Enzymatic synthesis of D-tagatose from lactose with the combination of β-galactosidase and L-arabinose isomerase

将大肠杆菌K-12中的β-半乳糖苷酶基因lacZ和L-阿拉伯糖异构酶基因araA以串联方式克隆到载体pET-28a(+)上,并转入大肠杆菌BL21( DE3)中进行表达.通过SDS-PAGE分析发现,重组菌株能表达出大量可溶性β-半乳糖苷酶蛋白和L-阿拉伯糖异构酶蛋白.以重悬菌液为酶源,可将乳糖降解为D-半乳糖,并将D-半乳糖转化为D-塔格糖.在温度为50℃,pH 7.0的缓冲液中,经一段时间反应后,D-塔格糖的转化率可达21％以上.加入Mn2+、Co2+和Fe2+均能够使D-塔格糖的转化率提高.     

Action of protein disulfide isomerase on proinsulin exit from endoplasmic reticulum of pancreatic β-cells

For insulin synthesis, the proinsulin precursor is translated at the endoplasmic reticulum (ER), folds to include its three native disulfide bonds, and is exported to secretory granules for processing and secretion. Protein disulfide isomerase (PDI) has long been assumed to assist proinsulin in this process. Herein we have examined the effect of PDI knockdown (PDI-KD) in β-cells. The data establish that upon PDI-KD, oxidation of proinsulin to form native disulfide bonds is unimpaired and in fact enhanced. This is accompanied by improved proinsulin exit from the ER and increased total insulin secretion, with no evidence of ER stress. We provide evidence for direct physical interaction between PDI and proinsulin in the ER of pancreatic β-cells, in a manner requiring the catalytic activity of PDI. In β-cells after PDI-KD, enhanced export is selective for proinsulin over other secretory proteins, but the same effect is observed for recombinant proinsulin trafficking upon PDI-KD in heterologous cells. We hypothesize that PDI exhibits unfoldase activity for proinsulin, increasing retention of proinsulin within the ER of pancreatic β-cells.     

Characterization of a monoclinic crystal form of an enzyme of steroid metabolism, Δ5-3-ketosteroid isomerase

A monoclinic crystal form ($\text{P2}{}_1\text{, a = 140.4}\text{A}\text{?}\text{, b = 85.0}\text{A}\text{?}\text{, c = 94.5}\text{A}\text{?}$, β= 130.1 °) of Δ⁵-3-ketosteroid isomerase from Pseudomonas testosteroni (EC 5.3.3.1), grown at pH 7.0, has been characterized. Crystal-density measurements show that the asymmetric unit contains 12 protomers (M_r = 13,394).     

The fermentation of xylose —an analysis of the expression of Bacillus and Actinoplanes xylose isomerase genes in yeast

Summary The genes encoding xylose isomerase from Bacillus subtilis and Actinoplanes missouriensis have been isolated by complementation of a xylose isomerase defective Escherichia coli mutant. The xylose isomerase gene from A. missouriensis could be expressed in E. coli under the control of its own promoter, whereas the cloned Bacillus gene was expressed in E. coli only after the spontaneous integration of the E. coli IS5 element. After fusion of the Bacillus gene to the yeast PDC1 promoter, transformants of Saccharomyces cerevisiae contained the xylose isomerase protein. Approx. 5% of the total cellular protein of transformants consisted of xylose isomerase that was found to be at least partly insoluble. Neither the insoluble protein nor Triton X-114 solubilized isomerase was catalytically active. To investigate whether the xylose isomerase of A. missouriensis can be expressed in S. cerevisiae the coding region was fused to the yeast GAL1 promoter. Analysis of total RNA from yeast transformants containing this construction showed a xylose isomerase specific mRNA.Dedicated to Professor Karl Esser on the occasion of his 60th birthday     

12pter → 12p 12.2: Possible assignment of human triose phosphate isomerase

Summary Red cell triose-phosphate isomerase (TPI) was determined, together with other enzymes, in three patients with chromosome 12 abnormalities.In patient No. 1 (trisomy of the segment 12pter 12q12) and in patient No. 2 (trisomy of the segment 12pter 12p12.1), the TPI activity was significantly increased. In patient No. 3 (deletion of the segment 12p11 12p12.2), the TPI activity was in the normal range. These results suggest that the human TPI locus is located on the chromosome 12 short arm, between 12pter and 12p12.2.Directeur de Recherches à l'I.N.S.E.R.M.     

