* Synopsis for the Graphical Abstract - 50 words max, NO UNDEFINED ABBREVIATIONS – below is my suggestion

Gaseous ligands (NO, CO) bind to the HmuY-heme. Binding to the heme iron is preceded by the dissociation of one coordinating histidine (possibly His166). The rupture of the coordination bond with His134 can be induced by binding to the HmuR outer-membrane transporter with subsequent heme release and transfer to HmuR.

* Highlights - 5 max, each with at most 90 characters including spaces, NO UNDEFINED ABBREVIATIONS OK

* Keywords - six words or very short phrases OK

* Abstract - 250 words max, one paragraph, NO UNDEFINED ABBREVIATIONS OK

* Abbreviations - all uncommon abbreviations must be defined in the text when first used. A Table of Abbreviations (located at end of text) is VERY STRONGLY ENCOURAGED if you have more than three or four. Not necessary