HEM4 encodes uroporphyrinogen-III synthase (UROS, EC 4.2.1.75), the fourth enzyme of tetrapyrrole biosynthesis. It cyclises the linear tetrapyrrole hydroxymethylbilane, made by Hem3p, into the asymmetric macrocycle uroporphyrinogen III, inverting ring D. Without the enzyme the substrate cyclises spontaneously to the non-physiological uroporphyrinogen I isomer. Uroporphyrinogen III is the last common precursor of heme and siroheme. Yeast UROS shares only 18-23% sequence identity with UROS from other organisms. hem4 deletion abolishes UROS activity and causes porphyrin accumulation, respiratory deficiency and heme auxotrophy. The hem4 null strain has served as a complementation host for identifying plant UROS. The gene was first isolated as a complementing locus for the ts2326 and cyt mutations and was originally thought to be involved in mitochondrial import, an attribution later corrected.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004852 uroporphyrinogen-III synthase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000273375 (UROS family, PTHR12390). Hem4p is a divergent but genuine UROS (18-23% identity) whose loss abolishes UROS activity; the inherited activity is correct. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt PANTHER; PTHR12390; UROPORPHYRINOGEN III SYNTHASE; 1. PMID:7597845 it encodes a protein with 18-23% identity to uroporphyrinogen III synthases from different sources PMID:7597845 Biochemical analysis confirmed the absence of uroporphyrinogen III synthase activity in both hem4 delta and cyt mutant strains. |
| GO:0004852 uroporphyrinogen-III synthase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Combined IEA (ARBA, InterPro IPR003754/IPR036108 UROS domain, RHEA:18965, EC 4.2.1.75). Correct mapping. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt Reaction=hydroxymethylbilane = uroporphyrinogen III + H2O; |
| GO:0004852 uroporphyrinogen-III synthase activity | IMP PMID:7597845 Isolation of the gene HEM4 encoding uroporphyrinogen III syn... | ACCEPT | Summary: Amillet & Labbe-Bois showed that hem4 deletion (and the allelic cyt mutation) abolish uroporphyrinogen III synthase activity and cause porphyrin accumulation and respiratory deficiency; supports the activity. Supporting Evidence: PMID:7597845 Biochemical analysis confirmed the absence of uroporphyrinogen III synthase activity in both hem4 delta and cyt mutant strains. PMID:7597845 This enzyme catalyses the fourth step in the heme biosynthetic pathway and we named its gene HEM4. |
| GO:0004852 uroporphyrinogen-III synthase activity | ISS PMID:7597845 Isolation of the gene HEM4 encoding uroporphyrinogen III syn... | ACCEPT | Summary: Same study: the HEM4 product shares 18-23% identity with UROS enzymes from other organisms. Low identity, but combined with the loss-of-activity mutant data the assignment is sound. Supporting Evidence: PMID:7597845 it encodes a protein with 18-23% identity to uroporphyrinogen III synthases from different sources |
| GO:0004852 uroporphyrinogen-III synthase activity | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA from PWY3O-50 (tetrapyrrole biosynthesis): preuroporphyrinogen (hydroxymethylbilane) -> uroporphyrinogen III + H2O (EC 4.2.1.75). Correct reaction for Hem4p. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt Reaction=hydroxymethylbilane = uroporphyrinogen III + H2O; |
| GO:0004852 uroporphyrinogen-III synthase activity | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA from PWY3O-69 (superpathway of heme and siroheme biosynthesis): preuroporphyrinogen (hydroxymethylbilane) -> uroporphyrinogen III + H2O (EC 4.2.1.75). Correct reaction for Hem4p. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt Reaction=hydroxymethylbilane = uroporphyrinogen III + H2O; |
| GO:0005829 cytosol | IBA GO_REF:0000033 | ACCEPT | Summary: IBA is_active_in cytosol from PTN000273375 (UROS family, PTHR12390). UROS is a soluble cytosolic enzyme in fungi and animals (the plastid-targeted plant enzyme notwithstanding). Hem4p has no targeting signal and works between the cytosolic Hem3p and Hem12p steps; no direct localisation of Hem4p is cached, so the row rests on the family inference, which is sound. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt PANTHER; PTHR12390; UROPORPHYRINOGEN III SYNTHASE; 1. file:yeast/HEM4/HEM4-uniprot.txt FUNCTION: Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, the fourth step in the heme biosynthetic pathway. |
