Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “ISOMERASES”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 235 records · Page 13Linked to original sources
PubMed2000

The platelet integrin alpha IIbbeta 3 has an endogenous thiol isomerase activity.

Integrins are cysteine-rich heterodimeric cell-surface adhesion molecules that alter their affinity for ligands in response to cellular activation. The molecular mechanisms involved in this activation of integrins are not understood. Treatment with the thiol-reducing agent, dithiothreitol, can induce an activation-like state in many integrins suggesting that cysteine-cysteine dithiol bonds are important for the receptor's tertiary structure and may be involved in activation-induced conformational changes. Here we demonstrate that the platelet-specific integrin, alpha(IIb)beta(3), contains an endogenous thiol isomerase activity, predicted from the presence of the tetrapeptide motif, CXXC, in each of the cysteine-rich repeats of the beta(3) polypeptide. This motif comprises the active site in enzymes involved in disulfide exchange reactions, including protein-disulfide isomerase (EC ) and thioredoxin. Intrinsic thiol isomerase activity is also observed in the related integrin, alpha(v)beta(3), which shares a common beta-subunit. Thiol isomerase activity within alpha(IIb)beta(3) is time-dependent and saturable, and is inhibited by the protein-disulfide isomerase inhibitor, bacitracin. Furthermore, this activity is calcium-sensitive and is regulated in the EDTA-stabilized conformation of the integrin. This novel demonstration of an enzymatic activity associated with an integrin subunit suggests that altered thiol bonding within the integrin or its substrates may be locally modified during alpha(IIb)beta(3) activation.

Amino Acid Sequence↗
PubMed2002

An atypical protein disulfide isomerase from the protozoan parasite Leishmania containing a single thioredoxin-like domain.

In higher eukaryotes, secretory proteins are under the quality control of the endoplasmic reticulum for their proper folding and release into the secretory pathway. One of the proteins involved in the quality control is protein disulfide isomerase, which catalyzes the formation of protein disulfide bonds. As a first step toward understanding the endoplasmic reticulum quality control of secretory proteins in lower eukaryotes, we have isolated a protein disulfide isomerase gene from the protozoan parasite Leishmania donovani. The parasite enzyme shows high sequence homology with homologs from other organisms. However, unlike the four thioredoxin-like domains found in most protein disulfide isomerases, of which two contain an active site, the leishmanial enzyme possesses only one active site present in a single thioredoxin-like domain. When expressed in Escherichia coli, the recombinant parasite enzyme shows both oxidase and isomerase activities. Replacement of the two cysteins with alanines in its active site results in loss of both enzymatic activities. Further, overexpression of the mutated/inactive form of the parasite enzyme in L. donovani significantly reduced their release of secretory acid phosphatases, suggesting that this single thioredoxin-like domain protein disulfide isomerase could play a critical role in the Leishmania secretory pathway.

Amino Acid Sequence↗
PubMed2003

Rpe65 is a retinyl ester binding protein that presents insoluble substrate to the isomerase in retinal pigment epithelial cells.

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of beta-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Leber's congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.

Animals↗
PubMed1994

Immobilization of D-xylose (D-glucose) isomerase from a Chainia species.

D-Xylose isomerase is a heat-stable enzyme which isomerizes D-xylose into D-xylulose. D-Xylose isomerase from various species also isomerizes D-glucose into D-fructose. This enzyme is used in industry for the production of high-fructose corn syrup. The enzyme is specific for both, xylose and glucose. In most species xylose isomerase is localized intracellularly. However, in a rare actinomycete, Chainia sp. (NCL 82-5-1), xylose isomerase is present in both intracellular and extracellular compartments. We have previously purified and characterized intracellular enzyme from Chainia sp. In the present paper, we describe a procedure for immobilization of intracellular xylose isomerase on INDION 48-R by ionic binding. This method is inexpensive, does not require cross-linking agents and results in firm binding of the enzyme with the resin. The properties of immobilized enzyme such as pH optimum, substrate specificity, Km and inhibition by various metabolites are described and compared with those of purified, nonimmobilized enzyme.

Actinomycetales↗
PubMed1993

Cloning, sequencing and biochemical characterization of xylose isomerase from Thermoanaerobacterium saccharolyticum strain B6A-RI.