Protein disulphide isomerase of Ostertagia ostertagi: an excretory-secretory product of L4 and adult worms?

A pepstatin A-agarose column was used in an attempt to purify a previously described antibody-degrading aspartyl proteinase from excretory-secretory material from the L4 and the adult stages of the bovine abomasal nematode Ostertagia ostertagi. However, no aspartyl proteinase activity was detected in the eluted fractions (L4Pepst and AdPepst). Screening of cDNA libraries with polyclonal antibodies raised against L4Pepst and AdPepst showed that a protein disulphide isomerase (Ost-PDI2) was present in both antigen fractions. This multifunctional enzyme was detected in extracts of L3, L4 and adult parasites and, interestingly, also in excretory-secretory material of L4 and adult O. ostertagi. By immunohistochemistry, the Ost-PDI2 enzyme was localised in some parts of the hypodermis of L4 and adult worms and in the intestinal cells of all three parasitic life stages. Two-dimensional Western blot analysis indicated that Ost-PDI2 is recognised by calves during a natural O. ostertagi infection, which suggests that Ost-PDI2 could be used for immunological control of ostertagiosis.     

Predominant expression of the β subunit of prolyl 4-hydroxylase (disulfide isomerase) in human extravillous trophoblasts

Prolyl 4-hydroxylase is a heterodimeric enzyme that is crucial in the biosynthesis of collagen. The subunit of this enzyme is a multifunctional protein which is also known as protein-disulfide isomerase. Immunofluorescence and monoclonal antibody (Mab) 5B5 were used to localize the subunit in human extraembryonic tissues. The strongest sites of 5B5 reactivity were extravillous cytotrophoblasts in the basal plate, uteroplacental arteries and amniochorion, syncytiotrophoblast displayed variable weaker reactivity. Only a small fraction of placental 5B5 antigen was detected as a component of prolyl-4-hydroxylase by affinity chromatography on immobilized polyproline. The results indicate a difference in the expression of an endoplasmic reticulum marker between villous and extravillous trophoblast. The predominance of 5B5 antigen in extravillous trophoblast could be associated with an increased ability to synthesize collagen or other enzymatic reactions associated with prolyl 4-hydroxylase subunit.     

Enhancing functional expression of β-glucosidase in Pichia pastoris by co-expressing protein disulfide isomerase

The expression of heterologous proteins may exert severe stress on the host cells at different levels. Protein folding and disulfide bond formation were identified as rate-limited steps in recombinant protein secretion in yeast cells. For the production of β-glucosidase in Pichia pastoris, final β-glucosidase activity reached 1,749 U/mL after fermentation optimization in a 3 L bioreactor, while the specific activity decreased from 620 to 467 U/mg, indicating a potential protein misfolding. To solve this problem, protein disulfide isomerase, a chaperone protein which may effectively regulate disulfide bond formation and protein folding, was co-expressed with β-glucosidase. In the co-expression system, a β-glucosidase production level of 2,553 U/mL was achieved and the specific activity of the enzyme reached 721 U/mg, which is 1.54 fold that of the control.     

Expression ofArabidopsis tryptophan biosynthetic pathway genes: effect of the 5′ coding region of phosphoribosylanthranilate isomerase gene

There are three non-allelic isogenes encoding phosphoribosylanthranilate isomerase (PAI) inArabidopsis thaliana. The expression plasmids were constructed by fusion of the GUS reporter gene to the three PAI promoters with or without the 5′ region encoding PAI N-terminal polypeptides and transferred into Arabidopsis plants byAgrobacterium tumefaciens. Analysis of GUS activity revealed that the PAI 5′ coding region was necessary for high expression of GUS activity. GUS activity in transgenic plants transformed with the expression plasmids containing the 5′ coding region of PAH or PAI3 was 60–100-fold higher than that without the corresponding 5′ region. However, the effect of 5’ coding region of PAI2 gene on the GUS activity was very small (only about 1 time difference). The GUS histochemical staining showed a similar result as revealed by GUS activity assay. It was expressed in the mesophyll cells and guard cells, but not in the epidermic cells, indicating that the N-terminal polypeptides encoded by the 5′ region of PAI genes have the function of PTP.     

