The effects in vivo of mutationally modified uroporphyrinogen decarboxylase in different hem12 mutants of baker's yeast (Saccharomyces cerevisiae)

Analysis of the role of the Aspergillus niger aminolevulinic acid synthase (hemA) gene illustrates the difference between regulation of yeast and fungal haem- and sirohaem-dependent pathways

To increase knowledge on haem biosynthesis in filamentous fungi like Aspergillus niger, pathway-specific gene expression in response to haem and haem intermediates was analysed. This analysis showed that iron, 5'-aminolevulinic acid (ALA) and possibly haem control haem biosynthesis mostly via modulating expression of hemA [coding for 5'-aminolevulinic acid synthase (ALAS)]. A hemA deletion mutant (ΔhemA) was constructed, which showed conditional lethality. Growth of ΔhemA was supported on standard nitrate-containing media with ALA, but not by hemin. Growth of ΔhemA could be sustained in the presence of hemin in combination with ammonium instead of nitrate as N-source. Our results suggest that a branch-off within the haem biosynthesis pathway required for sirohaem synthesis is responsible for lack of growth of ΔhemA in media containing nitrate as sole N-source, because of the requirement of sirohaem for nitrate assimilation, as a cofactor of nitrite reductase. In contrast to the situation in Saccharomyces cerevisiae, cysteine, but not methionine, was found to further improve growth of ΔhemA. These results demonstrate that A. niger can use exogenous hemin for its cellular processes. They also illustrate important differences in regulation of haem biosynthesis and in the role of haem and sirohaem in A. niger compared to S. cerevisiae.

Sampangine inhibits heme biosynthesis in both yeast and human

The azaoxoaporphine alkaloid sampangine exhibits strong antiproliferation activity in various organisms. Previous studies suggested that it somehow affects heme metabolism and stimulates production of reactive oxygen species (ROS). In this study, we show that inhibition of heme biosynthesis is the primary mechanism of action by sampangine and that increases in the levels of reactive oxygen species are secondary to heme deficiency. We directly demonstrate that sampangine inhibits heme synthesis in the yeast Saccharomyces cerevisiae. It also causes accumulation of uroporphyrinogen and its decarboxylated derivatives, intermediate products of the heme biosynthesis pathway. Our results also suggest that sampangine likely works through an unusual mechanism-by hyperactivating uroporhyrinogen III synthase-to inhibit heme biosynthesis. We also show that the inhibitory effect of sampangine on heme synthesis is conserved in human cells. This study also reveals a surprising essential role for the interaction between the mitochondrial ATP synthase and the electron transport chain.

Heme biosynthesis and its regulation: towards understanding and improvement of heme biosynthesis in filamentous fungi

Heme biosynthesis in fungal host strains has acquired considerable interest in relation to the production of secreted heme-containing peroxidases. Class II peroxidase enzymes have been suggested as eco-friendly replacements of polluting chemical processes in industry. These peroxidases are naturally produced in small amounts by basidiomycetes. Filamentous fungi like Aspergillus sp. are considered as suitable hosts for protein production due to their high capacity of protein secretion. For the purpose of peroxidase production, heme is considered a putative limiting factor. However, heme addition is not appropriate in large-scale production processes due to its high hydrophobicity and cost price. The preferred situation in order to overcome the limiting effect of heme would be to increase intracellular heme levels. This requires a thorough insight into the biosynthetic pathway and its regulation. In this review, the heme biosynthetic pathway is discussed with regards to synthesis, regulation, and transport. Although the heme biosynthetic pathway is a highly conserved and tightly regulated pathway, the mode of regulation does not appear to be conserved among eukaryotes. However, common factors like feedback inhibition and regulation by heme, iron, and oxygen appear to be involved in regulation of the heme biosynthesis pathway in most organisms. Therefore, they are the initial targets to be investigated in Aspergillus niger.

A conserved cysteine residue in yeast uroporphyrinogen decarboxylase is not essential for enzymatic activity

Uroporphyrinogen decarboxylase catalyzes the fifth step of heme biosynthesis in Saccharomyces cerevisiae. Studies utilizing sulfhydryl-specific reagents suggest that the enzyme requires a cysteine residue within the catalytic site. This hypothesis was tested directly by site-directed mutagenesis of highly conserved cysteine-52 to serine or alanine. Plasmids containing these mutations were able to complement a hem6 mutant strain. In addition, properties associated with decreased uroporphyrinogen decarboxylase activity were not detected in the mutant strain transformed with these mutant plasmids. These results suggest that the conserved cysteine-52 by itself is not essential for enzymatic activity.

