ACT_SITE 156; /evidence="ECO:0000250|UniProtKB:P27695"; ACT_SITE 197; /note="Proton donor/acceptor"; /evidence="ECO:0000250|UniProtKB:P27695"; ACT_SITE 304; /note="Proton acceptor"; /evidence="ECO:0000255|PROSITE-ProRule:PRU00764"