Reductive pyrimidine degradation

A three-reaction pathway that degrades the pyrimidine bases uracil and thymine to beta-alanine or 3-aminoisobutanoate, carbon dioxide and ammonium. Dihydropyrimidine dehydrogenase first reduces the C5=C6 bond of the pyrimidine ring, dihydropyrimidinase hydrolytically opens the saturated ring to an N-carbamoyl-beta-amino acid, and beta-ureidopropionase (beta-alanine synthase) releases the beta-amino acid. The chemistry is conserved across bacteria and eukaryotes, but the first and third reactions are carried out by structurally distinct enzymes: a single-chain NADPH-dependent dehydrogenase in eukaryotes versus a two-subunit NADH-dependent enzyme in bacteria, and a nitrilase-superfamily beta-alanine synthase in animals, plants and amoebae versus an unrelated di-zinc M20-family enzyme in fungi.

Scope. The module covers the three enzymatic steps from uracil (or thymine) to beta-alanine (or 3-aminoisobutanoate). Pyrimidine nucleotide dephosphorylation and nucleoside cleavage (pyrimidine_nucleotide_catabolism), base transport, beta-alanine and 3-aminoisobutanoate utilization, CoA synthesis and fluoropyrimidine pharmacology are outside the core. The oxidative Rut pathway of E. coli and other bacteria degrades uracil by a different route (ring opening by the RutA monooxygenase) and belongs in a separate module; it is not a variant of this one. Taxonomic generalization. Exemplars come from human (all three steps), P. putida KT2440 (steps 1 and 2), E. coli K-12 (step 1) and the yeast Lachancea kluyveri (step 3). E. coli PreT-PreA is the biochemically characterized bacterial first-step enzyme, but E. coli lacks the downstream steps, so it grounds the activity without satisfying the whole module. The pathway is drawn with the uracil-to-beta-alanine branch; thymine passes through the same three enzymes to 3-aminoisobutanoate. Grounding. The PANTHER families containing these enzymes are functionally heterogeneous: PTHR43073 also holds archaeal dihydroorotate dehydrogenase B subunits, PTHR11647 holds catalytically inactive CRMP/DPYSL proteins, D-hydantoinases and acyl-amino-acid deacylases, PTHR43674 holds amidases, formamidases and N-carbamoylputrescine amidases, and PTHR32494 holds allantoate amidohydrolases. Eukaryotic variants are therefore grounded on the PAINT ancestral nodes at which PAINT curators placed each activity (PTN000234105, PTN000182450, PTN000557641), and lineage-specific bacterial and fungal variants on ORTHOLOG_OF anchors with a required function. The PydXA subunits keep the PTHR43073 family term because no function is asserted for the individual subunits. The family reviews record where each pathway-specific clade sits in its family. Bacterial terminal step. No bacterial beta-ureidopropionase with experimentally established specificity is grounded here; in P. putida KT2440 the candidates Q88FQ3 (HyuC, PTHR32494) and PP_0614/Q88Q81 remain unresolved (see knowledge gaps), so the module is not satisfied end-to-end in any bacterium represented.