Ricin A-Chain: Kinetic Isotope Effects and Transition State Structure with Stem-Loop RNA †

Ricin toxin A-chain (RTA) depurinates 28 S ribosomal RNA and small stem-loop RNAs at the first adenosine residue in a 5‘-GAGA-3‘ tetraloop. The transition state for depurination of stem-loop RNA by RTA was determined from kinetic isotope effects (KIEs). A stem-loop RNA, called A-10 (5‘-GGCGAGAGCC-3‘), was synthesized using isotopically labeled ATP. KIEs were measured for RNA substrates with adenylates containing [1‘-14C], [9-15N], …