Abstract Methylation of glucosamine-6-phosphate isomerase deaminase (2-amino-2-deoxy- d-glucose-6-phosphate ketol-isomerase, deaminating, or glucosamine-6-phosphate deaminase, EC 126.96.36.199), from Escherichia coli produces a modified protein having two alkylated sulfhydryls per each polypeptide chain. The enzyme is still active and allosteric, but exhibits a lower homotropic cooperativity and its V max E total is almost exactly half that of the native enzyme. Arsenite produces comparable kinetic changes that can be reversed with ethanedithiol but not with 2-thioethanol or dialysis. Thiols can be oxidized by molecular oxygen using the (1,10-phenanthroline) 3-Cu(II) complex as catalyst; the enzyme obtained no longer has titrable SH groups with 5,5′-dithiobis(2-nitrobenzoic acid) and displays kinetic behavior similar to that of the other chemically modified forms of the deaminase using monofunctional or bifunctional reagents. The results reported indicate that the involved sulfhydryls are vicinal groups, and are located in a region of the molecule that moves as a whole in the allosteric transition.