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. Author manuscript; available in PMC: 2011 Jun 29.
Published in final edited form as: Biochemistry. 2010 Jun 29;49(25):5154–5159. doi: 10.1021/bi902172n

Scheme 1.

Scheme 1

Simplified catalytic cycle of SO and SDH. Stage 1 corresponds to the resting state of the enzyme. The pathway 1→2→3→1 corresponds to the normal catalytic turnover. Depending on the buffer pH, Stage 3 corresponds to the hpH or lpH form of the active center. Stage 3A (typically observed at low pH < 7) corresponds to the blocked form of the active center. In wt vertebrate SO, Stage 3A is generated in chloride-depleted samples at low pH. Addition of chloride results in hydrolysis of sulfate (transition 3A→3).