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Roles of Conserved P Domain Residues and Mg 2 + in ATP Binding in the Ground and Ca 2 +-activated States of Sarcoplasmic Reticulum Ca 2 +-ATPase *

Journal of Biological Chemistry, ISSN: 0021-9258, Vol: 279, Issue: 31, Page: 32515-32523
2004
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Residues in conserved motifs 625 T GD, 676 FAR XX P XX K, and 701 TG D GVN D in domain P of sarcoplasmic reticulum Ca 2+ -ATPase, as well as in motifs 601 D PPR and 359 N QR(/K)MSV in the hinge segments connecting domains N and P, were examined by mutagenesis to assess their roles in nucleotide and Mg 2+ binding and stabilization of the Ca 2+ -activated transition state for phosphoryl transfer. In the absence of Mg 2+, mutations removing the charges of domain P residues Asp 627, Lys 684, Asp 703, and Asp 707 increased the affinity for ATP and 2′,3′- O -(2,4,6-trinitrophenyl)-8-azidoadenosine 5′-triphosphate. These mutations, as well as Gly 626 → Ala, were inhibitory for ATP binding in the presence of Mg 2+ and for tight binding of the β,γ-bidentate chromium(III) complex of ATP. The hinge mutations had pronounced, but variable, effects on ATP binding only in the presence of Mg 2+. The data demonstrate an unfavorable electrostatic environment for binding of negatively charged nucleotide in domain P and show that Mg 2+ is required to anchor the phosphoryl group of ATP at the phosphorylation site. Mutants Gly 626 → Ala, Lys 684 → Met, Asp 703 → Ala/Ser/Cys, and mutants with alteration to Asp 707 exhibited very slow or negligible phosphorylation, making it possible to measure ATP binding in the pseudo-transition state attained in the presence of both Mg 2+ and Ca 2+. Under these conditions, ATP binding was almost completely blocked in Gly 626 → Ala and occurred with 12- and 7-fold reduced affinities in Asp 703 → Ala and Asp 707 → Cys, respectively, relative to the situation in the presence of Mg 2+ without Ca 2+, whereas in Lys 684 → Met and Asp 707 → Ser/Asn the affinity was enhanced 14- and 3–5-fold, respectively. Hence, Gly 626 and Asp 703 seem particularly critical for mediating entry into the transition state for phosphoryl transfer upon Ca 2+ binding at the transport sites.

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