Biochemistry - MCQ Practice Questions

Competitive inhibitors compete with substrate for the active site, increasing apparent Km while Vmax remains unchanged. Non-competitive inhibitors bind to a site other than the active site, decreasing Vmax without changing Km.

Collagen has a characteristic Gly-X-Y tripeptide repeat pattern where glycine appears at every third position. This allows tight packing in the triple helix structure. Hydroxyproline (formed by post-translational modification of proline) stabilizes the helix.

Aminoacyl-tRNA synthetases achieve high fidelity through a two-step selection process: initial selection during aminoacylation and a second proofreading step (editing) that hydrolyzes incorrectly charged tRNA-amino acid complexes before they leave the enzyme.

Allosteric enzymes like phosphofructokinase show cooperative binding where substrate binding at one subunit increases affinity at others. This produces an S-shaped curve rather than the hyperbolic Michaelis-Menten curve, allowing for better metabolic control.

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.

GroEL/GroES in prokaryotes forms a barrel-like structure that creates an isolated environment for protein folding. GroEL binds unfolded proteins using ATP hydrolysis, providing conformational assistance distinct from Hsp70's role in preventing aggregation.

Chymotrypsin is secreted as chymotrypsinogen. Trypsin (activated by enterokinase) cleaves a specific dipeptide from chymotrypsinogen to generate active chymotrypsin, which then undergoes autolytic cleavage for full activation.

Turnover number (kcat = Vmax/[E]total) represents the number of substrate molecules converted to product per enzyme molecule per unit time at maximum velocity. It indicates catalytic efficiency when substrate is saturating.

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.

Peptidyl transferase activity is catalyzed by the 23S rRNA (in prokaryotes) or 28S rRNA (in eukaryotes) component of the ribosome. This ribozyme catalyzes the formation of the peptide bond between the incoming aminoacyl-tRNA and the growing polypeptide chain.

IRE1α is a transmembrane protein with both kinase and RNase (endonuclease) domains. Upon ER stress (detected by dissociation from BiP), it autophosphorylates and uses its RNase domain to splice XBP1 mRNA, a key transcription factor in the UPR.

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.

The SCF (Skp1-Cullin1-F-box protein) complex, specifically SCF-β-TrCP, recognizes phosphorylated IκB and polyubiquitinates it for proteasomal degradation. This is a key regulatory step in the NF-κB inflammatory signaling pathway.

Transition state theory proposes that enzymes achieve catalysis by stabilizing the transition state more effectively than the substrate ground state. The differential stabilization lowers the activation energy barrier, accelerating the reaction.

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.

DNA ligase catalyzes the formation of phosphodiester bonds between adjacent DNA strands. In prokaryotes, it uses NAD+ as the energy source, while in eukaryotes, ATP is used. This is essential for DNA replication, repair, and recombination.