Hello all, i work on a protein which is a hydrolase and after experiments including pH titration via NMR we discovered, that one serine residue (in the active site) deprotonated at a pH of around 7. Usually Serine is neutral up to a pH of 10-11....i ruled out that serine deprotonates due to a catalytic triad, since i can see no Histidine/Lysin nor Aspartate/Glutamate close to this serine residue. Does anyone has an idea...what other reasons could lead to such a big pKa shift of serine?