosomes, particularly in macrophages, spleen, and liver.

Ubiquitin is a 76-amino acid protein that is conjugated to lysine residues of target proteins via E1, E2, and E3 enzymes, marking them for degradation by the 26S proteasome.

Glycine (every third residue) provides flexibility, proline stabilizes the polyproline II helix conformation, and hydroxyproline (formed by post-translational modification) stabilizes the triple helix through additional hydrogen bonding.

PFK exhibits allosteric regulation where AMP/ADP (signals of low energy) activate the enzyme, while ATP/citrate (signals of high energy/biosynthesis) inhibit it by binding to allosteric sites, changing enzyme conformation.

Ornithine transcarbamylase (OTC) catalyzes the condensation of carbamoyl phosphate (formed by CPS I) with ornithine to form citrulline, the second step of the urea cycle. OTC deficiency is the most common urea cycle disorder.

Alcohol dehydrogenase contains catalytic zinc that coordinates the hydroxyl group of ethanol/aldehyde substrate, activating it for hydride transfer to NAD+, and structural zinc that maintains protein stability.

Enzyme specificity varies: absolute (one substrate only), group (substrates with similar functional groups), linkage (specific types of bonds), and stereochemical (stereoisomers). Specificity results from 3D active site structure and orientation of catalytic residues.

The Cori cycle (glucose-lactate cycle) allows muscles undergoing anaerobic glycolysis to produce lactate, which is transported to liver and converted back to glucose via gluconeogenesis, maintaining blood glucose homeostasis during intense exercise.

Denaturation is disruption of non-covalent interactions (hydrogen bonds, hydrophobic interactions, ionic bonds) that maintain higher-order structures. Peptide bonds (primary structure) remain intact. Some proteins can refold (renature) if conditions permit, as demonstrated by Anfinsen's ribonuclease experiments.