Frontiers in metalloprotein crystallography and cryogenic electron microscopy.

Metalloproteins comprise at least a third of all proteins that utilize redox properties of transition metals on their own or as parts of cofactors. The development of third generation storage ring sources and X-ray free-electron lasers with femtosecond pulses in the first decade of the 21st century has transformed metalloprotein crystallography. In the past decade, cryogenic-electron microscopy single-particle analysis, which does not require crystallization of biological samples has been extensively utilized, particularly for membrane-bound metalloprotein systems. Here, we explore recent frontiers in metalloprotein crystallography and cryogenic electron microscopy, organized for convenience under three metalloprotein-centered biological cycles, focusing on contributions from each technique, their synergy and the ability to preserve metals' redox states when subjected to a particular probe.