passages.id: split_4_validation_1218_passage
passages.type: progene_text
passages.text: From the results , the conserved water molecule , corresponding to W206 in the E. coli DHFR complexes , that is H - bonded to both the OD2 oxygen atom of the carboxyl ( Asp ) side chain and O4 of the pterin / dihydropterin ring , appears critically important and may determine the protonation site for the enzyme - bound substrates .