A Novel Pathway for Bacterial Ethanolamine Metabolism

Abstract

Ethanolamine, a major component of the cell membranes of bacteria and animals in the form of phosphatidylethanolamine, can be a source of carbon and/or nitrogen for many bacterial species. The canonical model for bacterial ethanolamine utilization (eut) requires the adenosylcobalamin-dependent enzyme ethanolamine ammonia-lyase, which catalyzes the conversion of ethanolamine to ammonia and acetaldehyde for cellular nitrogen and carbon, respectively. Here, we report discovery and characterization of a novel pathway for bacterial ethanolamine metabolism in organisms that lack canonical eut genes. We use molecular docking, enzymology, microbiology, and mass spectrometry to delineate a four-enzyme pathway for ethanolamine catabolism that terminates in glycine, a source of cellular nitrogen. Initially, a gamma-glutamyl synthetase catalyzes the ATP-dependent condensation of ethanolamine with L-glutamate to form γ-glutamyl-ethanolamide. Oxidation of the hydroxyl group of γ-glutamyl-ethanolamide by an alcohol dehydrogenase and subsequent oxidation of the resulting aldehyde by an aldehyde dehydrogenase leads to formation of γ-glutamyl glycine. Finally, hydrolysis of the γ-glutamyl glycine peptide bond by an amidohydrolase yields L-glutamate and glycine, which is assimilated as a nitrogen source. Using the Enzyme Function Initiative Enzyme Similarity Tool (EFI-EST) and Genome Neighborhood Tool (EFI-GNT), we show that a diverse set of organisms ranging from the Actinobacteria to the Proteobacteria possess the genetic capability for non-eut ethanolamine metabolism, with multiple apparent instances of non-orthologous gene displacement (NOD) between different phyla. This research was supported by US National Institutes of Health grant U54GM093342.