Fig. 5. Natural enzymes with altered regioselectivity. Two natural enzymes, SxtT (green) and GxtA (purple) catalyze hydroxylation of β-saxitoxinol at two different sites. This is due to the different orientation of the substrate in the enzyme binding pocket. Residue R204 is involved in altering the orientation in SxtT relative to GxtA. Y198 is also positioned differently in SxtT enabling it to make a hydrogen bond with a water molecule that also interacts with the substrate.¹⁰⁷.