In comparison to previously studied halogenases, barriers for isomerization between axial and equatorial Fe-oxo isomers is higher for hydroxylases by ca. 5 kcal/mol, suggesting that isomerization from axial oxo to equatorial oxo is more difficult in hydroxylases relative to halogenases. As the size of the carboxylate ligand played a role in distinguishing the relative isomer energetics of halogenases and hydroxylases (see Sec. 4.1), it may similarly affect differences in isomerization barriers. Isomerization of the oxo moiety forces Glu to change its orientation along the RC. We will examine the possibility that this might result in Glu having unfavorable orientations during oxo isomerization, leading to higher isomerization barriers in Sec.