Human liver GTP cyclohydrolase I: purification and some properties

Biochemical characterization of the tetrahydrobiopterin synthesis pathway in the oleaginous fungus Mortierella alpina

We characterized the de novo biosynthetic pathway of tetrahydrobiopterin (BH₄) in the lipid-producing fungus Mortierella alpina. The BH₄ cofactor is essential for various cell processes, and is probably present in every cell or tissue of higher organisms. Genes encoding two copies of GTP cyclohydrolase I (GTPCH-1 and GTPCH-2) for the conversion of GTP to dihydroneopterin triphosphate (H₂-NTP), 6-pyruvoyltetrahydropterin synthase (PTPS) for the conversion of H₂-NTP to 6-pyruvoyltetrahydropterin (PPH₄), and sepiapterin reductase (SR) for the conversion of PPH₄ to BH₄, were expressed heterologously in Escherichia coli. The recombinant enzymes were produced as His-tagged fusion proteins and were purified to homogeneity to investigate their enzymic activities. Enzyme products were analysed by HPLC and electrospray ionization-MS. Kinetic parameters and other properties of GTPCH, PTPS and SR were investigated. Physiological roles of BH₄ in M. alpina are discussed, and comparative analyses between GTPCH, PTPS and SR proteins and other homologous proteins were performed. The presence of two functional GTPCH enzymes has, as far as we are aware, not been reported previously, reflecting the unique ability of this fungus to synthesize both BH₄ and folate, using the GTPCH product as a common substrate. To our knowledge, this study is the first to report the comprehensive characterization of a BH₄ biosynthesis pathway in a fungus.

Co-expression of the Plasmodium falciparum molecular chaperone, PfHsp70, improves the heterologous production of the antimalarial drug target GTP cyclohydrolase I, PfGCHI

Molecular chaperones have been used for the improved expression of target proteins within heterologous systems; however, the chaperone and target protein have seldom been matched in terms of origin. We have developed a heterologous co-expression system that allows independent expression of the plasmodial chaperone, PfHsp70, and a plasmodial target protein. In this study, the target was Plasmodium falciparum GTP cyclohydrolase I (PfGCHI), the first enzyme in the plasmodial folate pathway. The sequential expression of the molecular chaperone followed by the target protein increased the expression of soluble functional PfGCHI. His-tagged PfGCHI was successfully purified using nickel affinity chromatography, and the specific activity was determined by high performance liquid chromatography with spectrofluorometeric detection to be 5.93nmol/h/mg. This is the first report of a heterologous co-expression system in which a plasmodial chaperone is harnessed for the improved production and purification of a plasmodial target protein.

Cloning, expression, purification, and characterization of Nocardia sp. GTP cyclohydrolase I

The sequence of the gene from Nocardia sp. NRRL 5646 encoding GTP cyclohydrolase I (GCH), gch, and its adjacent regions was determined. The open reading frame of Nocardia gch contains 684 nucleotides, and the deduced amino acid sequence represents a protein of 227 amino acid residues with a calculated molecular mass of 24,563Da. The uncommon start codon TTG was identified by matching the N-terminal amino acid sequence of purified Nocardia GCH with the deduced amino acid sequence. A likely ribosomal binding site was identified 9bp upstream of the translational start site. The 3' end flank region encodes a peptide that shares high homology with dihydropteroate synthases. Nocardia GCH has 73 and 60% identity to the proteins encoded by the putative gch of Mycobacterium tuberculosis and Streptomyces coelicolor, respectively. Nocardia GCH was highly expressed in Escherichia coli cells carrying a pHAT10 based expression vector, and moderately expressed in Mycobacterium smegmatis cells carrying a pSMT3 based expression vector. Enterokinase digestion of recombinant Nocardia GCH, and in-gel digestion of Nocardia GCH and recombinant GCH followed by MALDI-TOF-MS analysis, confirmed that the actual subunit size of the enzyme was 24.5kDa. Thus, we conclude that the active form of native Nocardia GCH is a decamer. Our earlier incorrect conclusion was that the native enzyme was an octamer derived from the anomalous SDS-PAGE migration of the subunit.

GTP cyclohydrolase I: purification, characterization, and effects of inhibition on nitric oxide synthase in nocardia species

GTP cyclohydrolase I (GTPCH) catalyzes the first step in pteridine biosynthesis in Nocardia sp. strain NRRL 5646. This enzyme is important in the biosynthesis of tetrahydrobiopterin (BH4), a reducing cofactor required for nitric oxide synthase (NOS) and other enzyme systems in this organism. GTPCH was purified more than 5,000-fold to apparent homogeneity by a combination of ammonium sulfate fractionation, GTP-agarose, DEAE Sepharose, and Ultragel AcA 34 chromatography. The purified enzyme gave a single band for a protein estimated to be 32 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular mass of the native enzyme was estimated to be 253 kDa by gel filtration, indicating that the active enzyme is a homo-octamer. The enzyme follows Michaelis-Menten kinetics, with a Km for GTP of 6.5 micromoles. Nocardia GTPCH possessed a unique N-terminal amino acid sequence. The pH and temperature optima for the enzyme were 7.8 and 56 degrees C, respectively. The enzyme was heat stable and slightly activated by potassium ion but was inhibited by calcium, copper, zinc, and mercury, but not magnesium. BH4 inhibited enzyme activity by 25% at a concentration of 100 micromoles. 2,4-Diamino-6-hydroxypyrimidine (DAHP) appeared to competitively inhibit the enzyme, with a Ki of 0.23 mM. With Nocardia cultures, DAHP decreased medium levels of NO2- plus NO3-. Results suggest that in Nocardia cells, NOS synthesis of nitric oxide is indirectly decreased by reducing the biosynthesis of an essential reducing cofactor, BH4.

