Govt. Exams
Entrance Exams
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.
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 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.
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.
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.
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.
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.
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.
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.
Aconitase requires an iron-sulfur cluster [4Fe-4S] for catalyzing the isomerization of citrate to isocitrate. This is essential for its catalytic mechanism in the TCA cycle.
