Metal fluoride complexes mimic the transferring phosphoryl group in many enzyme-catalyzed reactions. We here employ the trifluoromagnesate transition state analog (TSA) to study a Zika virus NS3h helicase, which uses energy from ATP hydrolysis to reorganize ssRNA leading to completion of virus replication. The crystal structure of this TSA complex displays two conformations for a catalytically important loop, demonstrating how ATP hydrolysis can be coupled with RNA translocation. Unexpectedly,

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... he trifluoromagnesate core of this transition state complex is octahedral. It is identified as having an unprecedented MgF₃(Wat)^– ligand, confirmed by ¹⁹F NMR analysis. This structure was further probed by quantum mechanical calculations of the catalytic core (200 atoms), confirming the structural data interpretation and the concerted mechanism of ATP hydrolysis by this class of helicase. The formation of this MgF₃(Wat)^– ligand in helicase but not in other multiple MF_x structures for ATPases and GTPases strongly implies they cannot possess such an additional water in their active sites.