Biochemistry - MCQ Practice Questions

Phenylalanine hydroxylase requires tetrahydrobiopterin (BH4) as a cofactor for the hydroxylation of phenylalanine to tyrosine. Mutations in this enzyme or BH4 synthesis lead to PKU, causing intellectual disability if untreated.

PP2A is a major serine/threonine phosphatase that plays crucial roles in reversing kinase-mediated phosphorylation. Its dysfunction is associated with various cancers, making it an important tumor suppressor enzyme.

Vitamin K-dependent carboxylation of glutamate residues in prothrombin and other clotting factors creates γ-carboxyglutamate residues that coordinate Ca2+ ions, essential for binding to phospholipid membranes and clotting cascade progression.

A Hill coefficient of 1.0 indicates no cooperativity and follows simple Michaelis-Menten kinetics. n > 1 indicates positive cooperativity (like hemoglobin), while n < 1 indicates negative cooperativity.

In zero-order kinetics (when substrate >> Km), all enzyme active sites are saturated. Velocity is directly proportional to enzyme concentration since V = kcat[E]total when enzyme is the limiting factor.

PDI catalyzes the formation, reduction, and rearrangement of disulfide bonds. In the oxidizing ER environment, PDI helps misfolded proteins achieve correct disulfide bonding patterns, acting as both an isomerase and a chaperone to prevent aggregation.

In serine proteases like trypsin and chymotrypsin, histidine (His57) acts as a general acid-base catalyst. Its imidazole ring (pKa ~6) can both accept and donate protons, facilitating the nucleophilic attack by the serine hydroxyl group on the carbonyl carbon.

Hyperammonemia type 1 (Reye's syndrome-like) results from CPS I deficiency, the first enzyme in the urea cycle. CPS I catalyzes ammonia fixation to form carbamoyl phosphate. Its deficiency prevents ammonia detoxification, causing neurotoxic accumulation.

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.

In competitive inhibition, the inhibitor competes with substrate for the active site. This increases the apparent Km (requires more substrate to reach half-maximal velocity) while Vmax remains unchanged.

Kwashiorkor results from severe protein malnutrition involving deficiency of all essential amino acids, leading to loss of muscle mass and edema despite adequate calorie intake.

The Ramachandran plot shows allowed and disallowed combinations of φ and ψ angles for the polypeptide backbone, used to verify the stereochemical quality of 3D protein structures.