The xylose isomerase gene from Thermoanaerobacterium saccharolyticum strain B6A-RI was cloned by complementation using Escherichia coli xyl-5 mutant strain HB101. One positive clone was detected and the recombinant plasmid, pZX16, was isolated. The clone contained the vector pUC18 and an insert fragment of 4.5 kb. The cloned xylose isomerase gene (xylA) was expressed constitutively in E. coli. The gene contained one open reading frame (ORF) of 1317 bp, which corresponds to 439 amino acid residues. The molecular mass of the gene product was calculated to be 50474 Da from the deduced amino acid sequence. A putative promoter region (Pribnow box), TATAATATATAAT, which repeated twice at the -10 region in E. coli, was found 25 bp upstream of the ribosomal binding site. The deduced amino acid sequence of T. saccharolyticum strain B6A-RI xylose isomerase exhibited very high homology to those from Thermoanaerobacterium thermosulfurigenes 4B (formerly Clostridium thermosulfurogenes 4B) and Thermoanaerobacter ethanolicus 39E (formerly Clostridium thermohydrosulfuricum 39E). Codon usage in xynA, xynB and xylA showed a clear propensity for AT-containing isocodons. The native molecular mass of the purified recombinant thermostable xylose isomerase was 200 kDa, and the enzyme was a tetramer comprised of identical subunits. The apparent temperature and pH optima for activity of the cloned xylose isomerase were 80 degrees C and 7.0 to 7.5, respectively.

Aldose-Ketose Isomerases↗
PubMed2005

Molecular identification and characterization of the Arabidopsis delta(3,5),delta(2,4)-dienoyl-coenzyme A isomerase, a peroxisomal enzyme participating in the beta-oxidation cycle of unsaturated fatty acids.

Degradation of unsaturated fatty acids through the peroxisomal beta-oxidation pathway requires the participation of auxiliary enzymes in addition to the enzymes of the core beta-oxidation cycle. The auxiliary enzyme delta(3,5),delta(2,4)-dienoyl-coenzyme A (CoA) isomerase has been well studied in yeast (Saccharomyces cerevisiae) and mammals, but no plant homolog had been identified and characterized at the biochemical or molecular level. A candidate gene (At5g43280) was identified in Arabidopsis (Arabidopsis thaliana) encoding a protein showing homology to the rat (Rattus norvegicus) delta(3,5),delta(2,4)-dienoyl-CoA isomerase, and possessing an enoyl-CoA hydratase/isomerase fingerprint as well as aspartic and glutamic residues shown to be important for catalytic activity of the mammalian enzyme. The protein, named AtDCI1, contains a peroxisome targeting sequence at the C terminus, and fusion of a fluorescent protein to AtDCI1 directed the chimeric protein to the peroxisome in onion (Allium cepa) cells. AtDCI1 expressed in Escherichia coli was shown to have delta(3,5),delta(2,4)-dienoyl-CoA isomerase activity in vitro. Furthermore, using the synthesis of polyhydroxyalkanoate in yeast peroxisomes as an analytical tool to study the beta-oxidation cycle, expression of AtDCI1 was shown to complement the yeast mutant deficient in the delta(3,5),delta(2,4)-dienoyl-CoA isomerase, thus showing that AtDCI1 is also appropriately targeted to the peroxisome in yeast and has delta(3,5),delta(2,4)-dienoyl-CoA isomerase activity in vivo. The AtDCI1 gene is expressed constitutively in several tissues, but expression is particularly induced during seed germination. Proteins showing high homology with AtDCI1 are found in gymnosperms as well as angiosperms belonging to the Monocotyledon or Dicotyledon classes.

Amino Acid Sequence↗
PubMed1995

Triosephosphate isomerase deficiency: biochemical and molecular genetic analysis for prenatal diagnosis.

