Aryldithioethyloxycarbonyl (Ardec): a new family of amine protecting groups removable under mild reducing conditions and their applications to peptide synthesis

The development of phenyldithioethyloxycarbonyl (Phdec) and 2-pyridyldithioethyloxycarbonyl (Pydec) protecting groups, which are thiol-labile urethanes, is described. These new disulfide-based protecting groups were introduced onto the epsilon-amino group of L-lysine; the resulting amino acid derivatives were easily converted into N alpha-Fmoc building blocks suitable for both solid- and solution-phase peptide synthesis. Model dipeptide(Ardec)s were prepared by using classical peptide couplings followed by standard deprotection protocols. They were used to optimize the conditions for complete thiolytic removal of the Ardec groups both in aqueous and organic media. Phdec and Pydec were found to be cleaved within 15 to 30 min under mild reducing conditions: i) by treatment with dithiothreitol or beta-mercaptoethanol in Tris.HCl buffer (pH 8.5-9.0) for deprotection in water and ii) by treatment with beta-mercaptoethanol and 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) in N-methylpyrrolidinone for deprotection in an organic medium. Successful solid-phase synthesis of hexapeptides Ac-Lys-Asp-Glu-Val-Asp-Lys(Ardec)-NH2 has clearly demonstrated the full orthogonality of these new amino protecting groups with Fmoc and Boc protections. The utility of the Ardec orthogonal deprotection strategy for site-specific chemical modification of peptides bearing several amino groups was illustrated firstly by the preparation of a fluorogenic substrate for caspase-3 protease containing the cyanine dyes Cy 3.0 and Cy 5.0 as FRET donor/acceptor pair, and by solid-phase synthesis of an hexapeptide bearing a single biotin reporter group.     

Kinetic analysis of semisynthetic peroxidase enzymes containing a covalent DNA-heme adduct as the cofactor

The reconstitution of apo enzymes with DNA oligonucleotide-modified heme (protoporphyrin IX) cofactors has been employed as a tool to produce artificial enzymes that can be specifically immobilized at the solid surfaces. To this end, covalent heme-DNA adducts were synthesized and subsequently used in the reconstitution of apo myoglobin (aMb) and apo horseradish peroxidase (aHRP). The reconstitution produced catalytically active enzymes that contained one or two DNA oligomers coupled to the enzyme in the close proximity to the active site. Kinetic studies of these DNA-enzyme conjugates, carried out with two substrates, ABTS and Amplex Red, showed a remarkable increase in peroxidase activity of the DNA-Mb enzymes while a decrease in enzymatic activity was observed for the DNA-HRP enzymes. All DNA-enzyme conjugates were capable of specific binding to a solid support containing complementary DNA oligomers as capture probes. Kinetic analysis of the enzymes immobilized by the DNA-directed immobilization method revealed that the enzymes remained active after hybridization to the capture oligomers. The programmable binding properties enabled by DNA hybridization make such semisynthetic enzyme conjugates useful for a broad range of applications, particularly in biocatalysis, electrochemical sensing, and as building blocks for biomaterials.     

Direct and efficient monitoring of glycosyltransferase reactions on gold colloidal nanoparticles by using mass spectrometry

A simple and efficient assay for glycosyltransferase activity on gold colloidal nanoparticles (GCNPs) by using laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) is demonstrated by the enzymatic synthesis of the Lewis X trisaccharide on GCNPs containing GlcNAc residues. GCNPs containing multivalent sugars were well dispersed in aqueous solution and proved to be excellent acceptor substrates for the glycosyltransferase reaction. Direct LDI-TOF MS analysis of these GCNPs provided the ion peaks of the sugar derivatives, chemisorbed through S--Au linkages onto the GCNPs, even in the presence of contaminants such as proteins and salts. Thus, it enabled the rapid and direct detection of the enzymatic reaction on the GCNPs by subjecting a small amount (0.15 muL) of the reaction mixture to MS analysis without purification. Subsequent MS/MS analyses (LDI-LIFT-TOF/TOF method) of the product-carrying GCNPs enabled the structures of the sugar derivatives that had been constructed on the GCNPs by enzymatic glycosylation to be determined. A quantitative inhibition assay for glycosyltransferase by using LDI-TOF MS analysis on the GCNPs was demonstrated by using uridine 5'-diphosphate (UDP) as the inhibitor. This simple assay was then applied to the detection of the enzymatic activity of a crude cell extract of Escherichia coli, which produces Neisseria meningitidis beta-1,4-galactosyltransferase (beta-1,4-GalT). In this case, the GCNPs were roughly purified by means of ultrafiltration to remove the buffer and detergents before MS analysis. That the GCNPs are dissolved in solution in the reaction medium but are solid in the purification process is greatly advantageous for the simple and efficient detection of enzymatic activity in crude biological samples. Thus, GCNPs containing a variety of biomolecules may become a versatile and efficient tool for the rapid and direct monitoring of metabolism (metabolomics) in living cells when combined with LDI-TOF MS analysis.     

