Govt Exams
Sigmoidal (S-shaped) kinetics indicate positive cooperativity, typical of allosteric enzymes with multiple subunits (e.g., aspartate transcarbamoylase). Binding of substrate to one subunit increases affinity in others.
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.
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.
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.
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.
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.
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.
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.
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.
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.