Figure 3.
Ca2+-binding sites and potential MIDAS. A) The two Ca2+-binding sites in a C-type 3 repeat, as exemplified by repeat 1C. The first Ca2+ ion is completely coordinated by protein atoms, while the second Ca2+ ion has one coordinating water molecule that is buried. B) The 2N repeat is shown as an example of an N-type 3 repeat, which contains two Ca2+-binding sites. The first Ca2+ ion has one coordinating water molecule that is buried. The second Ca2+ ion has a similar coordinating water molecule, but the water molecule is opened to the solvent. This Ca2+ is lost in some N-type 3 repeats due to unfavorable amino acid substitutions in the coordinating residues. The water molecules shown in A and B were not resolved or assigned in the COMP structure due to its low-resolution limit. They were assigned in the higher-resolution structure of TSP-2 (19). C) The three Ca2+-binding sites in the CTD and their involvement in molecular packing. CTD_Ca1 is coordinated by N565, D593 (two bonds), S728 (two bonds including one to its carbonyl group), and the E341 (two bonds) from a neighboring molecule. CTD_Ca2 is coordinated by D593, D594, D595 (two bonds), Q619, and the E341 (weak bond) from a neighboring molecule. There is a water molecule below CTD_Ca3, completing its coordination (19). The water molecule was not resolved in the E4T3C5 structure and is not shown.