Reduction of Uroporphyrinogen Decarboxylase by Antisense RNA Expression Affects Activities of Other Enzymes Involved in Tetrapyrrole Biosynthesis and Leads to Light-Dependent Necrosis

We introduced a full-length cDNA sequence encoding tobacco (Nicotiana tabacum) uroporphyrinogen III decarboxylase (UROD; EC 4.1.1.37) in reverse orientation under the control of a cauliflower mosaic virus 35S promoter derivative into the tobacco genome to study the effects of deregulated UROD expression on tetrapyrrole biosynthesis. Transformants with reduced UROD activity were characterized by stunted plant growth and necrotic leaf lesions. Antisense RNA expression caused reduced UROD protein levels and reduced activity to 45% of wild type, which was correlated with the accumulation of uroporphyrin(ogen) and with the intensity of necrotic damage. Chlorophyll levels were only slightly reduced (up to 15%), indicating that the plants sustained cellular damage from accumulating photosensitive porphyrins rather than from chlorophyll deficiency. A 16-h light/8-h dark regime at high-light intensity stimulates the formation of leaf necrosis compared with a low-light or a 6-h high-light treatment. Transgenic plants grown at high light also showed inactivation of 5-aminolevulinate dehydratase and porphobilinogen deaminase, whereas the activity of coproporphyrinogen oxidase and the 5-aminolevulinate synthesizing capacity were not altered. We conclude that photooxidation of accumulating uroporphyrin(ogen) leads to the generation of oxygen species, which destabilizes other enzymes in the porphyrin metabolic pathway. This porphyrin-induced necrosis resembles the induction of cell death observed during pathogenesis and air pollution.

Saccharomyces cerevisiae mutants defective in heme biosynthesis as a tool for studying the mechanism of phototoxicity of porphyrins

Mutants of Saccharomyces cerevisiae accumulating uroporphyrin (UP) or protoporphyrin (PP) were used as a model for the in vivo phototoxic effect of porphyrins observed in the human skin photosensitivity associated with porphyrias (porphyria cutanea tarda and erythropoietic protoporphyria). We have found that UP is localized in vacuoles and PP is present in all compartments except vacuoles in yeast cells. Endogenous PP is much more effective as a photosensitizer of yeast cells than UP. Protoporphyrin action is strictly dependent on the presence of oxygen. In contrast, UP displays a phototoxic effect even if oxygen is not present in the suspension, implicating a free radical mechanism that operates in anaerobiosis upon photosensitization by UP. Catalase or superoxide dismutase deficiency affects photosensitization by UP. A possible mechanism of UP photosensitizing activity is discussed.

Isolation and characterization of extragenic mutations affecting the expression of the uroporphyrinogen decarboxylase gene (HEM12) in Sacharomyces cerevisiae

Uroporphyrinogen decarboxylase (Uro-d; EC 4.1.1.37), the fifth enzyme in the heme biosynthetic pathway, which catalyzes the sequential decarboxylation of uroporphyrinogen to coproporphyrinogen, is encoded by the HEM12 gene in Saccharomyces cerevisiae. The HEM12 gene is transcribed into a major short mRNA and a minor longer one, approximately 1.35 and 1.55 kb, respectively, in size, and that differ in the 5' untranslated region. "Uroporphyric" mutants, which have no mutations in the HEM12 gene but accumulate uroporphyrinogen, a phenotype characteristic of partial Uro-d deficiency, were investigated. Genetic analysis showed that the mutant phenotype depends on the combined action of two unlinked mutations, udt1 and either ipa1, ipa2, or ipa3. ipa1 is tightly linked to HEM12. The mutation udt1 apparently acts specifically on the HEM12 gene, and causes a six to tenfold decrease in the levels of the short HEM12 mRNA, in the beta-galactosidase activity of a HEM12-lacZ fusion, in immunodetectable protein and enzyme activity. But heme synthesis is normal and porphyrin accumulation was modest. The mutations ipa1, ipa2, and ipa3 had no phenotype on their own, but they caused an increase in porphyrin accumulation in a udt1 background. This multiplicity of genetic factors leading to uroporphyric yeast cells closely resembles the situation in human porphyria cutanea tarda.

Molecular analysis of HEM6 (HEM12) in Saccharomyces cerevisiae, the gene for uroporphyrinogen decarboxylase

HEM6 (HEM12) in Saccharomyces cerevisiae encodes uroporphyrinogen decarboxylase, the fifth enzyme in the heme biosynthetic pathway. The HEM6 (HEM12) gene was cloned by complementation of heme auxotrophy of a hem6 mutant. Sequence analysis revealed an open reading frame of 1086 nucleotides. The predicted amino acid sequence of HEM6 (HEM12) shows extensive homology to those reported for uroporphyrinogen decarboxylase from mammalian sources. Expression of HEM6 (HEM12) was investigated and was found to increase two-fold in a non-fermentable carbon source. However, HEM6 (HEM12) transcription was unaffected by heme or by intermediates in the heme biosynthetic pathway. In addition, HEM6 (HEM12) expression is not regulated by the transcriptional activator complex HAP2-3-4, as has been shown for some genes encoding heme biosynthetic enzymes.

