Govt. Exams
Entrance Exams
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.
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.
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.
Serine proteases have an extended substrate-binding site composed of multiple subsites (S1, S1', S2, etc.) that recognize and bind extended substrate peptides. The catalytic triad (Ser-His-Asp) performs the actual catalysis.
Turnover number (kcat) is the number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated. It equals Vmax/[E]total.
Negative cooperativity occurs when substrate binding to one subunit decreases the affinity of other subunits for substrate, resulting in a hyperbolic (rather than sigmoidal) binding curve.
When Km >> [S], the Michaelis-Menten equation simplifies to v = (Vmax/Km)[S], making the reaction essentially first-order. The enzyme has low affinity for substrate under these conditions.
