Two distinct alcohol dehydrogenase classes mediate bacterial 2-hydroxyethylphosphonate catabolism.

Phosphonates are molecules containing a direct carbon-phosphorus bond. In nature, these compounds are produced by a variety of organisms, and many environmental microbes are able to degrade specific phosphonates and feed on them. This work focuses on 2-hydroxyethylphosphonate (HEP)-a natural compound that is found frequently in the environment, but whose microbial catabolism has remained poorly characterized. Starting from genomic analyses, we report here that specific pathways for HEP degradation in bacteria do exist and rely on the oxidation of HEP into the activated compound phosphonoacetaldehyde (PAA), which can be further processed by known hydrolases. The key conversion of HEP to PAA can be carried out by two unrelated groups of dehydrogenases: one comprises homologs of the biosynthetic enzyme PhpC (PAA reductase), whereas the other consists of newly described enzymes that we termed PbfG. Despite sharing the same substrates and using Zn2+ as the metal cofactor, PhpC-type and PbfG-type dehydrogenases exhibit markedly different structural and catalytic features, outlining a clear example of convergent evolution. However, PbfG enzymes are catalytically more efficient and apparently more specialized for HEP degradation. Overall, our results reveal the ample distribution in bacteria of specific routes for HEP catabolism. Furthermore, although the degradation pathways for HEP and of 2-aminoethylphosphonate (AEP; the most common natural phosphonate) share some enzymes, the corresponding gene clusters are often distinct, suggesting a selective advantage in keeping the two catabolic processes separate.