  2.8.1.13 | a [protein]-S-sulfanyl-L-cysteine + uacil34 in tRNA + ATP + reduced acceptor = a [protein]-L-cysteine + 2-thiouracil34 in tRNA + AMP + diphosphate + acceptor |
on enzyme activation, an alpha-helix overhanging the active site is restructured into an idiosyncratic beta-hairpin-containing loop, which packs the flipped-out U34 deeply into the catalytic pocket and triggers the activation of the catalytic cysteine residues. The adenylated RNA intermediate is trapped. The active closed-conformation of the complex ensures accurate sulfur incorporation into the activated uridine carbon by forming a catalytic chamber to prevent solvent from accessing the catalytic site |
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