Microbiology
From the beginning and at the lowest levels of life, nature has controlled poisonous diatomics by transition metals; the wellestablished principles and tenets of organotransition metal chemistry, predictable electronic and geometrical structures, and mechanistic paths that correlate with the electronic structures may be applied to such enzyme active sites.
Abstract
studies, and computational mechanisms reaches a new level with this developing field. The Co-C bond in vitamin B12 stood as the sole bona fide organometallic in biology for decades, but discoveries in the last 20 years that are covered in this book have established examples of even more fundamental organometallics; i.e., diatomic ligands CO and CNas intrinsic ligands, insuring low spin iron in low ( 1 and 2) oxidation states in the hydrogenases. This amazing discovery, confirmed by classical vibrational spectroscopic analysis, suggests the following: from the beginning and at the lowest levels of life, nature has controlled poisonous diatomics by transition metals; the wellestablished principles and tenets of organotransition metal chemistry, predictable electronic and geometrical structures, and mechanistic paths that correlate with the electronic structures may be applied to such enzyme active sites; and hydrogenases most likely operate through the intermediacy of metal hydrides. The relation between classical organometallic reactivity, including CO insertions/deinsertions, oxidative addition, and C-C coupling reactions, is particularly clear in the nickel-mediated processes in the bifunctional or partnered enzymes, carbonmonoxide dehydrogenases (CODH) and acetyl coA-synthases (ACS). In these, the active nickel is not in the sophisticated corrinoid ligand environment of MCR, but in a more primitive iron-sulfur ligation. Chapter 4 lists evidence for no less than five Ni-Carbon species, including Ni-CO in both the CODH and ACS; Ni-CH3 in ACS; Ni-C(O)CH3 in ACS; and a nickel carboxylate, Ni-C(O)-O-Fe in CODH. Although some, if not most, of the evidence is indirect, the suggestions for further studies are seen (in my eyes) as fertile ground for bioinspired catalyst development. The beautiful stories of the hydrogenases are described by the most influential players of their structural analyses in Chapters 5, 6, and 7. In addition to definitive, state-of-the-art presentations of the structural and electronic makeup of the [NiFe]-, [FeFe]-, and mono-Fe hydrogenases, Chapter 6 discusses the biosynthesis of the H-cluster (the hydrogen-producing FeS cluster) of the [FeFe]hydrogenase as a major emerging area of research that is key both to technological applications and to the link of these enzymes to prebiotic life. Moving from prebiotic chemistry in anaerobes to current mammalian cells, the role of carbon monoxide spans toxicity to required signaling agent. Chapter 8 is an extensive review of the dual role of heme as cofactor and substrate in the biosynthesis of CO—in this case, the researchers look for release of CO from heme rather than the Fe-CO bond. A serious beginning to understanding the intricate role of H-bonding that controls heme hydroxylation and CO release while avoiding inhibition is described, yet “a significant portion of the path remains uncharted” (p. 285). Although extrinsic CO is a well-known reporter of metal-binding sites (including Cu), until recently no Cu-carbon bonds in biology have been described. Given the large organic synthesis industry that relies on copper-mediated catalysis, it is not surprising that evidence is accruing that suggests Cu(I) can “sense” ethylene, and a bacterial copper chaperone binds in a -fashion to a tryptophan amino acid residue. Final chapters on less abundant metals in biology find fewer and less common M-C bonds. This is not to say they, or possibilities of them, are not extremely interesting. Hille concludes in Chapter 11 that the evidence for a Mo-C bond involving an N-heterocyclic carbene intermediate in the xanthine oxidoreductase mechanism is lacking, but organometallic chemists would suggest that such well-known species will eventually surface in biology. All in all, this volume is a must for bioinorganic and organometallic chemists. It could easily be used as a textbook in graduate courses, and it will be an exceptional reference source. Marcetta Y. Darensbourg, Chemistry, Texas A&M University, College Station, Texas