Bacterial 4-hydroxybutyrate oxidation to succinate

A reusable two-reaction bacterial pathway in which 4-hydroxybutyrate dehydrogenase oxidizes 4-hydroxybutyrate to succinate semialdehyde and a succinate-semialdehyde dehydrogenase oxidizes that intermediate to succinate. The second reaction can use NAD+ or NADP+ depending on enzyme family and organism. Uptake, upstream 4-hydroxybutyrate production, the GABA shunt, and downstream TCA-cycle oxidation are outside the core.

Pseudomonas putida KT2440 supplies a candidate Gbd exemplar but current local evidence does not identify whether Sad-I, Sad-II, or another aldehyde dehydrogenase is the physiological partner during 4-hydroxybutyrate utilization. Their inclusion as candidate exemplars records coverage, not proof that only one paralog contributes. PTHR43217 does not reliably resolve NAD versus NADP specificity for the KT2440 Sad candidates. Q88EN2/GabD-II was excluded after its doe-like ectoine-catabolic locus provided a stronger competing substrate hypothesis than the submitted SSADH name. This exclusion records the absence of evidence for a physiological 4-hydroxybutyrate role; it does not establish that Q88EN2 lacks SSADH side activity. The listed KT2440 proteins are a focused product-name/family candidate set, not a proof that every relevant aldehyde dehydrogenase paralog has been enumerated. No additional gene named gabD-I was found in the simple UniProt gene-list query, but the gabD-II suffix should not be interpreted as evidence of completeness. PTHR11496:SF102 is also broader than Gbd: its entries include proteins named as maleylacetate reductases and propanediol, methanol, or threonine dehydrogenases, so membership alone does not establish GO:0047577.