**ATP Analogues for Structural Investigations: Case Studies of a DnaB Helicase and an ABC Transporter**

**Denis Lacabanne 1,2,**† **, Thomas Wiegand 1,\* ,**† **, Nino Wili <sup>1</sup> , Maria I. Kozlova <sup>3</sup> , Riccardo Cadalbert <sup>1</sup> , Daniel Klose <sup>1</sup> , Armen Y. Mulkidjanian 3,4, Beat H. Meier 1,\* and Anja Böckmann 5,\***


Academic Editor: Marilisa Leone

Received: 17 October 2020; Accepted: 9 November 2020; Published: 12 November 2020

**Abstract:** Nucleoside triphosphates (NTPs) are used as chemical energy source in a variety of cell systems. Structural snapshots along the NTP hydrolysis reaction coordinate are typically obtained by adding stable, nonhydrolyzable adenosine triphosphate (ATP) -analogues to the proteins, with the goal to arrest a state that mimics as closely as possible a physiologically relevant state, e.g., the pre-hydrolytic, transition and post-hydrolytic states. We here present the lessons learned on two distinct ATPases on the best use and unexpected pitfalls observed for different analogues. The proteins investigated are the bacterial DnaB helicase from *Helicobacter pylori* and the multidrug ATP binding cassette (ABC) transporter BmrA from *Bacillus subtilis*, both belonging to the same division of P-loop fold NTPases. We review the magnetic-resonance strategies which can be of use to probe the binding of the ATP-mimics, and present carbon-13, phosphorus-31, and vanadium-51 solid-state nuclear magnetic resonance (NMR) spectra of the proteins or the bound molecules to unravel conformational and dynamic changes upon binding of the ATP-mimics. Electron paramagnetic resonance (EPR), and in particular W-band electron-electron double resonance (ELDOR)-detected NMR, is of complementary use to assess binding of vanadate. We discuss which analogues best mimic the different hydrolysis states for the DnaB helicase and the ABC transporter BmrA. These might be relevant also to structural and functional studies of other NTPases.

**Keywords:** solid-state NMR; ELDOR-detected NMR; ATP hydrolysis; ATP analogues; DnaB helicase; ABC transporter
