An Antibody that Reconstitutes the “Base-On” Form of B12 Coenzymes

Abstract

The cobalt-coordinating 5,6-dimethylbenzimidazole (DMB) nucleotide is a structural feature of coenzyme B 12 and of methylcobalamin [1,2] that controls the reactivity of these organometallic cofactors. [3,4] The DMB nucleotide and the functionalized corrin ring are both important for tight and selective binding to B 12 apoenzymes. Several coenzyme B 12 dependent enzymes were recently shown to bind the B 12 cofactor in an unanticipated ™base-off/His-on∫ form, [5±7] in which the DMB nucleotide is displaced from the cobalt center by a histidine residue from the protein. In contrast, diol dehydratase binds coenzyme B 12 ™base-on∫, as described in a crystal structure at the end of the 1990s. [8] Here we report on a monoclonal antibody raised against coenzyme B 12 , which binds the ™base-on∫ form of natural coenzyme B 12 analogues that prefer to be ™base-off∫ in solution. This B 12 antibody thus causes an unprecedented ™reverse∫ coordinative ™base-off∫ to ™base-on∫ reconstitution, which also results in a significant change in the reactivity of the bound B 12 coenzyme.Over the last 15 years, antibodies have proved useful as catalysts for a set of remarkable chemical reactions and as receptors for probing ligand-binding mechanisms. [9±14] Versatile cofactors such as coenzyme B 12 have considerable potential to extend the chemistry of these systems. To investigate the ability of antibodies to recognize and modulate the chemical reactivity of corrinoid coenzymes, we have raised monoclonal antibodies against coenzyme B 12 (1). Although other antibodies that recognize B 12 derivatives are known, [15] their binding properties were never scrutinized spectroscopically.For immunization, coenzyme B 12 (1) was coupled to the carrier protein thyroglobulin (TG) with a succinic acid linker [16] (48 haptens/TG). An immune response was elicited against the TG±hapten conjugate, and a panel of monoclonal antibodies with good antigen recognition was prepared and purified by conventional means. [17] After initially screening 20 antigen binders by competition ELISA (enzyme-linked immunosorbent assay), [18] antibody 2C2 was selected for detailed characterization because it recognized both the DMB nucleotide and the corrin ring.Antibody 2C2 binds 1 with an estimated dissociation constant (K d ) of 9.8 AE 2.6 mm. As summarized in Table 1, vitamin B 12 (2; Scheme 1) and several B 12 derivatives compete effectively with the antigen. Dicyanocobinamide (3), an analogue of 1 and 2 lacking the nucleotide function, is the weakest corrinoid ligand tested, and neither benzimidazole nor 2-amino-isopropylribazole phosphate [19] alone compete with the antigen. These results suggest that the complete cobalt-coordinating nucleotide loop is required for formation of the B 12 ±antibody complex. To test this hypothesis, pseudocoenzyme B 12 (4) [20,21] and Co b -5'-deoxyadenosyl factor A (5) [21,22] were also investigated as competitive inhibitors. These natural coenzyme B 12 analogues differ from 1 by the replacement of the DMB base by adenine and 2-methylade...

A cyclodecapeptide ligand to vitamin B12

“…To date only very few model systems of B 12 -binding proteins have been reported in the literature. These include antibodies 16 and dendritic peptide ligands. 5 Cyclodecapeptide ligands reproduce essential features of previous model systems, yet display a much simpler structure and are particularly easy to synthesize.…”

Chemistry and Enzymology of Vitamin B12

“…Hannak et al raised a monoclonal antibody to AdoCbl. The antibody binds AdoCbl strongly ( K d = 10 μM) and CNCbl less tightly ( K d = 65 μM) but binds the nucleotide loop-free (CN) 2 Cbi weakly ( K d = 740 μM), suggesting that it recognizes base-on cobamides.…”

“…This is very provocative work considering the fact that it is now generally accepted that the carbon skeleton rearrangements catalyzed by AdoCbl-dependent enzymes occur via radical rearrangements while the rearrangement mechanism in play here evidently goes via an anion. Hannak et al 170 raised a monoclonal antibody to AdoCbl. The antibody binds AdoCbl strongly (K d ) 10 µM) and CNCbl less tightly (K d ) 65 µM) but binds the nucleotide loop-free (CN) 2 Cbi weakly (K d ) 740 µM), suggesting that it recognizes base-on cobamides.…”

B12‐retro‐Riboswitches: Guanosyl‐Induced Constitutional Switching of B12 Coenzymes

“…Our interest in this novel aspect of B 12 chemistry [33,34] was strengthened by the recent discovery of the relevance of direct interactions between B 12 derivatives and an RNA environment in B 12 riboswitches. [25,27,62,63] This latter finding established the direct interaction between RNA and B 12 coenzymes to represent a new mechanism of genetic control, as suspected in related genetic experiments. [64] In contrast to the studies of the B 12 riboswitches, in which mRNA was observed to restructure as the result of B 12 binding, [25,26] our investigation focuses on the reverse effect in the B 12 moieties.…”