Govt Exams
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.
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.
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.
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.
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.
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.
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.