Inherited deficiency of the glycolytic enzyme triosephosphate isomerase leads to a multisystem disorder characterized by progressive neuromuscular dysfunction, chronic nonspherocytic hemolytic anemia and increased susceptibility to severe infections. Most patients die within the first 6 years. We examined a family with severe triosephosphate isomerase deficiency. The 1-year-old index patient suffered from hemolytic anemia, neuromuscular impairment and pneumonias, with the necessity of intermitten mechanical ventilation. Triosephosphate isomerase activity in erythrocytes was reduced to about 20% of normal. Heat stability of the enzyme was strongly reduced; concentration of the physiological substrate, dihydroxyacetone phosphate was increased 20-fold due to the metabolic block. Direct sequencing of the triosephosphate isomerase gene revealed homozygosity for the formerly described GAG-->GAC-mutation changing 104 Glu-->Asp. During a 2nd pregnancy we examined a cord blood sample obtained in the 19th gestational week. The biochemical data on enzyme activity, heat stability of the enzyme and concentration of dihydroxyacetone phosphate were in the normal range. The molecular genetic analysis confirmed the presence of the normal triosephosphate isomerase alleles. Pregnancy was continued, resulting in the delivery of an unaffected, healthy newborn.

Anemia, Hemolytic, Congenital Nonspherocytic↗
PubMed1976

Matrix-bound phosphoglucose isomerase. Formation and properties of monomers and hybrids.

Dimeric phosphoglucose isomerase from rabbit muscle was immobilized by reaction with cyanogen-bromide-activated Sephrose 4B. The catalytic parameters and stability properties of the free and matrix-bound isomerases were essentially identical. Total monomerisation of the matrix-bound enzyme was achieved with 8 M urea as determined by a study in which one subunit was labelled with iodo[14C]acetate and the other with the 3H-labelled reagent. Although matrix-bound monomers were devoid of isomerase activity, they were still capable of binding the substrate. Matrix-bound monomers exhibited the ability to redimerize with soluble isomerase subunits from either rabbit or human yielding catalytically active dimers. Yeast isomerase monomers, in contrast, did not yield active hybrids with the rabbit monomers. Furthermore, soluble subunits, which had been inactivated with pyridoxal 5'-phosphate were also capable of hybridizing with and inducing catalytic activity in the matrix-bound monomers. These studies indicate the prerequisite of dimer formation for catalytic activity but the independent action of the catalytic centers of the dimer.

Animals↗
PubMed1997

Purification and characterization of two isoforms of isopentenyl-diphosphate isomerase from elicitor-treated Cinchona robusta cells.

In Cinchona robusta (Rubiaceae) cell suspension cultures, the activity of the enzyme isopentenyl-diphosphate isomerase (isopentenyl-POP isomerase) is transiently induced after addition of a homogenate of the phytopathogenic fungus Phytophthora cinnamomi. The enzyme catalyses the interconversion of isopentenyl-POP and dimethylallyl diphosphate (dimethylallyl-POP) and may be involved in the biosynthesis of anthraquinone phytoalexins that accumulate rapidly after elicitation of Cinchona cells. From elicitor-treated C. robusta cells, two isoforms of isopentenyl-POP isomerase have been purified to apparent homogeneity in four chromatographic steps. The purified forms are monomeric enzymes of 34 kDa (isoform I) and 29 kDa (isoform II), with Km values for isopentenyl-POP of 5.1 microM and 1.0 microM, respectively. Both isoforms require Mn2+ or Mg2+ as cofactor, isoform II showing a preference for Mn2+ with maximum activity at 1.5-2 mM. Isoform I was most active in the presence of 0.5-1.5 mM Mg2+ or in the presence of 0.5 mM Mn2+. A pH optimum of 7-7.8 was found for both forms and both were competitively inhibited by geranyl diphosphate (Ki 96 microM for isoform I) and the transition state analogue 2-(dimethylamino)ethyl diphosphate. Rechromatography of purified isoforms did not indicate any interconversion of both forms. Western blot analysis, using antibodies raised against isopentenyl-POP isomerase purified from Capsicum annuum, showed the presence of both isoforms in the crude protein extracts from C. robusta cells. Isoform II was specifically induced by elicitation, non-treated cells contained low activity of this isoform. The possible role of isopentenyl-POP isomerase in the biosynthesis of anthraquinones is discussed.

Anthraquinones↗
PubMed1989

Effect of 6-phosphogluconate on phosphoglucose isomerase in rat brain in vitro and in vivo.

