Biosynthetic studies on the alpha-glucosidase inhibitor acarbose: the chemical synthesis of dTDP-4-amino-4,6-dideoxy-alpha-D-glucose

Biochemical Characterization of GacI, a Bifunctional Glycosyltransferase-Phosphatase Enzyme Involved in Acarbose Biosynthesis in Streptomyces glaucescens GLA.O

Acarbose, a pseudotetrasaccharide produced by several strains of Actinoplanes and Streptomyces, is an α-glucosidase inhibitor clinically used to control type II diabetes. Bioinformatic analysis of the biosynthetic gene clusters of acarbose in Actinoplanes sp. SE50/110 (the acb cluster) and Streptomyces glaucescens GLA.O (the gac cluster) revealed their distinct genetic organizations and presumably biosynthetic pathways. However, to date, only the acarbose pathway in the SE50/110 strain has been extensively studied. Here, we report that GacI, one of the proteins that appear to be different between the two pathways, is a bifunctional glycosyltransferase family 5 (GT5)-phosphatase (PP) enzyme that functions at two different steps in acarbose biosynthesis in S. glaucescens GLA.O. In the acb pathway, the GT and the PP reactions are performed by two different enzymes. Truncated GacI proteins having only the GT or the PP domain showed comparable catalytic activity with the full-length GacI, indicating that domain separation does not significantly affect their respective catalytic activity. GacI, which is widely distributed in many Streptomyces, represents the first example of naturally occurring GT5-PP bifunctional enzymes biochemically characterized.

Complete biosynthetic pathway to the antidiabetic drug acarbose

Acarbose is a bacterial-derived α-glucosidase inhibitor clinically used to treat patients with type 2 diabetes. As type 2 diabetes is on the rise worldwide, the market demand for acarbose has also increased. Despite its significant therapeutic importance, how it is made in nature is not completely understood. Here, we report the complete biosynthetic pathway to acarbose and its structural components, GDP-valienol and O-4-amino-(4,6-dideoxy-α-D-glucopyranosyl)-(1→4)-O-α-D-glucopyranosyl-(1→4)-D-glucopyranose. GDP-valienol is derived from valienol 7-phosphate, catalyzed by three cyclitol modifying enzymes, whereas O-4-amino-(4,6-dideoxy-α-D-glucopyranosyl)-(1→4)-O-α-D-glucopyranosyl-(1→4)-D-glucopyranose is produced from dTDP-4-amino-4,6-dideoxy-D-glucose and maltose by the glycosyltransferase AcbI. The final assembly process is catalyzed by a pseudoglycosyltransferase enzyme, AcbS, which is a homologue of AcbI but catalyzes the formation of a non-glycosidic C-N bond. This study clarifies all previously unknown steps in acarbose biosynthesis and establishes a complete pathway to this high value pharmaceutical.

The market demand for acarbose, a drug used for treatment of patients affected by type-2 diabetes, has increased. In this article, the authors report the acarbose complete biosynthetic pathway, clarifying previously unknown steps and identifying a pseudoglycosyltransferase enzyme, AcbS, a homologue of AcbI that catalyzes the formation of a non-glycosidic C-N bond.

Highly improved acarbose production of Actinomyces through the combination of ARTP and penicillin susceptible mutant screening

Atmospheric and room temperature plasma (ARTP) was first employed to generate mutants of Actinomyces JN537 for improving acarbose production. To obtain higher acarbose producing strains, the method of screening the strains for susceptibility to penicillin was used after treatment with ARTP. The rationale for the strategy was that mutants showing penicillin susceptibility were likely to be high acarbose producers, as their ability to synthesize cell walls was weak which might enhance metabolic flux to the pathway of acarbose biosynthesis. Acarbose yield of the mutant strain M37 increased by 62.5 % than that of the original strain. The contents of monosaccharides and amino acids of the cell wall of M37 were lower than that of the original strain. The acarbose production ability in mutant strain remained relatively stable after 10 generations. This work provides a promising strategy for obtaining high acarbose-yield strains by combination of ARTP mutation method and efficient screening technique.