The reversibility of the Δ8-cholestenol–Δ7-cholestenol isomerase reaction in cholesterol biosynthesis

The incubation of Delta(7)-cholestenol with a 10000g supernatant or 105000g microsomes in the presence of tritiated water is studied. The reisolated Delta(7)-cholestenol contained up to 0.67g.atom of tritium/mole. This result can best be explained by assuming the reversibility of the reaction Delta(8)-cholestenol right harpoon over left harpoon Delta(7)-cholestenol.     

Mutation of Lys242 allows Δ-Δ-enoyl-CoA isomerase to acquire enoyl-CoA hydratase activity

We report here a novel example of generating hydratase activity through site-directed mutagenesis of a single residue Lys242 of rat liver mitochondrial Δ³-Δ²-enoyl-CoA isomerase, which is one of the key enzymes involved in fatty acid oxidation and a member of the crotonase superfamily. Lys242 is at the C-terminal of the enzyme, which is far from the active site in the crotonase superfamily and forms a salt bridge with Asp149. A variety of mutant expression plasmids were constructed, and it was observed that mutation of Lys242 to nonbasic residues allowed the mutants to have enoyl-CoA hydratase activity confirmed by HPLC analysis of the incubation mixture. Kinetic studies of these mutants were carried out for both isomerase and hydratase activities. Mutant K242C showed a k_cat value of 1.0 s^− 1 for hydration reaction. This activity constitutes about 10% of the total enzyme activity, and the remaining 90% is its natural isomerase activity. To the best of our knowledge, this is the first report on the generation of functional promiscuity through single amino acid mutation far from the active site. This may be a simple and efficient approach to designing a new enzyme based on an existing template. It could perhaps become a general methodology for facilitating an enzyme to acquire a type enzymatic activity that belongs to another member of the same superfamily, by interrupting a key structural element in order to introduce ambiguity, using site-directed mutagenesis.     

Is protein disulfide isomerase a redox-dependent molecular chaperone?

Protein disulfide isomerase (PDI) is a multifunctional protein catalysing the formation of disulfide bonds, acting as a molecular chaperone and being a component of the enzymes prolyl 4-hydroxylase (P4H) and microsomal triglyceride transfer protein. The role of PDI as a molecular chaperone or polypeptide-binding protein is mediated primarily through an interaction of substrates with its b' domain. It has been suggested that this binding is regulated by the redox state of PDI, with association requiring the presence of glutathione, and dissociation the presence of glutathione disulfide. To determine whether this is the case, we investigated the ability of PDI to bind to a folding polypeptide chain within a functionally intact endoplasmic reticulum and to be dissociated from the alpha-subunit of P4H in vitro in the presence of reducing or oxidizing agents. Our results clearly demonstrate that binding of PDI to these polypeptides is not regulated by its redox state. We also demonstrate that the dissociation of PDI from substrates observed in the presence of glutathione disulfide can be explained by competition for the peptide-binding site on PDI.     

Structure-based reconstruction of a Mycobacterium hypothetical protein into an active Δ5–3-ketosteroid isomerase

Protein engineering based on structure homology holds the potential to engineer steroid-transforming enzymes on demand. Based on the genome sequencing analysis of industrial Mycobacterium strain HGMS2 to produce 4-androstene-3,17-dione (4-AD), three hypothetical proteins were predicted as putative Δ⁵–3-ketosteroid isomerases (KSIs) to catalyze an intramolecular proton transfer involving the transformation of 5-androstene-3,17-dione (5-AD) into 4-AD, which were defined as mKSI228, mKSI291 and mKSI753. Activity assays indicated that mKSI228 and mKSI291 exhibited weak activity, as low as 0.7% and 1.5%, respectively, of a well-studied and highly active KSI from Pseudomonas putida KSI (pKSI), while mKSI753 had no activity similar to Mycobacterium tuberculosis KSI (mtKSI). Although the 3D structures of the putative mKSIs were homologous to pKSI, their amino acid sequences were significantly different from those of pKSI and tKSI. Thus, by use of these two KSIs as homology models, we were able to convert the low-active mKSI291 into a high-active active KSI by site-directed mutagenesis. On the other hand, an X-ray crystallographic structure of mKSI291 identified a water molecule in its active site. This unique water molecule might function as a bridge to connect Ser-OH, Tyr57-OH and C3O of the intermediate form a hydrogen-bonding network that was responsible for its weak activity, compared with that of mtKSI. Our results not only demonstrated the use of a protein engineering approach to understanding KSI catalytic mechanism, but also provided an example for engineering the catalytic active sites and gaining a functional enzyme based on homologous structures.     

