title: Channeling in sulfate activating complexes.
abstract: The synthesis of activated sulfate (adenosine 5'-phosphosulfate, APS) and inorganic pyrophosphate from ATP and SO4 is remarkably unfavorable: K(eq) approximately 10(-8) under presumed, near-physiological conditions. Consequently, ATP sulfurylases, which catalyze APS synthesis, suffer approximately 10(8)-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 III reveal a 75 A-long channel that interconnects active-site pairs in the complex and that opens and closes in response to occupancy of those sites.
authors.affiliation: The Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, New York 10461-1926, USA.
authors.forename: Meihao
authors.initials: M
authors.lastname: Sun
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Leyh
mesh_terms: D000250:Adenosine Phosphosulfate / Q000378:metabolism*; D000818:Animals; D006801:Humans; D007700:Kinetics; D008958:Models, Molecular; D009169:Mycobacterium tuberculosis / Q000201:enzymology; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000737:chemistry; D010724:Phosphoadenosine Phosphosulfate / Q000378:metabolism; D011817:Rabbits; D012242:Rhodobacter sphaeroides / Q000201:enzymology; D055672:Static Electricity; D013431:Sulfates / Q000378:metabolism*
chemical_list: D013431:Sulfates; D010724:Phosphoadenosine Phosphosulfate; D000250:Adenosine Phosphosulfate; D050862:Oxidoreductases Acting on Sulfur Group Donors; C019618:adenylylsulfate reductase