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Figure 2.
Figure 2. Crystallographic structures of BtuB. a, apo BtuB;
b, Ca^2+ -BtuB; and c, Ca^2+ -B[12] -BtuB.  -barrel
domains are shown in green; hatch domains, in purple. Bound
calcium ions are shown in yellow in the Ca^2+ -BtuB and Ca^2+
-B[12] -BtuB structures. The bound cyanocobalamin (vitamin
B[12]) substrate is shown in space-filling representation in the
Ca^2+ -B[12] -BtuB structure. The left column depicts the
structures with extracellular loops pointing upwards and
periplasmic turns downwards; the -strands
of the barrel domain span the outer membrane. The right column
presents views, normal to the surface of the outer membrane,
looking down into the extracellular side of the structures. The
hatch domains extend from residues 6 -132, with the Ton box
located at 6 -12. The hatch domain is formed around a core of
four -strands.
A short linker (133 -136) connects the hatch to the 22-stranded
-barrel
domain (137 -594). Extracellular loops that are disordered in
the apo BtuB structure become partially ordered in the Ca^2+
-BtuB structure and fully ordered in the Ca^2+ -B[12] -BtuB
structure. This ordering occurs in the vicinity of the bound
calcium ions. Waters, detergent molecules and other (weakly)
bound ions are not shown (for clarity).
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