Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)RecQ helicases are important maintainers of genome integrity with distinct roles in almost every cellular process requiring access to DNA. RECQL5 is one of five human RecQ proteins and is particularly versatile in this regard, forming protein complexes with a diverse set of cellular partners in order to coordinate its helicase activity to various processes including replication, recombination and DNA repair. In this study, we have determined crystal structures of the core helicase domain of RECQL5 both with and without the nucleotide ADP in two distinctly different ('Open' and 'Closed') conformations. Small angle X-ray scattering studies show that the 'Open' form of the protein predominates in solution and we discuss implications of this with regards to the RECQL5 mechanism and conformational changes. We have measured the ATPase, helicase and DNA binding properties of various RECQL5 constructs and variants and discuss the role of these regions and residues in the various RECQL5 activities. Finally, we have performed a systematic comparison of the RECQL5 structures with other RecQ family structures and based on these comparisons we have constructed a model for the mechano-chemical cycle of the common catalytic core of these helicases.45742314243AbbVieBayer Pharma AGBoehringer IngelheimCanada Foundation for InnovationEshelman Institute for InnovationGenome CanadaInnovative Medicines Initiative (EU/EFPIA) [ULTRA-DD 115766]JanssenMerck Co.Novartis Pharma AGOntario Ministry of Economic Development and InnovationPfizerSao Paulo Research Foundation-FAPESPTakedaWellcome Trust [106169/ZZ14/Z]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)