5‘-Peroxyadenosine and 5‘-Peroxyadenosylcobalamin as Intermediates in the Aerobic Photolysis of Adenosylcobalamin

Abstract

The photolysis of adenosylcobalamin (coenzyme B 12 ) results in homolytic cleavage of the Co-C5′ bond, forming cob(II)alamin and the 5′-deoxyadenosyl radical. In the presence of molecular oxygen, it has been proposed that the primary reaction is interception of the 5′-deoxyadenosyl radical by O 2 to form adenosine-5′-aldehyde as the product (Hogenkamp, H. P. C., Ladd, J. N., and Barker, H. A. (1962) J. Biol. Chem. 237, 1950-1952. 5′-Peroxyadenosine is here found to be the initial nucleoside product of this reaction and that it decomposes to adenosine-5′-aldehyde. Evidence indicates that 5′-peroxyadenosine arises from the hydrolysis of 5′-peroxyadenosylcobalamin, with the formation of cob(III)alamin. 5′-Peroxyadenosine undergoes further decomposition to adenosine-5′-aldehyde as the major final product of aerobic photolysis, as well as to adenosine and adenine as minor products. In a cobalamin-dependent process, 5′-peroxyadenosine becomes religated to cob(III)alamin to form 5′-peroxyadenosylcobalamin, which quickly decomposes to adenosine-5′-aldehyde and cob(III)alamin. This is supported by spectrophotometric observations of both rapidly photolyzed adenosylcobalamin and of the reaction of cob(III)alamin with excess 5′-peroxyadenosine. 5′-Peroxyadenosine also slowly undergoes cobalamin-independent decomposition to adenosine-5′-aldehyde and the minor products adenosine and adenine. The present study provides a detailed description of the products initially formed when aqueous, homolytically cleaved adenosylcobalamin reacts with molecular oxygen and of the behavior of those products subsequent to photolysis.Enzymes dependent upon the vitamin B 12 -coenzyme adenosylcobalamin 1 catalyze intriguingly diverse chemical reactions, including carbon-skeleton rearrangements, reductive heteroatom eliminations, and intramolecular 1,2-migrations of heteroatomic substituents; and they proceed by mechanisms involving organic free radicals as intermediates (1,2). Adenosylcobalamin incorporates a cobalt ion in an octahedral arrangement with the pyrroline and pyrrolidine groups of a corrin ring system making up the equatorial ligands, the nitrogen from a dimethylbenzimidazole moiety as the lower axial ligand, and a 5′-deoxadenosyl moiety forming a cobalt-carbon (Co-C5′) bond in the sixth, upper axial ligand position (3).The key to the function of adenosylcobalamin in enzymatic catalysis is the reversible homolytic scission of the Co-C5′ bond, generating cob(II)alamin and a transiently formed 5′- †

Dioldehydrase: An Essential Role for Potassium Ion in the Homolytic Cleavage of the Cobalt−Carbon Bond in Adenosylcobalamin

“…Most notably, the product profile for nucleoside products from oxygen-inactivated dioldehydrase is very similar to that obtained from aerobically photolyzed adenosylcobalamin (16).…”

“…Nucleoside products were separated by HPLC and quantified as described (16). Dioldehydrase concentration was measured spectrophotometrically by using an extinction coefficient of 0.527 mL mg −1 at 278 nm.…”

Light-dependent gene regulation by a coenzyme B 12 -based photoreceptor

“…Lightinduced homolytic cleavage of the AdoB 12 Co-C bond is very rapid (24)(25)(26). Hence, extreme care was taken to avoid light in the aforementioned experiments.…”

“…Our studies in vivo and in vitro uncover an ability of AdoB 12 to induce repressor oligomerization and markedly enhance operator DNA binding to bring about gene repression in the dark. Furthermore, the photosensitivity of AdoB 12 (24)(25)(26) is exploited in light-induced disassembly of active repressor oligomers to diminish operator binding and relieve P B repression in vivo, thus linking B 12 to the light response. AdoB 12 did not produce similar effects with CarA, thereby providing a rationale for its B 12 -independent action.…”

New light on vitamin B12: The adenosylcobalamin-dependent photoreceptor protein CarH

“…For AdoCbl-requiring enzymes, the 5'-deoxyadenosyl free radical takes part in rearrangement of substrate to product. For photolysis of free AdoCbl in the absence of oxygen, the 5'-deoxyadenosyl radical cyclises to give 5',8-cycloadenosine 8 and in the presence of oxygen it gives adenosine 5'-aldehyde and 5'-peroxyadenosine (Scheme 1) [9][10][11] . Dorothy Hodgkin determined the X-ray crystal structure of vitamin B 12 in 1955.…”