Structural and functional analysis of missense mutations in fumarylacetoacetate hydrolase, the gene deficient in hereditary tyrosinemia type 1

Hereditary tyrosinemia type 1 (HT1) is an autosomal recessive disease caused by a deficiency of the enzyme involved in the last step of tyrosine degradation, fumarylacetoacetate hydrolase (FAH). Thus far, 34 mutations in the FAH gene have been reported in various HT1 patients. Site-directed mutagenesis of the FAH cDNA was used to investigate the effects of eight missense mutations found in HTI patients on the structure and activity of FAH. Mutated FAH proteins were expressed in Escherichia coli and in mammalian CV-1 cells. Mutations N16I, F62C, A134D, C193R, D233V, and W234G lead to enzymatically inactive FAH proteins. Two mutations (R341W, associated with the pseudo-deficiency phenotype, and Q279R) produced proteins with a level of activity comparable to the wild-type enzyme. The N16I, F62C, C193R, and W234G variants were enriched in an insoluble cellular fraction, suggesting that these amino acid substitutions interfere with the proper folding of the enzyme. Based on the tertiary structure of FAH, on circular dichroism data, and on solubility measurements, we propose that the studied missense mutations cause three types of structural effects on the enzyme: 1) gross structural perturbations, 2) limited conformational changes in the active site, and 3) conformational modifications with no significant effect on enzymatic activity.     

Different molecular basis for fumarylacetoacetate hydrolase deficiency in the two clinical forms of hereditary tyrosinemia (type I).

Hereditary tyrosinemia is characterized by a deficiency of the enzyme fumarylacetoacetate hydrolase (FAH; E.C.3.7.1.2), the last enzyme in the catabolic pathway of tyrosine. FAH was purified from rat and human liver and was used to immunize rabbits. Specific antibodies were used to probe protein extracts of livers and other tissues of normal and tyrosinemic patients. No immunoreactive FAH band was observed on immunoblots of liver, kidneys, and lymphocytes from patients presenting with the acute form of hereditary tyrosinemia. Patients with the chronic form had immunoreactive FAH at a level approximately 20% of normal liver values, which was correlated with the measured enzymatic activity. Immunoblot analysis of aborted fetal tissues revealed normal FAH immunoreactivity in normal liver and kidneys. No FAH immunoreactivity was found in liver and kidneys of tyrosinemic fetuses. The presence of FAH immunoreactivity in normal fetal tissues suggests that deficient FAH activity in tyrosinemia is not simply related to a developmentally regulated expression of the enzyme. By this immunoblot assay, FAH was detected in most human tissues, with maximal immunoreactivity in liver and kidneys and with only trace amounts in chorionic villi and cultured amniocytes. These data confirm that the primary defect in the acute form of hereditary tyrosinemia is an absence of FAH. Moreover, these data suggest that both clinical forms of the disease have a different molecular basis.     

Structural organization and analysis of the human fumarylacetoacetate hydrolase gene in tyrosinemia type I

Fumarylacetoacetate hydrolase (FAH) is a metabolic enzyme functioning at the last steo of tyrosine catabolism. Deficiency in this enzyme activity is associated with tyrosinemia type I, characterized by hypertyrosinemia, liver dysfunction, renal tubular dysfunction, liver cirrhosis, and hepatic tumors. We isolated from a human gene library a chromosomal gene related to FAH. The human FAH gene is 30 kilobases long and is split into 14 exons. All of the splice donor and acceptor sites conform to the GT/AG rule. We also analyzed findings in a patient with tyrosinemia type I with respect to the mutation responsible for detects in the enzyme. A nucleotide change from T to G was found in the exon 2 of the gene and this change was accompanied by an amino acid substitution (Phe62Cys). Transfection and expression analysis of the cDNA is cultured BMT-10 cells with the nucleotide substitution demonstrated that the substitution was indeed responsible for the decreased activity of the enzyme in the patient. These results confirmed that the T to G mutation was one of the causes of tyrosinemia type I. Structure of the FAH gene and tests for expression of the mutant FAH will facilitate further understanding of various aspects of FAH.     