| GO:0005829 cytosol | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA cytosolic location from PWY3O-50 (tetrapyrrole biosynthesis). Consistent with Hem4p being a soluble enzyme acting between the cytosolic Hem3p and Hem12p steps; accepted on the same family inference as the IBA row. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt FUNCTION: Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, the fourth step in the heme biosynthetic pathway. |
| GO:0005829 cytosol | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA cytosolic location from PWY3O-69 (superpathway of heme and siroheme biosynthesis). Consistent with Hem4p being a soluble enzyme acting between the cytosolic Hem3p and Hem12p steps; accepted on the same family inference as the IBA row. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt FUNCTION: Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, the fourth step in the heme biosynthetic pathway. |
| GO:0006780 uroporphyrinogen III biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000273375 to uroporphyrinogen III biosynthetic process. Hem4p catalyses the step that forms uroporphyrinogen III, so this is a precise and correct process term. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt PANTHER; PTHR12390; UROPORPHYRINOGEN III SYNTHASE; 1. file:yeast/HEM4/HEM4-uniprot.txt Reaction=hydroxymethylbilane = uroporphyrinogen III + H2O; |
| GO:0006783 heme biosynthetic process | IMP PMID:14559249 Identification of rate-limiting steps in yeast heme biosynth... | ACCEPT | Summary: Hoffman et al. studied rate-limiting steps of yeast heme biosynthesis; the cached abstract only describes Hem2p/Hem3p overproduction, so the HEM4-specific data are presumably in the full text. Hem4p is unambiguously a heme pathway enzyme, so the curator annotation is accepted. Supporting Evidence: PMID:14559249 In an attempt to identify rate-limiting steps in heme synthesis, which may constitute potential regulatory points |
| GO:0006783 heme biosynthetic process | IMP PMID:18042043 Identification and characterization of the Arabidopsis gene ... | ACCEPT | Summary: Tan et al. constructed a hem4 deletion strain that requires heme for growth, and rescued it with Arabidopsis UROS cDNA; the hem4 null phenotype supports involvement in heme biosynthesis. Supporting Evidence: PMID:18042043 A mutant of Saccharomyces cerevisiae was constructed in which the HEM4 gene encoding UROS was deleted. PMID:18042043 two colonies were obtained that could grow in the absence of haem |
| GO:0006783 heme biosynthetic process | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA from PWY3O-69 (superpathway of heme and siroheme biosynthesis). Hem4p forms uroporphyrinogen III, the branch-point precursor of heme and siroheme; correct. Supporting Evidence: PMID:7597845 This enzyme catalyses the fourth step in the heme biosynthetic pathway and we named its gene HEM4. |
| GO:0033014 tetrapyrrole biosynthetic process | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the UROS domain (IPR003754, IPR036108). Correct: uroporphyrinogen III is the precursor of all tetrapyrroles (heme and siroheme in yeast). Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt FUNCTION: Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, the fourth step in the heme biosynthetic pathway. |
| GO:0033014 tetrapyrrole biosynthetic process | RCA GO_REF:0000123 | ACCEPT | Summary: YeastPathways RCA from PWY3O-50 (tetrapyrrole biosynthesis) (tetrapyrrole biosynthesis). Correct. Supporting Evidence: file:yeast/HEM4/HEM4-uniprot.txt FUNCTION: Catalyzes cyclization of the linear tetrapyrrole, hydroxymethylbilane, to the macrocyclic uroporphyrinogen III, the fourth step in the heme biosynthetic pathway. |
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Download this section (compressed HTML)Q: Given its low sequence identity to other UROS enzymes, does yeast Hem4p share the two-domain fold and active-site residues of human UROS, and is it, like the human enzyme, unusually thermolabile?
Experiment: Titrate HEM4 expression with a regulatable promoter and quantify uroporphyrin I vs III isomers and heme/siroheme output by HPLC.
Hypothesis: Hem4p availability limits the partition of hydroxymethylbilane between uroporphyrinogen III and spontaneous uroporphyrinogen I formation in vivo
Type: genetic titration / metabolite analysis
Uroporphyrinogen III synthase; catalyzes the conversion of hydroxymethylbilane to uroporphyrinogen III, the fourth step in heme biosynthesis; deficiency in the human homolog can result in the disease congenital erythropoietic porphyria
Uroporphyrinogen-III synthase (UROS).
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