Table 3.

Ca2+-Coordinating Ligandsa in the PelC R218K–(Ca2+)3–4–PentaGalpA Complexes

Ca2+LigandDistance (Å)
1Ca2+bLys-218 N-ζ2.6
GalpA1 O-6B2.9
GalpA2 O-52.9
GalpA2 O-6A2.6
1CaWat12.8
2Ca2+Asp-160 O-δ22.3
Asp-162 O-δ22.4
GalpA2 O-32.6
GalpA3 O-52.5
GalpA3 O-6A2.2
2CaWat12.6
2CaWat22.2
3Ca2+Glu-166 O-δ12.5
Glu-166 O-δ22.5
GalpA3 O-6B2.4
GalpA4 O-52.4
GalpA4 O-6A2.4
3CaWat12.5
3CaWat22.4
3CaWat32.4
4Ca2+Asp-129 O-δ12.3
Asp-131 O-δ12.5
Asp-131 O-δ22.4
Glu-166 O-δ12.4
Asp-170 O-δ22.4
GalpA4 O-6A2.5
4CaWat12.2
  • a zCa2+ refers to the Z position of the Ca2+ ion as defined in Figure 4. xWaty refers to the Y water molecule associated with either the X GalpA unit or the X Ca2+ ion. The labels for the oxygen atoms are defined in Figure 4.

  • b The observed electron density is best suited to a Ca2+ with a 50% occupancy rather than to a water molecule. The 1Ca2+ ion is in the same location, relative to the uronic acid of GalpA1, as are the other Ca2+ ions that coordinate GalpA units. However, unfavorable contact with the well-ordered and fully occupied lysine, Lys-218, was observed. It is not possible to determine whether 1Ca2+ interacts with an unprotonated lysine at pH 9.5 in the crystals in 50% of the molecules.