Scenari of the steps possibly inhibited by the low pH during the reassembly of CxtB.

Each monomer is represented by a circle. The deprotonated and the protonated CtxB monomers are indicated in white and in black, respectively. In scenari 1, 2 and 3, the β-strands constituting the two subunit interfaces (25–33 a.a. and 97–103 a.a.) are indicated by a line only when they are capable of associating. If the association is impaired by the low pH, the strands of the interfaces are not represented. The native CtxB₅ is represented as a ring of five monomers according to the x-ray crystallographic structure (10). 2A. Scenario 1. The folding of the CtxB monomer is inhibited by the low pH. The protonated (black square) and the deprotonated CtxB (white circle) monomers have two different folds, and only the deprotonated CtxB monomer persue the assembly process. The protonated CtxB monomer misfolds irreversibly. 2B. CtxB₅ interfaces and histidine residues. For simplicity, out of the five CtxB monomers that composed the native pentamer, only three are shown in strands. Each monomer has two interfaces (Interfaces 1 and 2) involving two different β-strands. The strand number 3 of M (residues 25 to 33) associates with the C-terminal end of the β-strand number 6 of monomer M+1 (residues 97 to 103) to form the interface 1 (I₁). The C-terminal end of the β-strand number 6 of monomer M associates with the strand number 3 of monomer M-1 to form the interface 2 (I₂). The four histidine residues are indicated as balls and sticks, histidines 18 and 94 which are located upstream the two β-strands of the interfaces are colored in black. The figure was made using rasmol and using the coordinates from the x-ray structure of CtxB₅ (10). 2C. Scenario 2. The formation of both the interfaces 1 and 2 is inhibited by the low pH. The protonated CtxB monomer is association-incompetent. Only the deprotonated CtxB monomer can associate. 2D. Scenario 3. The formation of either interface 1or 2 is inhibited by the low pH. The CtxB protonated can form only one of the two interfaces and is association-deficient. 2E. Scenario 4. An intramolecular rearrangement (folding) within the CtxB pentamer is inhibited by the low pH. Both the protonated and the deprotonated CtxB monomers can associate together (black and white) or separately (white-white or black-black). The formation of the native pentamer is pH-dependent. In each scenario, the steps which involve deprotonation/protonation of the CtxB subunit are indicated by a star (*). The native pentamer is considered the most stable species of the reaction and therefore its formation is assumed irreversible.