title: Fe(2+)-catalyzed oxidation and cleavage of sarcoplasmic reticulum ATPase reveals Mg(2+) and Mg(2+)-ATP sites.
abstract: Fe(2+) can substitute for Mg(2+) in activation of the sarcoplasmic reticulum (SR) ATPase, permitting approximately 25% activity in the presence of Ca(2+). Therefore, we used Fe(2+) to obtain information on the binding sites for Mg(2+) and the Mg(2+)-ATP complex within the enzyme structure. When the ATPase is incubated with Fe(2+) in the presence of H(2)O(2) and/or ascorbate, specific patterns of Fe(2+)-catalyzed oxidation and cleavage are observed in the SR ATPase, depending on its Ca(2+)-bound (E1-Ca(2)) or Ca(2+)-free conformation (E2-TG), as well as on the presence of ATP. The ATPase protein in the E1-Ca(2) state is cleaved efficiently by Fe(2+) with H(2)O(2) and ascorbate assistance, yielding a 70-75 kDa carboxyl end fragment. Cleavage of the ATPase protein in the E2-TG state occurs within the same region, but with a more diffuse pattern, yielding multiple fragments within the 65-85 kDa range. When Fe(2+) catalysis is assisted by ascorbate only (in the absence of H(2)O(2)), cleavage at the same protein site occurs much more slowly, and is facilitated by ATP (or AMP-PNP) and Ca(2+). Amino acid sequencing indicates that protein cleavage occurs at and near Ser346, and is attributed to Fe(2+) bound to a primary Mg(2+) site near Ser346 and neighboring Glu696. In addition, incubation with Fe(2+) and ascorbate produces Ca(2+)- and ATP-dependent oxidation of the Thr441 side chain, as demonstrated by NaB(3)H(4) incorporation and analysis of fragments obtained by extensive trypsin digestion. This oxidation is attributed to bound Fe(2+)-ATP complex, as shown by structural modeling of the Mg(2+)-ATP complex at the substrate site.
authors.affiliation: Institute of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, 21201, USA.
authors.forename: Suming
authors.initials: S
authors.lastname: Hua
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Inesi
authors.forename: Hiromi
authors.initials: H
authors.lastname: Nomura
authors.forename: Chikashi
authors.initials: C
authors.lastname: Toyoshima
mesh_terms: D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D017301:Ca(2+) Mg(2+)-ATPase / Q000737:chemistry* / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D004789:Enzyme Activation; D007501:Iron / Q000494:pharmacology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008958:Models, Molecular; D018482:Muscle, Skeletal / Q000201:enzymology; D010084:Oxidation-Reduction; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D000255:Adenosine Triphosphate; D007501:Iron; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium