ATP adenylyltransferase () is an enzyme that catalyzes the chemical reaction:
The enzyme characterised from Saccharomyces cerevisiae and Myxococcus xanthus combines adenosine triphosphate and adenosine triphosphate to give Ap4A, with orthophosphate (Pi) as a byproduct. Ap4A has been suggested to be an alarmone, a compound produced when an organism is damaged or exposed to heat stress.
This enzyme is a transferase, specifically one transferring phosphorus-containing nucleotide groups (nucleotidyltransferases). The systematic name of this enzyme class is ADP:ATP adenylyltransferase. Other names in common use include bis(5'-nucleosyl)-tetraphosphate phosphorylase (NDP-forming), diadenosinetetraphosphate alphabeta-phosphorylase, adenine triphosphate adenylyltransferase, diadenosine 5',5'"-P1,P4-tetraphosphate alphabeta-phosphorylase, (ADP-forming), and dinucleoside oligophosphate alphabeta-phosphorylase.
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