Human molybdopterin synthase gene: identification of a bicistronic transcript with overlapping reading frames. (1/1288)

A universal molybdenum-containing cofactor (MoCo) is essential for the activity of all human molybdoenzymes, including sulphite oxidase. The free cofactor is highly unstable, and all organisms share a similar biosynthetic pathway. The involved enzymes exhibit homologies, even between bacteria and humans. We have exploited these homologies to isolate a cDNA for the heterodimeric molybdopterin (MPT)-synthase. This enzyme is necessary for the conversion of an unstable precursor into molybdopterin, the organic moiety of MoCo. The corresponding transcript shows a bicistronic structure, encoding the small and large subunits of the MPT-synthase in two different open reading frames (ORFs) that overlap by 77 nucleotides. In various human tissues, only one size of mRNA coinciding with the bicistronic transcript was detected. In vitro translation and mutagenesis experiments demonstrated that each ORF is translated independently, leading to the synthesis of a 10-kDa protein and a 21-kDa protein for the small and large subunits, respectively, and indicated that the 3'-proximal ORF of the bicistronic transcript is translated by leaky scanning.  (+info)

Human molybdopterin synthase gene: genomic structure and mutations in molybdenum cofactor deficiency type B. (2/1288)

Biosynthesis of the molybdenum cofactor (MoCo) can be divided into (1) the formation of a precursor and (2) the latter's subsequent conversion, by molybdopterin synthase, into the organic moiety of MoCo. These two steps are reflected by the complementation groups A and B and the two formally distinguished types of MoCo deficiency that have an identical phenotype. Both types of MoCo deficiency result in a pleiotropic loss of all molybdoenzyme activities and cause severe neurological damage. MOCS1 is defective in patients with group A deficiency and has been shown to encode two enzymes for early synthesis via a bicistronic transcript with two consecutive open reading frames (ORFs). MOCS2 encodes the small and large subunits of molybdopterin synthase via a single transcript with two overlapping reading frames. This gene was mapped to 5q and comprises seven exons. The coding sequence and all splice site-junction sequences were screened for mutations, in MoCo-deficient patients in whom a previous search for MOCS1 mutations had been negative. In seven of the eight patients whom we investigated, we identified MOCS2 mutations that, by their nature, are most likely responsible for the deficiency. Three different frameshift mutations were observed, with one of them found on 7 of 14 identified alleles. Furthermore, a start-codon mutation and a missense mutation of a highly conserved amino acid residue were found. The locations of the mutations confirm the functional role of both ORFs. One of the patients with identified MOCS2 mutations had been classified as type B, in complementation studies. These findings support the hypothetical mechanism, for both forms of MoCo deficiency, that formerly had been established by cell-culture experiments.  (+info)

Human biotinidase isn't just for recycling biotin. (3/1288)

For years, the major role of biotin has been as the coenzyme for four carboxylases in humans. Although there has been evidence that biotin might have other functions, none has been firmly established. The discovery that human serum biotinidase has biotinyl-transferase activity, in addition to biotinidase hydrolase activity, presents new possibilities for the role of biotinidase in biotin metabolism. Specific transfer of biotin to histones by biotinidase provides a possible explanation for why biotin is found in the nucleus and the nature of its role in the regulation of protein transcription. Future studies will help to determine the functions of biotinidase in biotin metabolism and in disease states.  (+info)

Re-design of Rhodobacter sphaeroides dimethyl sulfoxide reductase. Enhancement of adenosine N1-oxide reductase activity. (4/1288)

The periplasmic DMSO reductase from Rhodobacter sphaeroides f. sp. denitrificans has been expressed in Escherichia coli BL21(DE3) cells in its mature form and with the R. sphaeroides or E. coli N-terminal signal sequence. Whereas the R. sphaeroides signal sequence prevents formation of active enzyme, addition of a 6x His-tag at the N terminus of the mature peptide maximizes production of active enzyme and allows for affinity purification. The recombinant protein contains 1.7-1.9 guanines and greater than 0.7 molybdenum atoms per molecule and has a DMSO reductase activity of 3.4-3.7 units/nmol molybdenum, compared with 3.7 units/nmol molybdenum for enzyme purified from R. sphaeroides. The recombinant enzyme differs from the native enzyme in its color and spectrum but is indistinguishable from the native protein after redox cycling with reduced methyl viologen and Me2SO. Substitution of Cys for the molybdenum-ligating Ser-147 produced a protein with DMSO reductase activity of 1.4-1.5 units/nmol molybdenum. The mutant protein differs from wild type in its color and absorption spectrum in both the oxidized and reduced states. This substitution leads to losses of 61-99% of activity toward five substrates, but the adenosine N1-oxide reductase activity increases by over 400%.  (+info)

The strict molybdate-dependence of glucose-degradation by the thermoacidophile Sulfolobus acidocaldarius reveals the first crenarchaeotic molybdenum containing enzyme--an aldehyde oxidoreductase. (5/1288)

