RESUMEN
The role of S-adenosylmethionine (SAM) as a precursor to organic radicals, generated by one-electron reduction of SAM and subsequent fission to form 5'-deoxyadenosyl radical and methionine, has been known for some time. Only recently, however, has it become apparent how widespread such enzymes are, and what a wide range of chemical reactions they catalyze. In the last few years several new SAM radical enzymes have been identified. Spectroscopic and kinetic investigations have begun to uncover the mechanism by which an iron sulfur cluster unique to these enzymes reduces SAM to generate adenosyl radical. Most recently, the first X-ray structures of SAM radical enzymes, coproporphyrinogen-III oxidase, and biotin synthase have been solved, providing a structural framework within which to interpret mechanistic studies.
Asunto(s)
Enzimas/metabolismo , S-Adenosilmetionina/metabolismo , Coproporfirinógeno Oxidasa/química , Coproporfirinógeno Oxidasa/metabolismo , Enzimas/química , Radicales Libres , Cinética , Modelos Moleculares , Conformación Molecular , Conformación Proteica , Estructura Terciaria de Proteína , S-Adenosilmetionina/química , Sulfurtransferasas/química , Sulfurtransferasas/metabolismoAsunto(s)
Cobamidas/metabolismo , Ácido Glutámico/metabolismo , Transferasas Intramoleculares/metabolismo , Análisis Espectral/métodos , Vitamina B 12/análogos & derivados , Acrilatos/metabolismo , Ácido Aspártico/análogos & derivados , Ácido Aspártico/metabolismo , Proteínas Bacterianas/química , Proteínas Bacterianas/metabolismo , Desoxiadenosinas/metabolismo , Transferasas Intramoleculares/química , Isomerismo , Estructura Molecular , Unión Proteica , Vitamina B 12/metabolismoRESUMEN
We describe a novel reaction of adenosylcobalamin that occurs when adenosylcobalamin-dependent glutamate mutase is reacted with the substrate analogue 2-methyleneglutarate. Although 2-methyleneglutarate is a substrate for the closely related adenosylcobalamin-dependent enzyme 2-methyleneglutarate mutase, it reacts with glutamate mutase to cause time-dependent inhibition of the enzyme. Binding of 2-methyleneglutarate to glutamate mutase initiates homolysis of adenosylcobalamin. However, instead of the adenosyl radical proceeding to abstract a hydrogen from the substrate, which is the next step in all adenosylcobalamin-dependent enzymes, the adenosyl radical undergoes addition to the exo-methylene group to generate a tertiary radical at C-2 of methyleneglutarate. This radical has been characterized by EPR spectroscopy with regiospecifically (13)C-labeled methyleneglutarates. Irreversible inhibition of the enzyme appears to be a complicated process, and the detailed chemical and kinetic mechanism remains to be elucidated. The kinetics of this process suggest that cob(II)alamin may reduce the enzyme-bound organic radical so that stable adducts between the adenosyl moiety of the coenzyme and 2-methyleneglutarate are formed.
