Bacterial free-acid 4-hydroxybutyrate oxidation to succinate

Two consecutive non-CoA reactions oxidize 4-hydroxybutyrate to succinate: NAD-dependent Gbd forms succinate semialdehyde, and a nonacylating succinate-semialdehyde dehydrogenase forms succinate. The experimentally supported implementation is grounded in selected or experimentally expressing Cupriavidus necator backgrounds. Pseudomonas putida candidates and a distinct 3-hydroxybutyrate enzyme provide comparative context without establishing a complete native free-acid route. Uptake, upstream diol oxidation, GABA supply, CoA-dependent beta-oxidation, PHA synthesis and reverse succinate degradation are outside the boundary.

One experimentally grounded species-level route is required; optional Pseudomonas comparisons cannot complete it, even when mixed with a single Cupriavidus enzyme. Exact selectors preserve all original accessions while avoiding a broad family-wide activity claim. Selected-strain expression and actual flux are not computed from sequence presence by the Boolean compiler. No claim is made that these proteins form a stable complex, that both Cupriavidus enzymes were jointly purified/reconstituted, or that the named Sad candidates exhaust all SSA enzymes. Every SSA-oxidation annoton uses the cofactor-general GO:0009013, following one convention for dual-acceptor succinate-semialdehyde dehydrogenases across this module and its member reviews. One active site running one reaction with either nicotinamide acceptor is stated with the parent term, which is what its definition says, and the cofactor-specific children are kept as annotations at their measured weights rather than as separate core functions. So the Cupriavidus GabD core is GO:0009013 with NADP as the measured major branch and NAD the minor one, the E. coli Sad core is GO:0009013 with the preference reversed, and for the optional Pseudomonas Sad candidates the acceptor preference is simply unresolved. This is not an assertion of cofactor exclusivity in any of the four cases. On taxonomic scope, the second reaction is kept species-neutral and sampled in three genera, because it is broadly distributed and experimentally settled outside Cupriavidus, with Escherichia coli Sad added as a characterized exemplar. The first reaction is not generalized. Free-acid 4HB oxidation is experimentally grounded only in selected Cupriavidus backgrounds, the Pseudomonas candidate for it rests on an unchecked submitter name that its gene review marks as an over-annotation, and E. coli has no demonstrated 4HB dehydrogenase. The module therefore stays CONCRETE, with a bacterial envelope and named exemplars, rather than asserting a conserved pan-bacterial route.