 "(In panel 3 of Figure 5)"
The COOH group of serine is positioned tightly in the SDH enzyme so that the serine molecule is perpendicular to the PLP pi system.The R group OH group participates in two hydrogen bonds with SDH's Ala222 and the protonated phosphate of PLP.The protonated phosphate of PLP then acts as an acid and donates its proton to hydroxyl of serine.