The activity of phosphoglucose isomerase, its kinetic properties, and the effect of 6-phosphogluconate on its activity in the forward (glucose 6-phosphate----fructose 6-phosphate) and the reverse (fructose 6-phosphate----glucose 6-phosphate) reactions were determined in adult rat brain in vitro. The activity of phosphoglucose isomerase (in nmol/min/mg of whole brain protein) was 1,865 +/- 20 in the forward reaction and 1,756 +/- 32 in the reverse reaction at pH 7.5. It was 1,992 +/- 28 and 2,620 +/- 46, respectively, at pH 8.5. The apparent Km and Vmax of phosphoglucose isomerase were 0.593 +/- 0.031 mM and 2,291 +/- 61 nmol/min/mg of protein, respectively, for glucose 6-phosphate and 0.095 +/- 0.013 mM and 2,035 +/- 98 nmol/min/mg of protein, respectively, for fructose 6-phosphate. The activity of phosphoglucose isomerase was inhibited intensely and competitively by 6-phosphogluconate, with an apparent Ki of 0.048 +/- 0.005 mM for glucose 6-phosphate and 0.042 +/- 0.004 mM for fructose 6-phosphate as the substrate. With glucose 6-phosphate as the substrate, at concentrations from 0.05 to 0.5 mM, the activity of the enzyme was inhibited completely in the presence of 0.5-2.0 mM 6-phosphogluconate. With 0.05-0.2 mM fructose 6-phosphate as the substrate, it was inhibited greater than or equal to 85% at the same concentrations of the inhibitor. No significant changes were observed in the values of Km, Vmax, and Ki for phosphoglucose isomerase in the brain of 6-aminonicotinamide-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

6-Aminonicotinamide↗
PubMed1971

Metabolism of D-arabinose by Aerobacter aerogenes: purification of the isomerase.

In Aerobacter aerogenes, the mutational event permitting the utilization of d-arabinose as a source of carbon and energy is a regulatory mutation resulting in the constitutive synthesis of certain enzymes of the l-fucose catabolic pathway. l-Fucose isomerase catalyzes the isomerization of d-arabinose to d-ribulose. This enzyme was purified to homogeneity as indicated by a single band in disc-gel electrophoretic columns and single peaks with column chromatography and ultracentrifugation from the wild-type PRL-R3 strain, induced with l-fucose and two constitutive mutants, 502 and 510. The ratios of the activities of this isomerase on d-arabinose and l-fucose remained constant throughout all purifications. The apparent K(m) of the isomerase from the wild-type strain induced with l-fucose and from the constitutive mutant strains was 5.0 x 10(-2)m for l-fucose and 1.5 x 10(-1)m for d-arabinose. A strain 531 possessing an apparent alteration in the isomerase was isolated from the strain 502. This altered isomerase exhibited a lowered K(m) for d-arabinose.

Ammonium Sulfate↗
PubMed1991

Cloning, characterization, and nucleotide sequence analysis of a Zymomonas mobilis phosphoglucose isomerase gene that is subject to carbon source-dependent regulation.

The Zymomonas mobilis gene encoding phosphoglucose isomerase (pgi) was cloned by genetic complementation of an Escherichia coli pgi mutant. An enzyme assay and sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirmed the presence of excess amounts of phosphoglucose isomerase in E. coli clones carrying the Z. mobilis pgi gene. The pgi gene is present in only one copy on the Z. mobilis genome. Nucleotide sequence analysis of the pgi region revealed an open reading frame of 1,524 bp preceded by a strong Shine-Dalgarno sequence. The pgi gene encodes a 507-amino-acid protein with a predicted molecular weight of 55,398. Z. mobilis phosphoglucose isomerase is between 38 and 43% identical to the enzyme from other species. Northern (RNA) blot analysis showed that the pgi transcript is 1.8 kb in length. The level of the pgi transcript was found to be influenced by the phase of growth and by the carbon and energy sources. Transcript levels increased with respect to total RNA during logarithmic growth and were threefold higher when grown on fructose than on glucose. These changes in transcript levels paralleled phosphoglucose isomerase activities in the cultures. Differential mRNA stability was not a factor, since the half-life of the pgi transcript was 6.3 min in glucose-grown cells and 6.0 min in fructose-grown cells. Thus, an increase in the rate of transcription appears to be at least partially responsible for the increased levels of phosphoglucose isomerase observed for Z. mobilis grown on fructose.

