Photocycle alteration and increased enzymatic activity in genetically modified photoactivated adenylate cyclase OaPAC

Abstract

Photoactivable adenylate cyclases (PAC) are light activated enzymes that combine blue light sensing capacity with the ability to convert adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) and pyrophosphate (PPi) in a light-dependent manner. In most of the known PACs blue light regulation is provided by a BLUF domain which undergoes a structural reorganization after blue-light absorption. This minor structural change then is translated towards the C-terminal of the protein, inducing a larger conformational change that results in the ATP conversion to cAMP. As cAMP is a key second messenger in numerous signal transduction pathways regulating various cellular functions, photoactivable adenylate cyclases are of great interest in optogenetic studies. The optimal optogenetic device must be "silent" in the dark and highly responsive upon light illumination. OaPAC is a very good candidate as its basal activity is very small in the dark and the conversion rates increase 20-fold upon light illumination. We studied the effect of replacing D67 to N, in the BLUF domain. This mutation was found to accelerate the primary electron transfer process in the photosensing domain of the protein, as has been predicted. Furthermore, it resulted in a longer lived signalling state, which was formed with a lower quantum yield. Our studies show that the overall effects of the D67N mutation lead to a slightly higher conversion of ATP to cAMP, which points in the direction that by fine tuning the kinetic properties more responsive PACs and optogenetic devices can be generated.