S-Adenosylmethionine:tRNA ribosyltransferase-isomerase (, QueA enzyme, queuosine biosynthesis protein QueA) is an enzyme with systematic name S-adenosyl-L-methionine:7-aminomethyl-7-deazaguanosine ribosyltransferase (ribosyl isomerizing; L-methionine, adenine releasing). This enzyme catalyses the following chemical reaction
S-adenosyl-L-methionine + 7-aminomethyl-7-carbaguanosine34 in tRNA L-methionine + adenine + epoxyqueuosine34 in tRNA
The reaction is a combined transfer and isomerization of the ribose moiety of S-adenosyl-L-methionine to the modified guanosine base in the wobble position in tRNAs specific for Tyr, His, Asp or Asn.
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