| Description | This clade of sequences are the closest homologues to the PhnX enzyme, phosphonoacetaldehyde (Pald) hydrolase (phosphonatase, ). This phosphonatase-like enzyme and PhnX itself are members of the haloacid dehalogenase (HAD) superfamily having a a number of distinctive features that set them apart from typical HAD enzymes. The typical HAD N-terminal motif DxDx(T/V) here is DxAGT and the usual conserved lysine prior to the C-terminal motif is instead an arginine. Also distinctive of phosphonatase, and particular to its bi-catalytic mechanism is a conserved lysine in the variable "cap"domain [ ]. This lysine forms a Schiff base with the aldehyde of phosphonoacetaldehyde, providing, through the resulting positive charge, a polarization of the C-P bond necesary for cleavage as well as a route to the initial product of cleavage, an ene-amine. The conservation of these elements in this phosphonatase-like enzyme suggests that the substrate is also, like Pald, a 2-oxo-ethylphosphonate. Despite this, the genomic context of members of this family are quite distinct from PhnX, which is almost invariably associated with the 2-aminoethylphosphonate transaminase PhnW (), the source of the substrate Pald. Members of this clade are never associated with PhnW, but rather associate with families of FAD-dependent oxidoreductases related to deaminating amino acid oxidases () as well as zinc-dependent dehydrogenases ( ). Notably, family members from Arthrobacter aurescens TC1 and Nocardia farcinica IFM 10152 are adjacent to the PhnCDE ABC cassette phosphonates transporter ( ) typically found in association with the phosphonates C-P lyase system ( ). These observations suggest two possibilities. First, the substrate for this enzyme family is also Pald, the non-association with PhnW not withstanding. Alternatively, the substrate is something very closely related such as hydroxyphosphonoacetaldehyde (Hpald). Hpald could come from oxidative deamination of 1-hydroxy-2-aminoethylphosphonate (HAEP) by the associated oxidase. HAEP would not be a substrate for PhnW due to its high specificity for AEP. HAEP has been shown to be a constituent of the sphingophosphonolipid of Bacteriovorax stolpii [ ], and presumably has other natural sources. If Hpald is the substrate, the product would be glycoaldehyde (hydroxyacetaldehyde), and the associated alcohol dehydrogenase may serve to convert this to glycol. | Name | Phosphonatase-like hydrolase |
| Short Name | PhnX-like | Type | Family |
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