Abstract
BtuB is a TonB-dependent transporter that permits the high-affinity binding and transport of cobalamin (CBL), or vitamin B(12), across the asymmetric outer membrane (OM) of Gram-negative bacteria. It has been shown that Ca(2+) binding is necessary for high-affinity binding of CBL to BtuB, and earlier simulations suggested that calcium ions serve to stabilize key substrate-binding extracellular loops. However, those simulations did not account for the lipopolysaccharides in the OM. To illuminate the roles of both Ca(2+) and lipopolysaccharides in protein functionality, we performed simulations of apo and Ca(2+)-loaded BtuB in symmetric and asymmetric bilayers. The simulations reveal that the oligosaccharides of LPS stabilize the extracellular loops to some degree, apparently obviating the need for Ca(2+). However, it is shown that Ca(2+) ions stabilize a key substrate-binding loop to an even greater degree, as well as reposition specific CBL-binding residues, bringing them closer to the organization found in the CBL-bound structure. These results indicate the importance of including realistic membrane models when simulating outer-membrane proteins.