The narrow substrate specificity of human tyrosine aminotransferase--the enzyme deficient in tyrosinemia type II

Human tyrosine aminotransferase (hTATase) is the pyridoxal phosphate-dependent enzyme that catalyzes the reversible transamination of tyrosine to p-hydrophenylpyruvate, an important step in tyrosine metabolism. hTATase deficiency is implicated in the rare metabolic disorder, tyrosinemia type II. This enzyme is a member of the poorly characterized Igamma subfamily of the family I aminotransferases. The full length and truncated forms of recombinant hTATase were expressed in Escherichia coli, and purified to homogeneity. The pH-dependent titration of wild-type reveals a spectrum characteristic of family I aminotransferases with an aldimine pK(a) of 7.22. I249A mutant hTATase exhibits an unusual spectrum with a similar aldimine pK(a) (6.85). hTATase has very narrow substrate specificity with the highest enzymatic activity for the Tyr/alpha-ketoglutarate substrate pair, which gives a steady state k(cat) value of 83 s(-1). In contrast there is no detectable transamination of aspartate or other cosubstrates. The present findings show that hTATase is the only known aminotransferase that discriminates significantly between Tyr and Phe: the k(cat)/K(m) value for Tyr is about four orders of magnitude greater than that for Phe. A comparison of substrate specificities of representative Ialpha and Igamma aminotransferases is described along with the physiological significance of the discrimination between Tyr and Phe by hTATase as applied to the understanding of the molecular basis of phenylketonuria.     

The Caenorhabditis elegans K10C2.4 gene encodes a member of the fumarylacetoacetate hydrolase family: a Caenorhabditis elegans model of type I tyrosinemia

In eukaryotes and many bacteria, tyrosine is degraded to produce energy via a five-step tyrosine degradation pathway. Mutations affecting the tyrosine degradation pathway are also of medical importance as mutations affecting enzymes in the pathway are responsible for type I, type II, and type III tyrosinemia. The most severe of these is type I tyrosinemia, which is caused by mutations affecting the last enzyme in the pathway, fumarylacetoacetate hydrolase (FAH). So far, tyrosine degradation in the nematode Caenorhabditis elegans has not been studied; however, genes predicted to encode enzymes in this pathway have been identified in several microarray, proteomic, and RNA interference (RNAi) screens as perhaps being involved in aging and the control of protein folding. We sought to identify and characterize the genes in the worm tyrosine degradation pathway as an initial step in understanding these findings. Here we describe the characterization of the K10C2.4, which encodes a homolog of FAH. RNAi directed against K10C2.4 produces a lethal phenotype consisting of death in young adulthood, extensive damage to the intestine, impaired fertility, and activation of oxidative stress and endoplasmic stress response pathways. This phenotype is due to alterations in tyrosine metabolism as increases in dietary tyrosine enhance it, and inhibition of upstream enzymes in tyrosine degradation with RNAi or genetic mutations reduces the phenotype. We also use our model to identify genes that suppress the damage produced by K10C2.4 RNAi in a pilot genetic screen. Our results establish worms as a model for the study of type I tyrosinemia.     

Liver transplantation for hereditary tyrosinemia: the Quebec experience.

Sixteen tyrosinemic patients were evaluated in our institution for a possible liver transplantation. All patients showed biochemical and/or radiological evidence of liver dysfunction. Renal involvement was found to be more abnormal than expected. Seven patients have been transplanted, with two patients receiving a combined liver-kidney transplant. Hepatocarcinoma was detected in two of eight patients in whom the whole liver was examined. Six (37.5%) of the initial 16 patients have died since evaluation, one of the six dying after combined liver-kidney transplantation. Posttransplantation survival was 86%, with normal liver function, normal growth, and no recurrence of neurological crises on a normal diet.     

Purification of mRNA coding for rat-liver fructose-1,6-bisphosphatase by polysome immunoabsorption

The mRNA coding for rat liver fructose-1,6-bisphosphatase, which represents approx. 0.46% of total hepatic mRNA, has been purified to near homogeneity. Polysomes from rat liver were allowed to react with antibodies to rabbit anti-fructose-1,6-bisphosphatase purified by affinity chromatography. The complex was immobilized on a protein A-Sepharose column. After the removal of unabsorbed polysomes, the specific mRNA was eluted and chromatographed on an oligo(dT)-cellulose column. This method gave a 183-fold enrichment of the fructose-1,6-bisphosphatase mRNA to greater than 80% homogeneity as determined by electrophoreses of immunoprecipitated in vitro translation products on polyacrylamide slab gels in the presence of sodium dodecyl sulphate.     

Gamma-glutamyl hydrolase conjugase). Purification and properties of the bovine hepatic enzyme.

