Abstract
Phosphonates represent an important source of bioavailable phosphorus in certain environments. Accordingly, many microorganisms (particularly marine bacteria) possess catabolic pathways to degrade these molecules. One example is the widespread hydrolytic route for the breakdown of 2-aminoethylphosphonate (AEP, the most common biogenic phosphonate). In this pathway, the aminotransferase PhnW initially converts AEP into phosphonoacetaldehyde (PAA), which is then cleaved by the hydrolase PhnX to yield acetaldehyde and phosphate. This work focuses on a pyridoxal 5′-phosphate-dependent enzyme that is encoded in >13% of the bacterial gene clusters containing the phnW–phnX combination. This enzyme (which we termed PbfA) is annotated as a transaminase, but there is no obvious need for an additional transamination reaction in the established AEP degradation pathway. We report here that PbfA from the marine bacterium Vibrio splendidus catalyzes an elimination reaction on the naturally occurring compound (R)-1-hydroxy-2-aminoethylphosphonate (R-HAEP). The reaction releases ammonia and generates PAA, which can be then hydrolyzed by PhnX. In contrast, PbfA is not active toward the S enantiomer of HAEP or other HAEP-related compounds such as ethanolamine and d,l-isoserine, indicating a very high substrate specificity. We also show that R-HAEP (despite being structurally similar to AEP) is not processed efficiently by the PhnW–PhnX couple in the absence of PbfA. In summary, the reaction catalyzed by PbfA serves to funnel R-HAEP into the hydrolytic pathway for AEP degradation, expanding the scope and the usefulness of the pathway itself.
Highlights
Phosphonates represent an important source of bioavailable phosphorus in certain environments
AEP degradation relies on an initial transamination catalyzed by PhnW, a pyridoxal 5′-phosphate (PLP)-dependent aminotransferase, that converts AEP into phosphonoacetaldehyde (PAA);[9,10] in turn, PAA is transformed into acetaldehyde and inorganic phosphate by the hydrolase PhnX11 (Figure 1A)
Because it is certain that additional enzyme systems exist for the catabolism of phosphonates, we performed a detailed analysis of the genomes of many microorganisms that degrade phosphonates, looking in particular for genes that encode PLP-dependent enzymes and are associated with gene clusters for aminophosphonate breakdown
Summary
Phosphonates represent an important source of bioavailable phosphorus in certain environments. One example is the widespread hydrolytic route for the breakdown of 2aminoethylphosphonate (AEP, the most common biogenic phosphonate) In this pathway, the aminotransferase PhnW initially converts AEP into phosphonoacetaldehyde (PAA), which is cleaved by the hydrolase PhnX to yield acetaldehyde and phosphate. The degradation of phosphonates is advantageous for microbes living in marine environments, where the bioavailability of phosphorus is often a limiting factor for growth.[7] In particular, many marine bacteria possess a specialized “hydrolytic” pathway for the breakdown of the natural compound 2-aminoethylphosphonate (AEP; known as ciliatine), which is the most widely distributed biogenic phosphonate in the environment.[4,8] AEP degradation relies on an initial transamination catalyzed by PhnW, a pyridoxal 5′-phosphate (PLP)-dependent aminotransferase, that converts AEP into phosphonoacetaldehyde (PAA);[9,10] in turn, PAA is transformed into acetaldehyde and inorganic phosphate by the hydrolase PhnX11 (Figure 1A). Because it is certain that additional enzyme systems (beyond those characterized so far) exist for the catabolism of phosphonates, we performed a detailed analysis of the genomes of many microorganisms that degrade phosphonates, looking in particular for genes that encode PLP-dependent enzymes and are associated with gene clusters for aminophosphonate breakdown
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