Figure 3.

Hydrogen-bonding interactions between the protein and membrane. Specific protein-lipid hydrogen bonds are indicated as orange dashed lines and BtuB is shown in black ribbon. (A) Apo-sym system, showing a phospholipid (gray sticks) hydrogen bonding (orange dashes) to loop 3/4 of BtuB. Loop 3/4 becomes deformed as Asp193 and Asp195 are attracted toward the zwitterionic POPE headgroup. This interaction causes loop 3/4 to substantially deviate from the conformation observed in the CBL-bound structure. (B) Apo2-OM system, with an LPS molecule (gray sticks) hydrogen bonding (orange dashes) to loop 3/4 of BtuB. The oligosaccharides in the OM systems exhibit hydrogen bonding along the EC loops, including loop 3/4 residues Asp179 and Asp193, in the absence of ${\text{Ca}}^{2+}$. Additional hydrogen-bonding interactions can be found in Tables S3–S5. To see this figure in color, go online.