3.A.1.13.1
Vitamin B₁₂ porter. The 3-D structure of BtuCDF has been solved to 2.6 Å (Hvorup et al., 2007). The conformational transition pathways of BtuCD has been revealed by targeted molecular dynamics simulations (Weng et al., 2012). Asymmetric states of BtuCD are not discriminated by its cognate substrate binding protein BtuF (Korkhov et al., 2012).  ATP hydrolysis occurs at the nucleotide-binding domain (NBD) dimer interface, whereas substrate translocation takes place at the translocation pathway between the TM subunits, which is more than 30 angstroms away from the NBD dimer interface.  Hydrolysis of ATP appears to facilitate substrate translocation by opening the cytoplasmic end of translocation pathway (Pan et al. 2016). The molecular mechanism of ATP hydrolysis by BtuCD-F may proceeds in a stepwise manner (Prieß et al. 2018). First, nucleophilic attack of an activated lytic water molecule at the ATP gamma-phosphate yields ADP + HPO₄^2-. A conserved glutamate located close to the gamma-phosphate transiently accepts a proton acting as a catalytic base. In the second step, the proton transfers back from the catalytic base to the gamma-phosphate, yielding ADP + H₂PO₄^-. These two reaction steps are followed by rearrangements of the hydrogen bond network and the coordination of the Mg²⁺ ion. The overall free energy change of the reaction is close to zero, suggesting that ATP binding is essential for tight dimerization of the nucleotide-binding domains and the transition of the transmembrane domains from inward- to outward-facing. ATP hydrolysis resets the conformational cycle (Prieß et al. 2018).