ContentsI. Introduction 705 II. General Principles 706 A. One Electron Oxidized Amino Acids Thus Far Identified in Proteins 706 B. Biosynthesis of (Modified) Amino Acid Radicals 706 C. Emerging General Chemical Properties of Enzymes Utilizing Protein Radicals for Catalysis 708 D. Methods To Examine Radical Dependent Reactions 710 III. Background on Ribonucleotide Reductases 710 IV. Class I Ribonucleotide Reductases 712 A. Formation of the Tyrosyl Radical 713 B. Function of the Tyrosyl Radical 714 C. Thiyl Radical Involvement in Catalysis 716 D. Evidence for Enzyme-Mediated Radical Chemistry Using Nucleotide Analogues 717 E. Structure 719 V. Class II Ribonucleotide Reductases 719 A. Exchange Reaction: Detection of the Elusive Thiyl Radical 719 B. Role of the Thiyl Radical in Catalysis 721 VI. Pyruvate Formate Lyase 722 A. Characterization of the Glycyl Radical 723 B. Formation of the Glycyl Radical 723 C. Catalytic Mechanism 724 D. Enzyme-Mediated Radical Chemistry with Pyruvate Analogues 727 VII. Anaerobic Ribonucleotide Reductase 728 A. Background 728 B. Requirement for a Glycyl Radical 728 C. Mechanism of Nucleotide Reduction 729 VIII. Cytochrome c Peroxidase 730 A. Formation of the Ferryl Heme/Tryptophan Cation Radical 730 B. Proposed Function of the Tryptophan Radical in Electron Transfer 731 IX. Prostaglandin H Synthase 733 A. Structure: A Tyrosine Residue Revealed 733 B. Proposed Role of the Tyrosyl Radical in Catalysis 734 X. Photosynthetic Oxygen Evolution 736 A. Background 736 B. Proposed Roles of Tyrosyl Radicals Y Z • and Y D • 737 C. Spectroscopic Studies on Y Z • and Y D • 738 D. Proposed Mechanisms for Water Oxidation and O 2 Evolution 739 XI. Galactose Oxidase 741 A. Different Redox States of Galactose Oxidase 741 B. Structure: A Modified Tyrosine Residue Revealed 742 C. Is the Oxidized Amino Acid Associated with Apo Oxidized GAO the Novel Ortho-Thiol-Substituted Tyrosine? 743 D. Catalytic Mechanism 743 XII. Quinoproteins 744 A. Copper-Dependent Amine Oxidases 744 B. Methylamine Dehydrogenase 749 XIII. Other Systems in Which Protein-Based Radicals Have Been Proposed or Detected 751 A. Bovine Liver Catalase 751 B. DNA Photolyase 751 C. Dopamine β Monooxygenase 752 XIV. Summary and Outlook 754 XV. Abbreviations 754 XVI. Acknowledgments 755 XVII. References 755 JoAnne Stubbe is the Novartis professor of Chemistry and Biology at the Massachusetts Institute of Technology. She received her undergraduate degree from the University of Pennsylvania working with Ed Thornton and did NSF-sponsored undergraduate research with Ed Trachtenberg at Clark University. These mentors played a strong role in her interest in physical organic chemistry. She received her Ph.D.

Figure 9. Structures of lacticin 481, 97 mutacin II, 99 sublancin 168, 105 actagardine, 14 and cypemycin. 103 The natural derivative Ala(0)-actagardine is shown as a dotted circle.

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) that display a wide variety of biological activities, from antimicrobial to antiallodynic. Lanthipeptides that display antimicrobial activity are called lantibiotics. The post-translational modification reactions of lanthipeptides include dehydration of Ser and Thr residues to dehydroalanine and dehydrobutyrine, a transformation that is carried out in three unique ways in different classes of lanthipeptides. In a cyclization process, Cys residues then attack the dehydrated residues to generate the lanthionine and methyllanthionine thioether cross-linked amino acids from which lanthipeptides derive their name. The resulting polycyclic peptides have constrained conformations that confer their biological activities. After installation of the characteristic thioether cross-links, tailoring enzymes introduce additional post-translational modifications that are unique to each lanthipeptide and that fine-tune their activities and/or stability. This review focuses on studies published over the past decade that have provided much insight into the mechanisms of the enzymes that carry out the post-translational modifications.

The current need for antibiotics with novel target molecules has coincided with advances in technical approaches for the structural and functional analysis of the lantibiotics, which are ribosomally synthesized peptides produced by gram-positive bacteria. These peptides have antibiotic or morphogenetic activity and are structurally defined by the presence of unusual amino acids introduced by posttranslational modification. Lantibiotics are complex polycyclic molecules formed by the dehydration of select Ser and Thr residues and the intramolecular addition of Cys thiols to the resulting unsaturated amino acids to form lanthionine and methyllanthionine bridges, respectively. Importantly, the structural and functional diversity of the lantibiotics is much broader than previously imagined. Here we discuss this growing collection of molecules and introduce some recently discovered peptides, review advances in enzymology and protein engineering, and discuss the regulatory networks that govern the synthesis of the lantibiotics by the producing organisms.