Metal-carbon bonds in enzymes and cofactors

This volume is the 50th book in the two Sigels' series (44 in MIBS and 6 in MILS) and this is celebrated with a compre­hensive Author Index given at the end of this Volume 6. Supported by nearly 1700 references and about 160 illustrations (many in color), Metal-Carbon Bonds in Enzymes and Cofactors highlights the function of these metal-carbon bonds in life, but also their importance in research. Written by 20 internationally recognized experts, 12 stimulating chapters provide an authoritative and timely resource for scientists working in the wide range from physical and inorganic biochemistry all the way through to physiology and medicine. This volume is devoted to naturally occurring metal-carbon bonds, a topic recently obtaining (again) significant momentum, largely – but not only – due to new insights gained with hydrogenases. The field started out about 50 years ago when coenzyme B12 was identified as organometallic derivative of vitamin B12. This moved the cobalt-carbon bond into the center of interest and consequently, the first two chapters of this book are devoted to the organometallic chemistry of B12 coenzymes and to the biochemistry of cobalamin- and corrinoid-dependent enzymes. B12 coenzymes are required in the metabolism of a broad range of organisms including humans; however, only microorganisms have the ability to biosynthesize B12 and other natural corrinoids. This fact alone, together with new metabolic insights (e.g., riboswitches), guarantees a continued fascination – not only for the B12 community. Related to Co-corrin, the Ni-porphinoid unit (F430) is the prosthetic group of methyl-coenzyme M reductase. This enzyme, the topic of Chapter 3, catalyzes the methane-forming step in methanogenic archaea and most probably also the methane-oxidizing step in methanotrophic archaea. Chapter 4 deals with acetyl-coenzyme A synthases/carbon monoxide dehydrogenases, i.e., bifunctional nickel-containing enzymes, which catalyze the synthesis of acetyl-CoA and the reversible reduction of CO2 to CO in anaerobic, mostly thermophilic, organisms, able to grow chemiauto­trophically on simple inorganic compounds like CO2. Ni-C bonds with methyl, acetyl, carbonyl, and carboxylate groups are evidenced. [NiFe]-, [FeFe]-, and [Fe]-hydrogenases are detailed in the next three chapters. These enzymes, present in many microorganisms, catalyze the oxidation of molecular hydrogen or the reduction of protons. All of them have a Fe(CO)x unit in their active site. Iron-cyanide units occur in [NiFe]- and [FeFe]-hydrogenases. However, despite the indicated similarities they clearly have independent evolutionary origins. The participation of the commonly considered toxic ligands CO and CN– in the active sites of hydrogenases is still a surprise to many; yet, exactly their occurrence incites a great interest in physical chemists as well as evolutionary biologists. The dual role of heme as cofactor and substrate in the biosynthesis of carbon monoxide is the topic of Chapter 8. Carbon monoxide is a ubiquitous molecule in the atmosphere but it is also produced in mammalian, plastidic, and bacterial cells as a byproduct in the catalytic cycle of heme degradation as catalyzed by the enzyme heme oxygenase. Most fascinating is the fact that the biological role of CO spans the range from toxic to cytoprotective, depending on its concentration. CO generated by heme oxygenase is now known to function in several important physiological processes, including vasodilation, apoptosis, inflammation, and possibly neurotransmission. The relevance of the copper-carbon bond in biological inorganic chemistry will probably not easily come to the mind of most biochemical and inorganic researchers. However, there is a vast amount of literature, cunningly presented in Chapter 9. CO as well as CN– have proven very useful in obtaining insights into the active site structures and mechanisms of copper proteins. Naturally, in these instances both ligands are inhibitors and used as probes. However, there is also the recently described copper-carbon unit present in a carbon monoxide dehydrogenase, which contains a novel molybdenum-copper catalytic site, or the copper(I)-arene unit, which was evidenced in a bacterial copper chaperone. Apparently also a plant receptor site (ETR1) utilizes Cu(I) to sense the growth hormone ethylene. Chapter 10 focuses on the interaction of CN– with enzymes containing vanadium, manganese, non-heme iron, and zinc, and the inhibiting properties of this ligand, allowing its use as a probe. The reaction mechanism of the molybdenum hydroxylase xanthine oxidoreductase is revisited in Chapter 11; previously a molybdenum-carbon bond was postulated but now proof is presented against its formation. The terminating Chapter 12 reviews briefly the most popular computational approaches employed in theoretical studies of bioorganometallic species by providing detailed examples.