The structure of a truncated phosphoribosylanthranilate isomerase suggests a unified model for evolution of the (βα)8 barrel fold

The (βα)₈ barrel is one of the most common protein folds, and enzymes with this architecture display a remarkable range of catalytic activities. Many of these functions are associated with ancient metabolic pathways, and phylogenetic reconstructions suggest that the (βα)₈ barrel was one of the very first protein folds to emerge. Consequently, there is considerable interest in understanding the evolutionary processes that gave rise to this fold. In particular, much attention has been focused on the plausibility of (βα)₈ barrel evolution from homodimers of half barrels. However, we previously isolated a three-quarter-barrel-sized fragment of a (βα)₈ barrel, termed truncated phosphoribosylanthranilate isomerase (trPRAI), that is soluble and almost as thermostable as full-length N-(5′-phosphoribosyl)anthranilate isomerase (PRAI). Here, we report the NMR-derived structure of trPRAI. The subdomain is monomeric, is well ordered and adopts a native-like structure in solution. Side chains from strands β₁ (Glu3 and Lys5), β₂ (Tyr25) and β₆ (Lys122) of trPRAI repack to shield the hydrophobic core from the solvent. This result demonstrates that three-quarter barrels were viable intermediates in the evolution of the (βα)₈ barrel fold. We propose a unified model for (βα)₈ barrel evolution that combines our data, previously published work and plausible scenarios for the emergence of (initially error-prone) genetic systems. In this model, the earliest proto-cells contained diverse pools of part-barrel subdomains. Combinatorial assembly of these subdomains gave rise to many distinct lineages of (βα)₈ barrel proteins, that is, our model excludes the possibility that there was a single (βα)₈ barrel from which all present examples are descended.     

1H, 13C and 15N resonance assignments of the bb′ domains of human protein disulfide isomerase

Protein disulfide isomerase (PDI) participates in protein folding and catalyses formation of disulfide bonds. The b′ domain of human PDI contributes to binding unfolded proteins; its structure is stabilized by the b domain. Here, we report NMR chemical shift assignments for the bb′ fragment.     

Probing the equilibrium unfolding of ketosteroid isomerase through xenon-perturbed 1H–15N multidimensional NMR spectroscopy

We used xenon-perturbed ¹H–¹⁵N multidimensional NMR to investigate the structural changes in the urea-induced equilibrium unfolding of the dimeric ketosteroid isomerase (KSI) from Pseudomonas putida biotype B. Three limited regions located on the β3-, β5- and β6-strands of dimeric interface were significantly perturbed by urea in the early stage of KSI unfolding, which could lead to dissociation of the dimer into structured monomers at higher denaturant concentration as the interactions in these regions are weakened. The results indicate that the use of xenon as an indirect probe for multidimensional NMR can be a useful method for the equilibrium unfolding study of protein at residue level.     

High-level functional expression of a fungal xylose isomerase: the key to efficient ethanolic fermentation of xylose by Saccharomyces cerevisiae?

Evidence is presented that xylose metabolism in the anaerobic cellulolytic fungus Piromyces sp. E2 proceeds via a xylose isomerase rather than via the xylose reductase/xylitol-dehydrogenase pathway found in xylose-metabolising yeasts. The XylA gene encoding the Piromyces xylose isomerase was functionally expressed in Saccharomyces cerevisiae. Heterologous isomerase activities in cell extracts, assayed at 30 degrees C, were 0.3-1.1 micromol min(-1) (mg protein)(-1), with a Km for xylose of 20 mM. The engineered S. cerevisiae strain grew very slowly on xylose. It co-consumed xylose in aerobic and anaerobic glucose-limited chemostat cultures at rates of 0.33 and 0.73 mmol (g biomass)(-1) h(-1), respectively.     

Chemical shift assignments of a reduced N-terminal truncation mutant of the disulfide bond isomerase TrbB from plasmid F,TrbBΔ29

TrbB from the conjugative plasmid F is a 181-residue disulfide bond isomerase that plays a role in the correct folding and maintenance of disulfide bonds within F plasmid encoded proteins in the bacterial periplasm. As a member of the thioredoxin-like superfamily, TrbB has a predicted thioredoxin-like fold that contains a C–X–X–C active site required for performing specific redox chemistries on protein substrates. Here we report the sequence-specific assignments of the reduced form of the N-terminally truncated TrbB construct, TrbBΔ29.