The three-dimensional structure of bovine calcium ion-bound osteocalcin using HNMR spectroscopy

Structural information on osteocalcin or other noncollagenous bone proteins is very limited. We have solved the three-dimensional structure of calcium bound osteocalcin using H-1 2D NMR techniques and proposed a mechanism for mineral binding. The protons in the 49 amino acid sequence were assigned using standard two-dimensional homonuclear NMR experiments. Distance constraints, dihedral angle constraints, hydrogen bonds, and H-1 and C-13 chemical shifts were all used to calculate a family of 13 structures. The tertiary structure of the protein consisted of an unstructured N terminus and a C-terminal loop (residues 16-49) formed by long-range hydrophobic interactions. Elements of secondary structure within residues 16-49 include type III turns (residues 20-25) and two (x-helical regions (residues 2735 and 41-44). The three Gla residues project from the same face of the helical turns and are surface exposed. The genetic algorithm-molecular dynamics simulation approach was used to place three calcium atoms on the NMR-derived structure. One calcium atom was coordinated by three side chain oxygen atoms, two from Asp30, and one from Gla24. The second calcium atom was coordinated to four oxygen atoms, two from the side chain in Gla 24, and two from the side chain of Gla 21. The third calcium atom was coordinated to two oxygen atoms of the side chain of Gla17. The best correlation of the distances between the uncoordinated Gla oxygen atoms is with the intercalcium distance of 9.43 Angstrom in hydroxyapatite. The structure may provide further insight into the function of osteocalcin.