Glycopeptide synthesis through endo-glycosidase-catalyzed oligosaccharide transfer of sugar oxazolines: probing substrate structural requirement

An array of sugar oxazolines was synthesized and tested as donor substrates for the Arthrobacter endo-beta-N-acetylglucosaminidase (Endo-A)-catalyzed glycopeptide synthesis. The experiments revealed that the minimum structure of the donor substrate required for Endo-A catalyzed transglycosylation is a Man beta1-->4-GlcNAc oxazoline moiety. Replacement of the beta-D-Man moiety with beta-D-Glc, beta-D-Gal, and beta-D-GlcNAc monosaccharides resulted in the loss of substrate activity for the disaccharide oxazoline. Despite this, the enzyme could tolerate modifications such as attachment of additional sugar residues or a functional group at the 3- and/or 6-positions of the beta-D-Man moiety, thus allowing a successful transfer of selectively modified oligosaccharides to the peptide acceptor. On the other hand, the enzyme has a great flexibility for the acceptor portion and could take both small and large GlcNAc-peptides as the acceptor. The studies implicate a great potential of the endoglycosidase-catalyzed transglycosylation for constructing both natural and selectively modified glycopeptides.     

Flavin-dependent biocatalysts in synthesis

The diverse chemistry possible with flavin cofactors positions flavin-dependent enzymes as versatile synthetic tools. This focused review highlights applications of flavin-dependent enzymes in organic synthesis. Select examples of monoamine oxidases, ene-reductases, monooxygenases and halogenases in target-oriented synthesis are presented.     

光固定辣根过氧化酶修饰电极的直接电化学

用聚氨酯丙烯酸酯预聚体将辣根过氧化物酶用光固定法制备了辣根过氧化物酶修饰电极.采用循环伏安法研究了该电极在没有媒介体的条件下的电化学性质,并考察了其对H_2O_2的生物电催化活性.结果表明,该电极可以在没有媒介体存在的条件下进行直接而稳定的电子转移.扫描100圈后,扫描时的峰电流仍然几乎不变;在缓冲溶液中保存30天后,其伏安响应仍能保持保存前的约95%以上,且仍保持对H_2O_2电化学还原的生物电催化作用.这为酶电极的制备及酶的固定化提出了新方法.     

Study of cell killing effect on S180 by ultrasound activating protoporphyrin IX

The present study was initiated to investigate the potential biological mechanism of cell killing effect on isolate sarcoma 180 (S180) cells induced by ultrasound activating protoporphyrin IX (PPIX). S180 cells were exposed to ultrasound for 30 s duration, at a frequency of 2.2 MHz and an acoustic power of 3 W/cm² in the presence of 120 μM PPIX. The viability of cells was evaluated using trypan blue staining. The generation of oxygen free radicals in cell suspensions was detected immediately after treatment using a reactive oxygen detection kit. A copper reagent colorimetry method was used to measure the level of FFAs released into cell suspensions by the process of cell damage induced by ultrasound and PPIX treatment. Oxidative stress was assessed by measuring the activities of key antioxidant enzymes (i.e., SOD, CAT, GSH-PX) in S180 tumor cells. Treatment with ultrasound and PPIX together increased the cell damage rate to 50.91%, while treatment with ultrasound alone gave a cell damage rate to 24.24%, and PPIX alone kept this rate unchanged. Colorimetry and enzymatic chemical methods showed that the level of FFAs in cell suspension increased significantly after the treatment, while the activity of all the above enzymes decreased in tumor cells at different levels, and were associated with the generation of oxygen free radicals in cell suspension after treatment. The results indicate that oxygen free radicals may play an important role in improving the membrane lipid peroxidation, degrading membrane phospholipids to release FFAs, and decreasing the activities of the key antioxidant enzymes in cells. This biological mechanism might be involved in mediating the effects on S180 cells and resulting in the cell damage seen with SDT.