In order to investigate the effects of trace elements on different metabolic pathways, the thermoacidophilic Crenarchaeon Sulfolobus acidocaldarius (DSM 639) has been cultivated on various carbon substrates in the presence and absence of molybdate. When grown on glucose (but neither on glutamate nor casein hydrolysate) as sole carbon source, the lack of molybdate results in serious growth inhibition. By analysing cytosolic fractions of glucose adapted cells for molybdenum containing compounds, an aldehyde oxidoreductase was detected that is present in the cytosol to at least 0.4% of the soluble protein. With Cl2Ind (2,6-dichlorophenolindophenol) as artificial electron acceptor, the enzyme exhibits oxidizing activity towards glyceraldehyde, glyceraldehyde-3-phosphate, isobutyraldehyde, formaldehyde, acetaldehyde and propionaldehyde. At its pH-optimum (6.7), close to the intracellular pH of Sulfolobus, the glyceraldehyde-oxidizing activity is predominant. The protein has an apparent molecular mass of 177 kDa and consists of three subunits of 80.5 kDa (alpha), 32 kDa (beta) and 19.5 kDa (gamma). It contains close to one Mo, four Fe, four acid-labile sulphides and four phosphates per protein molecule. Methanol extraction revealed the existence of 1 FAD per molecule and 1 molybdopterin per molecule, which was identified as molybdopterin guanine dinucleotide on the basis of perchloric acid cleavage and thin layer chromatography. EPR-spectra of the aerobically prepared enzyme exhibit the so-called 'desulpho-inhibited'-signal, known from chemically modified forms of molybdenum containing proteins. Anaerobically prepared samples show both, the signals arising from the active molybdenum-cofactor as well as from the two [2Fe-2S]-clusters. According to metal-, cofactor-, and subunit-composition, the enzyme resembles the members of the xanthine oxidase family. Nevertheless, the melting point and long-term thermostability of the protein are outstanding and perfectly in tune with the growth temperature of S. acidocaldarius (80 degrees C). The findings suggest the enzyme to function as a glyceraldehyde oxidoreductase in the course of the nonphosphorylated Entner-Doudoroff pathway and thereby may attribute a new physiological role to this class of enzyme.  (+info)

A function for the vitamin E metabolite alpha-tocopherol quinone as an essential enzyme cofactor for the mitochondrial fatty acid desaturases. (6/1288)

A critical analysis of the changes in fatty acid patterns and their metabolism elicited by vitamin E deficiency leads to the proposal that a major role of dietary RRR-alpha-tocopherol (alpha-TOC) is as an enzymatic precursor of alpha-tocopherolquinone (alpha-TQ) whose semiquinone radical functions as an essential enzyme cofactor for the fatty acid desaturases of the recently elucidated carnitine-dependent, channeled, mitochondrial desaturation-elongation pathway; a detailed mechanism for its function is proposed. Pathophysiological states produced by vitamin E deficiency and alpha-TOC transfer protein defects, such as ataxia, myopathy, retinopathy, and sterility are proposed to develop from the effects of impaired alpha-TQ-dependent desaturases and the resulting deficiency of their polyenoic fatty acid products.  (+info)

Structure and function of a cysBJIH gene cluster in the purple sulphur bacterium Thiocapsa roseopersicina. (7/1288)

A gene cluster containing homologues of the genes cysB, cysJI and cysH was found in the genome of the sulphur-oxidizing purple bacterium Thiocapsa roseopersicina. The nucleotide sequence indicated four open reading frames encoding homologues of 3'-phosphoadenylylsulphate (PAPS) reductase (CysH), sulphite reductase flavoprotein (CysJ) and haem protein (CysI) subunits, and a transcriptional regulator (CysB). Genes cysJIH are separated by a short cis-active intergenic region from cysB which is transcribed divergently. cysB encodes a polypeptide of 35.9 kDa consisting of 323 amino acid residues with 40% identity to the CysB regulator from enterobacteria. cysH encodes a protein with 239 amino acid residues and a calculated mass of 27.7 kDa; cysJ encodes a protein with 522 amino acid residues and a mass of 57.8 kDa; and cysI encodes a protein with 559 amino acid residues and a mass of 62.3 kDa. The cysJIH gene products have been expressed and used for complementation of cys mutants from Escherichia coli Biochemical analysis. The gene product CysH is a thioredoxin-dependent PAPS reductase (EC 1.8.99.4). It was repressed under photoautotrophic growth using hydrogen sulphide as electron donor and derepressed under conditions of sulphate deficiency. Products of the cysJI genes were identified as the two subunits of NADPH-sulphite reductase (EC 1.8.1.2). cysJ encoded the flavoprotein, with > or = 39% identity to the protein from E. coli, and cysI encoded the haem protein, with > or = 53% identity. A cysI clone was used to complement the corresponding mutant from E. coli and to express enzymically active methylviologen-sulphite reductase.  (+info)

Specificity of coenzyme analogues and fragments in promoting or impeding the refolding of clostridial glutamate dehydrogenase. (8/1288)

NAD+ facilitates high-yield reactivation of clostridial glutamate dehydrogenase (GDH) after unfolding in urea. The specificity of this effect has been explored by using analogues and fragments of NAD+. The adenine portion, unlike the nicotinamide portion, is important for reactivation. Alteration in the nicotinamide portion, in acetylpyridine adenine dinucleotide, has little effect, whereas loss of the 6-NH2 substitution on the adenine ring, in 6-deamino NAD, diminishes the effectiveness of the nucleotide in promoting refolding. Also ADP-ribose, lacking nicotinamide, promotes reactivation whereas NMN-phosphoribose, lacking the adenine, does not. Of the smaller fragments, those containing an adenosine moiety, and especially those with one or more phosphate groups, impede the refolding ability of NAD+, and are able to bind to the folding intermediate though unable to facilitate refolding. These results are interpreted in terms of the known 3D structure for clostridial glutamate dehydrogenase. It is assumed that the refolding intermediate has a more or less fully formed NAD+-binding domain but a partially disordered substrate-binding domain and linking region. Binding of NAD+ or ADP-ribose appears to impose new structural constraints that result in completion of the correct folding of the second domain, allowing association of enzyme molecules to form the native hexamer.  (+info)