sentences:  Additionally, tyrosine residues are crucial to this mechanism in acting as stabilizing hydrogen acceptors.Class II proteins use a different mechanism which polarizes the carbonyl group with a divalent cation like Zn2+.The "Escherichia coli" galactitol operon protein, gatY, and N-acetyl galactosamine operon protein, agaY, which are tagatose-bisphosphate aldolase, are homologs of class II fructose-bisphosphate aldolase.