Figure 6. Conceptual model of TCS unidirectional signaling.
(A) P~DesR dephosphorylation catalysis is favored by the opening of the RR phosphorylation lid, via Phe82(RR) insertion into an HK hydrophobic pocket, and an ionic interaction between Asp189(HK) and Arg84(RR). The Phe82(RR) movement allows for entry of a nucleophilic water (yellow sphere) positioned by Gln193(HK). In the phosphatase state the phosphorylatable His188(HK) is buried within the DHp domain core, avoiding autophosphorylation and phosphoryl back-transfer reactions. (B) Signal-mediated pathway activation triggers the cogwheel rotation of the HK DHp α-helices, via coiled-coil disruption. The His188(HK) becomes exposed to the solvent, CA domains are released, and autophosphorylation thus enabled. Binding to unphosphorylated RR positions the P~His188(HK) into a suitable orientation for phosphoryl-transfer to occur through a loose (dissociative) nucleophilic substitution. Movement of Thr80(RR) is coupled with phosphoryl group migration, triggering RR’s phosphate lid closure and P~RR release. (C) Reaction scheme summarizing the autophosphorylation and phosphotransfer reactions. Stabilization of a protonated Nδ1 tautomer of the reactive histidine, is required in autophosphorylation. In keeping with the imidazole aromaticity, the nucleophilicity of Nε2 is thus finely regulated, allowing for phospho-acceptor/donor roles of the His.