FAHD2B

Model summary

A soluble mitochondrial enzyme that assembles as an oligomer to catalyze the hydro-lyase step that converts fumarylacetoacetate into fumarate and acetoacetate during tyrosine and phenylalanine catabolism. By feeding fumarate into central carbon metabolism and supplying acetoacetate that interfaces with short-chain acyl-CoA pools, it functionally links amino acid degradation to mitochondrial energy and lipid-derived pathways. Its activity depends on a conserved catalytic scaffold and self-association that stabilizes the active site within the mitochondrial matrix.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The polypeptide core is defined by IPR051121 (Fumarylacetoacetate hydrolase family, residues 19–314), which establishes membership in the canonical hydrolase family that catalyzes the hydro-lyase step of fumarylacetoacetate breakdown. This family-defining region is embedded within IPR036663 (Fumarylacetoacetase-like, C-terminal domain superfamily, residues 51–314) and capped by IPR011234 (Fumarylacetoacetase-like, C-terminal domain, residues 107–314). The ordered layout—an N-terminal family signature leading into a conserved C-terminal catalytic scaffold—creates a soluble, metal-assisted active site typical of the FAH fold. This architecture causes hydro-lyase chemistry that cleaves fumarylacetoacetate into fumarate and acetoacetate, matching GO:0016836 hydro-lyase activity. FAH-family enzymes frequently assemble as homodimers or higher-order oligomers; the conserved C-terminal interface supports GO:0042802 identical protein binding as a functional requirement for active-site formation and allosteric stability.

2. 02

   From this catalytic base, I infer the biological role.

   Hydro-lyase activity on fumarylacetoacetate is a committed step in the catabolism of tyrosine and phenylalanine, funneling carbon skeletons into central metabolism. The immediate products—acetoacetate and fumarate—tie the enzyme to broader pathways: acetoacetate interconverts with acetyl-CoA and short-chain acyl-CoA pools, and fumarate feeds the tricarboxylic acid cycle. These connections rationalize inclusion in GO:0006637 acyl-CoA metabolic process and GO:0019626 short-chain fatty acid catabolic process, as the enzyme’s output drives downstream handling of acyl intermediates.

3. 03

   Cellular placement follows from the soluble FAH-like fold and the metabolic context.

   The absence of transmembrane features and the soluble FAH scaffold argue for a matrix-localized enzyme. Its coupling to fumarate handling and short-chain acyl-CoA flux situates it in the mitochondrial matrix, consistent with GO:0005759 mitochondrial matrix. A matrix residence also explains how its activity directly supports oxidative metabolism.

4. 04

   Mechanistically, I propose a homodimeric FAH-like assembly that coordinates a divalent metal at the C-terminal catalytic pocket to polarize and hydrolyze fumarylacetoacetate.

   Dimerization (identical protein binding) stabilizes the active site geometry and may enable cooperative regulation. In the matrix, the enzyme likely forms transient assemblies with mitochondrial one-carbon and acyl-CoA–handling enzymes to channel products efficiently. Candidate partners include serine hydroxymethyltransferase and folate-cycle enzymes that balance one-carbon units, as well as acyl-CoA thiolases and citrate synthase that receive fumarate and short-chain acyl flux. Together, these features define a soluble mitochondrial hydro-lyase that anchors tyrosine/phenylalanine catabolism to central energy and acyl-CoA metabolism through oligomeric assembly and matrix localization.