Nature’s Anaerobic Toolkit: Glycyl Radical Enzymes and Their Expanding Functional and Mechanistic Diversity

-adenosyl-L-methionine (RS) activating enzyme (GRE-AE; activase) specific to each GRE. The catalytic repertoire of GREs is diverse, spanning ribonucleotide reduction for DNA synthesis, acetyl-CoA formation in anaerobic primary metabolism, dehydration reactions in secondary metabolism, decarboxylation reactions in aromatic amino acid fermentation, and hydrocarbon activation in anaerobic bacteria, enabling their survival on crude oil as a carbon source. Pyruvate formate lyase (PFL), the founding member of the GRE superfamily, was first identified several decades ago. In recent years, sequencing technology has enabled the annotation of thousands of unique GRE-encoding sequences. Several dozen of the identified GREs have been characterized further, providing insight into the structure, mechanism, and regulation of this multitudinous and diverse enzyme family. Here, we introduce the superfamily, summarize what is known about their activation and catalysis, note conserved structural features, expand on the previously identified GRE classes, highlight GRE-associated bacterial microcompartments, and discuss the future and outlook of the GRE superfamily.