Genetic engineering of Escherichia coli for production of tetrahydrobiopterin

Tetrahydrobiopterin (BH4) is an essential cofactor for various enzymes in mammals. In vivo, it is synthesized from GTP via the three-step pathway of GTP cyclohydrolase I (GCHI), 6-pyruvoyl-tetrahydropterin synthase (PTPS) and sepiapterin reductase (SPR). BH4 is a medicine used to treat atypical hyperphenylalaninemia. It is currently synthesized by chemical means, which consists of many steps, and requires costly materials and complicated procedures. To explore an alternative microbial method for BH4 production, we utilized recombinant DNA technology to construct recombinant Escherichia coli (E. coli) strains carrying genes expressing GCHI, PTPS and SPR enzymes. These strains successfully produced BH4, which was detected as dihydrobiopterin and biopterin, oxidation products of BH4. In order to increase BH4 productivity we made further improvements. First, to increase the de novo GTP supply, an 8-azaguanine resistant mutant was isolated and an additional guaBA operon was introduced. Second, to augment the activity of GCHI, the folE gene from E. coli was replaced by the mtrA gene from Bacillus subtilis. These modifications provided us with a strain showing significantly higher productivity, up to 4.0 g of biopterin/L of culture broth. The results suggest the possibility of commercial BH4 production by our method.

Tetrahydrobiopterin cofactor biosynthesis: GTP cyclohydrolase I mRNA expression in rat brain and superior cervical ganglia

GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin, the reduced pteridine cofactor required for catecholamine (CA), indoleamine, and nitric oxide biosynthesis. We have used the reverse transcription-polymerase chain reaction technique, based on the published cDNA sequence for rat liver GTPCH, to clone a portion of the GTPCH transcript from rat adrenal gland mRNA and have used this clone for the analysis of GTPCH mRNA in brain and other tissues of the rat by northern blot, nuclease protection assay, and in situ hybridization. Two GTPCH mRNA transcripts of 1.2 and 3.8 kb in length were detected by northern blot, with the 1.2-kb form predominating in the liver and the 3.8-kb form in the pineal gland, adrenal gland, brainstem, and hypothalamic neurons maintained in culture. In situ hybridization studies localized GTPCH mRNA to CA-containing perikarya in the locus ceruleus, ventral tegmental area, and substantia nigra, pars compacta. Levels of GTPCH mRNA in central and peripheral catecholamine neurons determined by nuclease protection assay were increased twofold 24 h after a single injection of the CA-depleting drug reserpine; both the 1.2- and 3.8-kb transcripts were increased in the adrenal gland. Low levels of GTPCH mRNA were also detected by nuclease protection assay in the striatum, hippocampus, and cerebellum, brain regions that do not contain monoaminergic perikarya.

Differential metabolism of tetrahydrobiopterin in monoamine neurons: a hypothesis based upon clinical and basic research

This chapter has attempted to describe and integrate some of the clinical and basic research that support our hypothesis that the metabolism of BH4 is normally heterogeneous across different populations of monoamine-containing neurons. Based upon this hypothesis, there may now be reason to support the idea that certain neuropsychiatric illnesses, which are though to be the result (at least in part) of altered monoamine metabolism, might find their roots in an abnormal metabolism of BH4 within specific monoaminergic cell groups. Such a specific dysfunction might not be apparent in the rest of the brain or peripheral nervous system, thereby being difficult to detect. Perhaps the application of molecular biological techniques to studies of BH4 metabolism in man will shed new light on these problems.

Species and tissue specificity of mammalian GTP cyclohydrolase I messenger RNA

Northern blot analysis of rat RNA from cell lines and isolated organs with a specific rat cDNA probe detected two GTP cyclohydrolase I mRNA species of approx. 1.4 and 3.6 kb. The ratio between these two species varies between 0.6 and 2.4 in different rat organs. Using primers derived from highly conserved regions in the rat and Escherichia coli cDNA sequences a human GTP cyclohydrolase I probe was obtained by means of reverse transcription and PCR (polymerase chain reaction). The human PCR product consisting of 555 bp was cloned and sequenced. It shows a 92% identity with the published sequence of the rat gene. The analysis of various human cell lines with this specific probe shows only one species of GTP cyclohydrolase I mRNA with an approximate size of 3.6 kb.