The flavin-containing enzyme monoamine oxidase (MAO) is essential for the enzymatic decomposition of amine neurotransmitters. The exact mechanism of the oxidative deamination of amines to aldehydes by the enzyme has not yet been fully understood despite extensive research on the area. The rate limiting step is the reductive half-reaction where the Ha together with two electrons of the amine substrate is transferred to the flavin cofactor. However, it is still not known whether the hydrogen is transferred as a proton or a hydride. Experimental results cannot be fully explained by either of those mechanisms. In our previous work, theoretical results based on QM/MM calculations of the full enzyme show an intermediate situation between these two cases. In this paper, we report on an in-depth computational analysis concerning the role of the enzymatic environment for the reaction mechanism of human MAO-B with different p-substituted benzylamines as substrates. Our results show that steric and electrostatic effects from the active site environment turn the mechanism closer to an asynchronous polar nucleophilic mechanism. We found indications that the protein environment of MAO-A enhances the polar nucleophilic character of the mechanism compared to that of MAO-B.