The synthesis of activated sulfate (adenosine 5′-phosphosulfate, APS) and inorganic pyrophosphate from ATP and SO4 is remarkably unfavorable: Keq ∼ 10-8 under presumed, near-physiological conditions. Consequently, ATP sulfurylases, which catalyze APS synthesis, suffer ∼108-fold losses in catalytic efficiency in the forward (APS-synthesis) versus reverse reaction. Losses of this magnitude place this catalyst at risk of being unable to supply its nutrients to the cell in a timely fashion. ATP sulfurylase domains are often embedded in multifunctional complexes that are capable of also catalyzing the second of two steps in the sulfate activation pathway: the phosphorylation of APS to produce PAPS (3′-phosphoadenosine 5′-phosphosulfate). The colocalization of these activities in a single scaffold suggests that evolution might have worked around the inefficiency problem by fashioning a system capable of transferring APS directly between the active sites of the complex, thereby avoiding the solution-phase energetics. For these reasons, representatives from each of the three types of sulfate activating complex (SAC) [Homo sapiens (type I); Mycobacterium tuberculosis (type II); and Rhodobacter sphaeroides (type III)] were tested for the ability to channel APS. A channeling assay that optically detects solution-phase APS was devised with APS reductase from M. tuberculosis, a previously uncharacterized enzyme. Channeling was not detected in two of the three types of SAC; however, the type III SAC channels with high efficiency. Structural models of type HI reveal a 75 Å-long channel that interconnects active-site pairs in the complex and that opens and closes in response to occupancy of those sites.
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