I will discuss differences in the strategies used by archaea and bacteria to salvage the coenzyme B12 precursor cobinamide (Cbi) from their environments. Cbi is an incomplete corrinoid that lacks the structure known as the nucleotide loop, which tethers the lower ligand base to the corrin macrocycle. Cbi also lacks the 5'-deoxyadenosine (Ado) ligand, which needs to be attached to Cbi before it can be converted into coenzyme B12 (aka AdoB12 or AdoCbl). From our studies of Cbi salvaging, we conclude that Cbi must be present in diverse ecosystems, given the fact that bacteria and archaea have evolved different enzymes to transport it into the cell and convert it into its complete coenzymic form. Cbi salvaging is important to all cells, because this capability saves them a great deal of energy, which cells would otherwise have to spend synthesizing the enzymatic machinery needed to assemble the corrin ring (~20 enzymes).
I will discuss results of structure-function studies of a type of ATP:Co(I)rrinoid adenosyltransferase enzyme found in humans, archaea and bacteria. These results have shed extraordinary insights into how this type of widely distributed enzyme works. Bacteria and archaea employ different pathways for the synthesis of the nucleotide loop. The discovery of an amidohydrolase in archaea capable of using Cbi as substrate, led to the elucidation of the new pathway for Cbi salvaging. However, we have found homologs of the gene encoding the archaeal amidohydrolase in several bacteria. Genetic and biochemical data will be presented to demonstrate the functionality of the bacterial genes. It is intriguing that some bacteria have the archaeal amidohydrolase-dependent pathway for Cbi salvaging and the enzymes known in Salmonella enterica to be important for Cbi salvaging. Finally, I will discuss recent findings that fill important gaps of knowledge of how archaea and bacteria synthesize this important coenzyme.