Identification of amino acid changes affecting yeast uroporphyrinogen decarboxylase activity by sequence analysis of hem12 mutant alleles

The molecular basis of the uroporphyrinogen decarboxylase defect in eleven yeast 'uroporphyric' mutants was investigated. Uroporphyrinogen decarboxylase, an enzyme of the haem-biosynthetic pathway, catalyses the decarboxylation of uroporphyrinogen to coproporphyrinogen and is encoded by the HEM12 gene in the yeast Saccharomyces cerevisiae. The mutations were identified by sequencing the mutant hem12 alleles amplified in vitro from genomic DNA extracted from the mutant strains. Four mutations leading to the absence of enzyme protein were found: one mutation caused the substitution of the translation initiator Met to Ile, a two-base deletion created a frameshift at codon 247 and two nonsense mutations were found at codons 50 and 263. Four different point mutations were identified in seven 'leaky' mutants with residual modified uroporphyrinogen decarboxylase activity; each of three mutations was found in two independently isolated mutants. The nucleotide transitions resulted in the amino acid substitutions Ser-59 to Phe, Thr-62 to Ile, Leu-107 to Ser, or Ser-215 to Asn, all located in or near highly conserved regions. The results suggest that there is a single active centre in uroporphyrinogen decarboxylase, the geometry of which is affected in the mutant enzymes.

Uroporphyrinogen decarboxylase in Saccharomyces cerevisiae. HEM12 gene sequence and evidence for two conserved glycines essential for enzymatic activity

The HEM12 gene from Saccharomyces cerevisiae encodes uroporphyrinogen decarboxylase which catalyzes the sequential decarboxylation of the four acetyl side chains of uroporphyrinogen to yield coproporphyrinogen, an intermediate in protoheme biosynthesis. The gene was isolated by functional complementation of a hem12 mutant. Sequencing revealed that the HEM12 gene encodes a protein of 362 amino acids with a calculated molecular mass of 41,348 Da. The amino acid sequence shares 50% identity with human and rat uroporphyrinogen decarboxylase and shows 40% identity with the N-terminus of an open reading frame described in Synechococcus sp. We determined the sequence of two hem12 mutations which lead to a totally inactive enzyme. They correspond to the amino acid changes Gly33----Asp and Gly300----Asp, located in two evolutionarily conserved regions. Each of these substitutions impairs binding of substrates without affecting the overall conformation of the protein. These results argue that a single active center exists in uroporphyrinogen decarboxylase.

Purification and properties of uroporphyrinogen decarboxylase from Saccharomyces cerevisiae. Yeast uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (EC 4.1.1.37) was purified about 14000-fold to homogeneity from the yeast Saccharomyces cerevisiae with a 70% overall yield. The purification included affinity chromatography on uroporphyrin-I-Affi-Gel 102. The specific activity of the final preparation was 1750 nmol coproporphyrinogen formed.h-1.(mg protein)-1 at pH 7.5 and 37 degrees C using 4 microM uroporphyrinogen I as substrate. The purified enzyme has a minimum molecular mass of 38 kDa by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and 46 kDa by gel filtration, suggesting that yeast uroporphyrinogen decarboxylase is a monomer. Chromatofocusing gave a pI of 6.0. Enzyme activity was inhibited by metals, such as Cu2+, Zn2+, Fe2+, Fe3+ and by sulfhydryl-specific reagents, but no cofactor requirement could be demonstrated. The optimum pH was pH 5.7 for uroporphyrinogens I and III and heptacarboxylate porphyrinogen I as estimated by coproporphyrinogen formation. The optimum pH for substrate decarboxylation was pH 5.7 for uroporphyrinogen I, but pH 6.8 for the two other substrates. The Km values at pH 5.7 were 10 nM for uroporphyrinogen I, 6 nM for uroporphyrinogen III and 7 nM for heptacarboxylate porphyrinogen I as measured by coproporphyrinogen formation. The pattern of accumulation of intermediate and final decarboxylation products and the rates of the successive decarboxylations were determined for the three substrates at different concentrations at pH 5.7 and pH 6.8. The rate-limiting step at 4 microM substrate concentration was the elimination of the second carboxyl group of uroporphyrinogen III and the fourth carboxyl of uroporphyrinogen I. An antiserum to purified yeast uroporphyrinogen decarboxylase was used to characterize the protein in several mutants.