ACT_SITE 155; /evidence="ECO:0000250|UniProtKB:P27695"; ACT_SITE 196; /note="Proton donor/acceptor"; /evidence="ECO:0000250|UniProtKB:P27695"; ACT_SITE 303; /note="Proton acceptor"; /evidence="ECO:0000255|PROSITE-ProRule:PRU00764"