On the basis of a large amount of experimental information, the reductive half-reaction of PSAO has been studied quantum mechanically using hybrid density functional theory (B3LYP). The suggested mechanism agrees very well with the one suggested experimentally and incorporates the observation of two different Schiff bases. The half-reaction is suggested to occur in six steps, and transition state structures connecting the different intermediates have been optimized. Two of these steps are further subdivided into two separate parts. The rate-limiting step is found to be the C-H activation of the substrate in agreement with experimental findings, and this step is found to be driven by a large gain of aromaticity in the TPQ ring. To make full use of this driving force, O2 of TPQ is suggested to be protonated prior to the C-H activation. A mechanism for how this protonation might occur is suggested. The roles of the TPQ cofactor and the critical Asp300 are discussed and compared to experimental observations and suggestions.