The mechanism of cobalamin adenosylation by the ACAT enzymes CobA and EutT
Moore, Theodore Carlton
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Cobalamin is an essential cofactor for many organisms across the three domains of life. Cobalamin is comprised of a central cobalt ion in an octahedral complex, ligated along the equatorial by the tetrapyrollic corrin ring. There are several biologically active varieties of cobalamin, each defined by the identities of their axial ligands. Among the most studied is adenosylcobalamin (AdoCbl, Coenzyme B12), which has a lower (α) ligand of 5,6-dimethylbenzimidazole (DMB) and an upper (β) ligand of 5’-deoxyadenosine. This adenosyl ligand is used to generate a radical Ado· species for use by isomerase enzymes. Cobalamin is only synthesized by bacteria and archaea, yet all organisms that use AdoCbl must be capable of synthesizing the α ligand. They do this through the use of ATP:Co(I)rrinoid adenosyltransferase (ACAT) enzymes, which overcome a thermodynamically difficult barrier to first reduce and then adenosylate a bound cobalamin molecule. There are three distinct ACAT families, CobA, EutT and PduO. CobA is regarded as the “housekeeping” ACAT in the model organism Salmonella enterica, and can catalyze adenosylation of cobalamin and several of its precursors. The first section of this work describes a structure-function analysis of a CobA crystal structure in complex with a cobalamin substrate and proposes a mechanism of action. Of the three ACAT families, little is known about EutT. In S. enterica, it participates in ethanolamine catabolism. The second section of this work describes biochemical studies of homogeneous EutT. EutT appears to be unique among the ACATs in that its activity is dependent on a divalent metal cation as a cofactor, which is likely used for proper orientation of the substrate in the active site. The third section of this work examines two proteins of unknown function in the eut operon, which were suspected of interacting with EutT. This hypothesis was incorrect, yet EutP and EutQ were shown to be a novel class of acetate kinase. Additionally, a phenotype was described for a eutQ strain of S. enterica growing anaerobically on ethanolamine. These data suggest that EutP and EutQ are important for substrate-level phosphorylation, cofactor recycling, carbon flux, or some combination of the three.