DIFFERENCES IN THE METAL-ION STRUCTURE BETWEEN SR- AND CA-PROTHROMBINFRAGMENT-1

The structure of Sr-prothrombin fragment 1 has been solved and refined by restrained least-squares methods at 2.5-Angstrom resolution to a c rystallographic R value of 0.167. The protein structure is very simila r to that of Ca-fragment 1. A polymeric array of five Sr2+ ions separa ted by about 4.0 Angstrom is buried. among six gamma-carboxyglutamic a cid (Gla) residues; three other Sr2+ ions interact with other Gla resi dues and are located further apart. One of these was not found in the Ca-fragment 1 structure. The coordination of the Sr2+ ions resembles t hat of Ca2+, but there are some significant differences between them. The most notable is the lack of water coordination with Sr2+ ions and two conformations for Gla 8, which change the coordination of Sr-2 and Sr-3. A hexose moiety of an oligosaccharide was located in the vicini ty of Asn101 that was flexibly disordered in Ca-fragment 1. The new Sr 2+ ion found may be involved in metal ion phospholipid binding interac tions along with Sr-1, and Sr-7, Sr-8.