As shown in Figure , the L314H mutation of SeR positions the histidine sidechain in the substrate entry channel. The Histidine sidechain is oriented toward the enzyme's putative disulfide active site. Only 3 other residues (T310, H236, and S14) are closer to the active side disulfide, at distances of 3.5 Å, 3.8 Å, and 4.8 Å respectively. All 3 of these residues bear functional groups implicated in enzymatic catalysis.