If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Synonyms: purine fermentation
|Superclasses:||Degradation/Utilization/Assimilation → Nucleosides and Nucleotides Degradation → Purine Nucleotides Degradation|
|Generation of Precursor Metabolites and Energy → Fermentation|
Expected Taxonomic Range: Firmicutes
The anaerobic fermentation of purines was first observed in the organisms Gottschalkia acidurici and Clostridium cylindrosporum in 1942 [Barker42]. These organisms can utilize only a small number of purine derivatives including purine, 8-hydroxypurine, 6,8-dihydroxypurine, hypoxanthine, guanine and xanthine under anaerobic conditions as carbon, nitrogen, and energy sources, and are unable to ferment any other organic compounds. The purines urate, guanine and xanthine are readily decomposed by these organisms, while growth on hypoxanthine is appreciably slower, and adenine does not support growth at all [Barker42]. The slow growth on hypoxanthine was explained upon the characterization of the enzyme xanthine dehydrogenase from Clostridium cylindrosporum. The enzyme was shown to catalyze not only the reversible conversion of urate to xanthine, but also the oxidation of most other substrates, sometimes requiring multiple steps, eventually converging at xanthine. While the enzyme is able to oxidize hypoxanthine to xanthine, this reaction proceeded very slowly [Rabinowitz56a].
The products of this fermentation were identified as ammonia, CO2 and acetate. Early studies using tracer experiments revealed that glycine, formate and L-serine were intermediates of the pathway [Barker41, Radin53, Rabinowitz56b, Beck56].
Work by Rabinowitz and coworkers helped discover many of the intermediates of this pathway. The intermediates 4-amino-5-imidazole carboxylate and 4-aminoimidazole were discovered first [Rabinowitz56c], followed by 4-ureido-5-imidazole carboxylate [Rabinowitz56d] and N-formimino-glycine [Pricer56].
The next step of the pathway is the transfer of a formimino group from N-formimino-glycine to tetrahydropteroyl mono-L-glutamate, generating glycine and a 5-formiminotetrahydrofolate. The discovery of this step eventually lead Rabinowitz to focus on folate metabolism, leading to significant progress in the understanding of that field.
Deciphering the later steps of the pathway was more complex. Formate is generated via the intermediates 5,10-methenyltetrahydrofolate mono-L-glutamate and 10-formyl-tetrahydrofolate mono-L-glutamate, by the enzymes formimidoyltetrahydrofolate cyclodeaminase (EC 126.96.36.199), methenyltetrahydrofolate cyclohydrolase (EC 188.8.131.52) and formate--tetrahydrofolate ligase (EC 184.108.40.206) [Himes62]. Formate is then converted to CO2 by an NAD-dependent formate dehydrogenase.
Acetate is generated from pyruvate as described below.
Rresearch conducted about two decades after the intial characterization of the pathway has shown that several branches of this pathway are modified when the cells are supplied with selenium, due to the activation of certain L-selenocysteine-containing enzymes. While in the pathway described here purines are oxidized largly via urate by the enzyme xanthine dehydrogenase, in selenium-supplemented cells a second enzyme, purine hydroxylase, oxidizes purines directly to xanthine. In addition, a selenium-dependent glycine reductase reduces glycine directly to acetyl phosphate, bypassing the branch of the pathway described below. The selenium-dependent pathway is described in purine nucleobases degradation I (anaerobic).
About This Pathway
Early studies showed that one of the carbons in the acetate that is produced originates from glycine, while the other carbon originates from carbon C5 of urate [Rabinowitz56b]. In addition, pyruvate and L-serine were also shown to be intermediates in the pathway [Radin53, SchieferUllrich84]. A preliminary suggestion for a pathway that accomodates all of these observations has been suggested in 1956 [Beck56], followed by a more refined model in 1961 [Sagers61]. As mentioned above, the splitting of N-formimino-glycine generates a glycine molecule and a formimino group that is transferred to tetrahydropteroyl mono-L-glutamate, forming a 5-formiminotetrahydrofolate [Uyeda65]. According to the model, a 5-formiminotetrahydrofolate is converted to 5,10-methenyltetrahydrofolate mono-L-glutamate [Uyeda67] and then to 5,10-methylenetetrahydropteroyl mono-L-glutamate [Uyeda67a]. The later transfers the C1 group to the glycine molecule formed earlier, releasing the folate cofactor and generating L-serine [Hougland79]. L-serine is then split into ammonia and pyruvate [Benziman60], which is converted to acetate via acetyl-CoA and acetyl phosphate [Sagers61]. this last step, catalyzed by acetate kinase, probably represents the major energy-yielding reaction during purine fermentation by these organisms.
Variants: pseudouridine degradation, purine deoxyribonucleosides degradation I, purine deoxyribonucleosides degradation II, purine nucleobases degradation I (anaerobic), purine nucleotides degradation I (plants), purine nucleotides degradation II (aerobic), purine ribonucleosides degradation, urate biosynthesis/inosine 5'-phosphate degradation
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