Amino Acid Sequence↗
PubMed1999

Escherichia coli open reading frame 696 is idi, a nonessential gene encoding isopentenyl diphosphate isomerase.

Isopentenyl diphosphate isomerase catalyzes the interconversion of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). In eukaryotes, archaebacteria, and some bacteria, IPP is synthesized from acetyl coenzyme A by the mevalonate pathway. The subsequent isomerization of IPP to DMAPP activates the five-carbon isoprene unit for subsequent prenyl transfer reactions. In Escherichia coli, the isoprene unit is synthesized from pyruvate and glyceraldehyde-3-phosphate by the recently discovered nonmevalonate pathway. An open reading frame (ORF696) encoding a putative IPP isomerase was identified in the E. coli chromosome at 65.3 min. ORF696 was cloned into an expression vector; the 20.5 kDa recombinant protein was purified in three steps, and its identity as an IPP isomerase was established biochemically. The gene for IPP isomerase, idi, is not clustered with other known genes for enzymes in the isoprenoid pathway. E. coli FH12 was constructed by disruption of the chromosomal idi gene with the aminoglycoside 3'-phosphotransferase gene and complemented by the wild-type idi gene on plasmid pFMH33 with a temperature-sensitive origin of replication. FH12/pFMH33 was able to grow at the restrictive temperature of 44 degrees C and FH12 lacking the plasmid grew on minimal medium, thereby establishing that idi is a nonessential gene. Although the V(max) of the bacterial protein was 20-fold lower than that of its yeast counterpart, the catalytic efficiencies of the two enzymes were similar through a counterbalance in K(m)s. The E. coli protein requires Mg(2+) or Mn(2+) for activity. The enzyme contains conserved cysteine and glutamate active-site residues found in other IPP isomerases.

Carbon-Carbon Double Bond Isomerases↗
PubMed2002

trans-Complementation rescue of cyclophilin A-deficient viruses reveals that the requirement for cyclophilin A in human immunodeficiency virus type 1 replication is independent of its isomerase activity.

Human immunodeficiency virus type 1 (HIV-1) requires the incorporation of cyclophilin A (CypA) for replication. CypA is packaged by binding to the capsid (CA) region of Gag. This interaction is disrupted by cyclosporine (CsA). Preventing CypA incorporation, either by mutations in the binding region of CA or by the presence of CsA, abrogates virus infectivity. Given that CypA possesses an isomerase activity, it has been proposed that CypA acts as an uncoating factor by destabilizing the shell of CA that surrounds the viral genome. However, because the same domain of CypA is responsible for both its isomerase activity and its capacity to be packaged, it has been challenging to determine if isomerase activity is required for HIV-1 replication. To address this issue, we fused CypA to viral protein R (Vpr), creating a Vpr-CypA chimera. Because Vpr is packaged via the p6 region of Gag, this approach bypasses the interaction with CA and allows CypA incorporation even in the presence of CsA. Using this system, we found that Vpr-CypA rescues the infectivity of viruses lacking CypA, either produced in the presence of CsA or mutated in the CypA packaging signal of CA. Furthermore, a Vpr-CypA mutant which has no isomerase activity and no capacity to bind to CA also rescues HIV-1 replication. Thus, this study demonstrates that the isomerase activity of CypA is not required for HIV-1 replication and suggests that the interaction of the catalytic site of CypA with CA serves no other function than to incorporate CypA into viruses.

Cell Line↗
PubMed1996

The immunosuppressant SR 31747 blocks cell proliferation by inhibiting a steroid isomerase in Saccharomyces cerevisiae.

SR 31747 is a novel immunosuppressant agent that arrests cell proliferation in the yeast Saccharomyces cerevisiae, SR 31747-treated cells accumulate the same aberrant sterols as those found in a mutant impaired in delta 8- delta 7-sterol isomerase. Sterol isomerase activity is also inhibited by SR 31747 in in vitro assays. Overexpression of the sterol isomerase-encoding gene, ERG2, confers enhanced SR resistance. Cells growing anaerobically on ergosterol-containing medium are not sensitive to SR. Disruption of the sterol isomerase-encoding gene is lethal in cells growing in the absence of exogenous ergosterol, except in SR-resistant mutants lacking either the SUR4 or the FEN1 gene product. The results suggest that sterol isomerase is the target of SR 31747 and that both the SUR4 and FEN1 gene products are required to mediate the proliferation arrest induced by ergosterol depletion.

