Single Amino Acid Mutation Decouples Photochemistry of the BLUF Domain from the Enzymatic Function of OaPAC and Drives the Enzyme to a Switched-on State

•The connection between the proton-coupled electron transfer process and the enzymatic activity of a photoactivated adenylate cyclase was studied by the means of ultrafast spectroscopy.•We characterized the impact of the replacement of a key glutamine amino acid which has a crucial role transmitting the signal from the BLUF domain of the protein towards the C-terminal where the ATP conversion takes place.•The replacement of this glutamine to a glutamic acid locked the protein in a switched on state producing cAMP in a light independent manner.•As cAMP is involved in numerous signal transduction pathways, modulating its cellular concentration puts forward the photoactivated adenylyl cyclases (PACs) in the optogenetics toolbox.