Abstract The combination of metal ions with proteins offers unique chemical reactivities that are at the heart of many of Nature’s most important and amazing chemical transformations. For example, metalloenzymes catalyze the reduction of ribonucleotides to deoxyribonucleotides, a rate-limiting step in DNA biosynthesis. They biosynthesize anticancer and antiviral compounds that have unusual scaffolds and catalyze the carbon-carbon bond forming reactions that afford life on CO2 and H2 gases. Our lab employs structural methods to interrogate how metalloenzymes are able to perform this incredible chemistry. We seek to understand how the architecture of metalloenzymes allows for radical species to be controlled – i.e. turned off, turned on and harnessed – to enable the reaction at hand. We also strive to understand how proteins are designed to enable long-range electron transfer without protein damage or radical loss. In this proposal, we describe structural studies of our metalloenzyme model systems, including class Ia (diiron-dependent) and class III (glycyl radical- dependent) ribonucleotide reductases that allow us to interrogate the molecular basis of radical-based chemistry. We also describe efforts to understand how protein scaffolds facilitate organometallic chemistry, especially in regard to microbial carbon dioxide fixation and methane production. These studies leverage both our expertise in working with O2-sensitive metalloenzymes and in cryogenic-electron microscopy (cryo-EM). Although we will continue to employ X-ray crystallography, cryo-EM is proving to be a game-changer for many of our metalloenzyme systems. In particular, the resolution revolution of cryo-EM provides us with the means to obtain long-awaited structures of both large (2000 kDa) and transient metalloprotein complexes and to determine structures of metalloenzymes in functionally-essential conformational states that were previously unattainable by crystallography. The results of our structural studies will enable structure-based design of novel antibiotics targeting, for example, microbial ribonucleotide reductases. These structural data will also guide efforts to exploit radical enzymes for the production of medically important compounds with unusual scaffolds. These data will additionally facilitate the application of metalloenzymes or their bioinspired inorganic counterparts in the production of high-value carbon compounds from the greenhouse gas CO2 and, ideally, improve our understanding of some of the more enigmatic aspects of metalloprotein biochemistry.