Intriguingly, in the structure of SsTroA, we observed two distinctive flexible loops mainly composed of charged residues, and these two flexible loops were designated as Loop 1 (Asn 126-Pro 38) and Loop 2 (Tyr 279-Glu 285) ( Fig. 5C and D) . Loop 1 is locateed between the b2-1 and b2-2 (refer to the TpTroA [21] topology nomenclature). The TpTroA and S. pneumoniae PsaA structures display a short loop compared to Loop 1 of SsTroA. Among the ZnuA proteins examined, this loop is longer (histidinerich) and it is proposed that this region is possibly involved in the capture of the Zn around the metal binding site or plays an important role in the process of docking with permeases [26] . Nevertheless, the significance of this loop remains unclear because there are limited experimental results to confirm these hypotheses. Regardless, it is speculated that the mobility of this loop may possess a biological function in SsTroA [26] .