CRYSTAL STRUCTURE OF CALBINDIN D_9k BINDING MG^2+. Maria Andersson¹, L. Anders Svensson^1, Sara Linse^2, 1Molecular Biophysics and ²Physical Chemistry 2 at the University of Lund, Box 124, S-221 00 Lund, Sweden

Calbindin D_9k is thought to transport Ca²⁺/Mg²⁺ in mammalians^1,2 and is present in bone and in the intestine. Calbindin D_9k has two EF-hand subunits, binding calcium cooperatively³. In this structure only one of two EF-hand loops binds Mg²⁺ and the other loop is devoid of metal ions. Significant structural differences exist between the (Ca²⁺)₂-calbindin structure⁴ and the Mg²⁺-calbindin structure. Due to these structural differences molecular replacement with (Ca²⁺)₂-calbindin as a model failed to solve the strucure and instead isomorphous replacement techniques had to be used. The final structure includes all 75 aminoacids, Mg²⁺, 50 water molecules and is refined to 1.6 Å (87 % completeness). The final Rfactor is 19.6 %. The Mg²⁺-calbindin structure further supports the idea of cooperativity. As Ca²⁺ levels increase intracellularly, Ca²⁺ goes into the empty site, inducing a conformational change which releases Mg²⁺. Thereafter Ca²⁺ binds to the second site.

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