Bovine hepatic gamma-glutamyl hydrolase (conjugase) has been purified to homogeneity. A feature of the purification procedure was the use of high affinity macromolecular polyanion enzyme inhibitors which formed tight complexes with the enzyme altering its solubility, gel filtration, and ion exchange properties. The enzyme, which cleaves the gamma-glutamyl bonds of pteroylpolyglutamates, has a molecular weight of 108,000. It is a glycoprotein with an acid pH optimum, properties consistent with its lysosomal localization. Zinc is essential for enzyme stability. The presence of highly reactive sulfhydryl groups was evident from the extreme sensitivity to oxidizing agents and organomercurials. Very little thermal denaturation occurs below 65 degrees, but the enzyme is extremely sensitive to 0uffer anions, in keeping with the polyanionic nature of the substrate. In order to study the mechanism of action of the enzyme, a wide range of pteroylpolyglutamates, N-t-Boc polyglutamates and free polyglutamates were synthesized containing L-[U-14C]glutamic acid residues in different positions. Two pteroyltriglutamate derivatives were also synthesized in which an alpha bond replaced one of the two available gamma bonds. Time course studies of the products of the action of conjugase on these various substrates enabled us to draw the following conclusions about the enzyme: (a) peptide bond cleavage occurred only at gamma-glutamyl bonds and the presence of a COOH-terminal gamma bond was essential for enzyme action; (b) bond cleavage occurred with equal facility at internal points of the peptide chain and the enzyme should therefore be more appropriately classified as an acid hydrolase; (c) longer chain gamma-glutamyl peptides were preferentially attacked by the enzyme, the cleavage of diglutamyl peptides being extremely slow; and (d) cleavage of gamma bonds was independent of the NH2-terminal pteroyl moiety. Studies with polyanions such as the glycosaminoglycans and dextran sulfate supported the concept that the polyanion structure of the substrate was a major factor in substrate-active site interaction.     

Purification of human liver cytosolic epoxide hydrolase and comparison to the microsomal enzyme

Epoxide hydrolase (EC 3.3.2.3) was purified to electrophoretic homogeneity from human liver cytosol by using hydrolytic activity toward trans-8-ethylstyrene 7,8-oxide (TESO) as an assay. The overall purification was 400-fold. The purified enzyme has an apparent monomeric molecular weight of 58 000, significantly greater than the 50 000 found for human (or rat) liver microsomal epoxide hydrolase or for another TESO-hydrolyzing enzyme also isolated from human liver cytosol. Purified cytosolic TESO hydrolase catalyzes the hydrolysis of cis-8-ethylstyrene 7,8-oxide 10 times more rapidly than does the microsomal enzyme, catalyzes the hydrolysis of TESO and trans-stilbene oxide as rapidly as the microsomal enzyme, but catalyzes the hydrolysis of styrene 7,8-oxide, p-nitrostyrene 7,8-oxide, and naphthalene 1,2-oxide much less effectively than does the microsomal enzyme. Purified cytosolic TESO hydrolase does not hydrolyze benzo[a]pyrene 4,5-oxide, a substrate for the microsomal enzyme. The activities of the purified enzymes can explain the specific activities observed with subcellular fractions. Anti-human liver microsomal epoxide hydrolase did not recognize cytosolic TESO hydrolase in purified form or in cytosol, as judged by double-diffusion immunoprecipitin analysis, precipitation of enzymatic activity, and immunoelectrophoretic techniques. Cytosolic TESO hydrolase and microsomal epoxide hydrolase were also distinguished by peptide mapping. The results provide evidence that physically different forms of epoxide hydrolase exist in different subcellular fractions and can have markedly different substrate specificities.     

Mechanistic inferences from the crystal structure of fumarylacetoacetate hydrolase with a bound phosphorus-based inhibitor

Fumarylacetoacetate hydrolase (FAH) catalyzes the hydrolytic cleavage of a carbon-carbon bond in fumarylacetoacetate to yield fumarate and acetoacetate as the final step of Phe and Tyr degradation. This unusual reaction is an essential human metabolic function, with loss of FAH activity causing the fatal metabolic disease hereditary tyrosinemia type I (HT1). An enzymatic mechanism involving a catalytic metal ion, a Glu/His catalytic dyad, and a charged oxyanion hole was previously proposed based on recently determined FAH crystal structures. Here we report the development and characterization of an FAH inhibitor, 4-(hydroxymethylphosphinoyl)-3-oxo-butanoic acid (HMPOBA), that competes with the physiological substrate with a K(i) of 85 microM. The crystal structure of FAH complexed with HMPOBA refined at 1.3-A resolution reveals the molecular basis for the competitive inhibition, supports the proposed formation of a tetrahedral alkoxy transition state intermediate during the FAH catalyzed reaction, and reveals a Mg(2+) bound in the enzyme's active site. The analysis of FAH structures corresponding to different catalytic states reveals significant active site side-chain motions that may also be related to catalytic function. Thus, these results advance the understanding of an essential catabolic reaction associated with a fatal metabolic disease and provide insight into the structure-based development of FAH inhibitors.