Amino Acid Sequence↗
PubMedDate not supplied

[Comparative study of xylose(glucose) isomerase synthesis in Arthrobacter strains].

The substrate specificity of isomerases produced by six strains of Arthrobacter sp. was studied. The role of utilizable carbon sources in controlling enzyme biosynthesis was established. All of the strains studied were found to produce xylose isomerases efficiently, converting D-xylose into D-xylulose and D-glucose into D-fructose. All but A. ureafaciens B-6 strains showed low activity toward D-ribose, Arthrobacter sp. B-5 was slightly active toward L-arabinose, and A. ureafaciens B-6 and Arthrobacter sp. B-2239, toward L-rhamnose. In Arthrobacter sp. B-5, the synthesis of xylose/glucose isomerase was constitutive (i.e., it was not suppressed by readily metabolizable carbon sources). The synthesis of xylose/glucose isomerase induced by D-xylose in Arthrobacter sp. strains B-2239, B-2240, B-2241, and B-2242 and by D-xylose and xylitol in A. ureafaciens B-6 was suppressed by readily metabolizable carbon sources in a concentration-dependent manner. The data obtained suggest that D-xylose and/or its metabolites are involved in the regulation of xylose/glucose isomerase synthesis in the Arthrobacter sp. strains B-5, B-2239, B-2240, and B-2241.

Aldose-Ketose Isomerases↗
PubMed1975

Concentration-dependent association of delta5-3-ketosteroid isomerase of Pseudomonas testosteroni.

Gel chromatography and ultracentrifugation studies show that delta5-3-ketosteroid isomerase of Pseudomonas testosteroni a dimer with a molecular weight of 26,800 at concentrations below 1 mg per ml, undergoes reversible, concentration-dependent association at higher enzyme concentrations. In the concentration range between 0.04 and 15.6 mg per ml, apparent molecular radii of 23 A to 36 A and molecular weights of 26,000 to 69,000 were observed. The latter value represents the weight average molecular weight of two or more ploymerization species in rapid equilibrium, rather than a discrete polymeric form of the enzyme. The isomerase dimer has been found to be unusually stable to dissociation upon dilution, even at concentrations in the nanogram per ml range. Evidence is presented which suggests that the enzyme is present as a dimer in P. testosteroni cells and that this is a catalytically active species. The isomerase monomer has been obtained and its molecular weight studied by gel electrophoresis in the presence of sodium dodecyl sulfate. A new determination of the extinction coefficient of the isomerase gives the value of 0.336 for the absorbance at 280 nm in a 1-cm light path of a solution containing 1 mg of the isomerase per ml.

Binding Sites↗
PubMed1975

Subunit structure and amino acid composition of xylose isomerase from Streptomyces albus.

The subunit structure and amino acid composition of xylose isomerase from Streptomyces albus have been examined. A native molecular weight of 165,000 determined by sedimentation equilibrium was reduced to 43,000 when the protein was treated with 6 M guanidine hydrochloride. No further reduction in molecular weight was observed when potential disulfide bridges of xylose isomerase were reduced and alkylated, indicating that the protein was devoid of interchain disulfide bonds. NH2-terminal analysis using [3H]dansyl chloride showed 0.86 residues of methionine per Mr equals 41,500 unit. Analysis of the native protein with an automated protein sequenator revealed the presence of only one degradable polypeptide chain. Fractionation of the soluble tryptic peptides of S-[14C]carboxymethyl xylose isomerase by ion exchange chromatography and one-dimensional paper electrophoresis yielded 37 to 43 peptides. When the acid-insoluble tryptic peptides were dissolved and analyzed using gel filtration techniques, and additional four peptides were found. A unique radioactive tryptic peptide containing S-carboxymethylcysteine was found among the soluble peptides, confirming cysteine as the limiting amino acid residue in the amino acid composition of xylose isomerase. On the basis of its lysine and arginine content, the number of tryptic peptides is consistent with the hypothesis that the native xylose isomerase is a tetramer of four very similar or identical subunits of Mr equals 41,500, associated by noncovalent bonds.

Aldose-Ketose Isomerases↗