Biosynthesis and Biological Activity of Carbasugars

The first synthesis of carbasugars, compounds in which the ring oxygen of a monosaccharide had been replaced by a methylene moiety, was described in 1966 by Professor G. E. McCasland’s group. Seven years later, the first true natural carbasugar (5a-carba-R-D-galactopyranose) was isolated from a fermentation broth of Streptomyces sp. MA-4145. In the following decades, the chemistry and biology of carbasugars have been extensively studied. Most of these compounds show interesting biological properties, especially enzymatic inhibitory activities, and, in consequence, an important number of analogues have also been prepared in the search for improved biological activities. The aim of this review is to give coverage on the progress made in two important aspects of these compounds: the elucidation of their biosynthesis and the consideration of their biological properties, including the extensively studied carbapyranoses as well as the much less studied carbafuranoses.

Synthesis and biological evaluation of prodrugs of 2-fluoro-2-deoxyribose-1-phosphate and 2,2-difluoro-2-deoxyribose-1-phosphate

We report in this Letter the synthesis of prodrugs of 2-fluoro-2-deoxyarabinose-1-phosphate and 2,2-difluoro-2-deoxyribose-1-phosphate. We demonstrate the difficulty of realising a phosphorylation step on the anomeric position of 2-deoxyribose, and we discover that introduction of fluorine atoms on the 2 position of 2-deoxyribose enables the phosphorylation step: in fact, the stability of the prodrugs increases with the degree of 2-fluorination. Stability studies of produgs of 2-fluoro-2-deoxyribose-1-phosphate and 2,2-difluoro-2-deoxyribose-1-phosphate in acidic and neutral conditions were conducted to confirm our observation. Biological evaluation of prodrugs of 2,2-difluoro-2-deoxyribose-1-phosphate for antiviral and cytotoxic activity is reported.

Actinoplanes utahensis ZJB-08196 fed-batch fermentation at elevated osmolality for enhancing acarbose production

Acarbose, a potent α-glucosidase inhibitor, is as an oral anti-diabetic drug for treatment of the type two, noninsulin-dependent diabetes. Actinoplanes utahensis ZJB-08196, an osmosis-resistant actinomycete, had a broad osmolality optimum between 309 mOsm kg(-1) and 719 mOsm kg(-1). Utilizing this unique feature, an fed-batch culture process under preferential osmolality was constructed through intermittently feeding broths with feed medium consisting of 14.0 g l(-1) maltose, 6.0 g l(-1) glucose and 9.0 g l(-1) soybean meal, at 48 h, 72 h, 96 h and 120 h. This intermittent fed-batch culture produced a peak acarbose titer of 4878 mg l(-1), increased by 15.9% over the batch culture.

Enzymatic synthesis of dTDP-4-amino-4,6-dideoxy-D-glucose using GerB (dTDP-4-keto-6-deoxy-D-glucose aminotransferase)

Over-expressed GerB (dTDP-4-keto-6-deoxy-d-glucose aminotransferase) of Streptomyces sp. GERI-155 was used in the enzymatic synthesis of dTDP-4-amino-4,6-dideoxy-D-glucose (2) from dTDP-4-keto-6-deoxy-D-glucose (1). [Carbohydrate structure: see text]. Five enzymes including dTMP kinase (TMK), acetate kinase (ACK), dTDP-glucose synthase (TGS), dTDP-glucose 4,6-dehydratase (DH), and dTDP-4-keto-6-deoxy-d-glucose aminotransferase (GerB) were used to synthesize 2 on a large scale from glucose-1-phosphate and TMP. A conversion yield of up to 57% was obtained by HPLC peak integration given a reaction time of 270min. After purification by two successive preparative HPLC systems, the final product was identified by HPLC and then analyzed by (1)H, (13)C, (1)H-(1)H COSY NMR spectrometry.

A retro-evolution study of CDP-6-deoxy-D-glycero-L-threo-4-hexulose-3-dehydrase (E1) from Yersinia pseudotuberculosis: implications for C-3 deoxygenation in the biosynthesis of 3,6-dideoxyhexoses

CDP-6-deoxy-l-threo-d-glycero-4-hexulose-3-dehydrase (E1), which catalyzes C-3 deoxygenation of CDP-4-keto-6-deoxyglucose in the biosynthesis of 3,6-dideoxyhexoses, shares a modest sequence identity with other B6-dependent enzymes, albeit with two important distinctions. It is a rare example of a B6-dependent enzyme that harbors a [2Fe-2S] cluster, and a highly conserved lysine that serves as an anchor for PLP in most B6-dependent enzymes is replaced by histidine at position 220 in E1. Since alteration of His220 to a lysine residue may produce a putative progenitor of E1, the H220K mutant was constructed and tested for the ability to process the predicted substrate, CDP-4-amino-4,6-dideoxyglucose, using PLP as the coenzyme. Our data showed that H220K-E1 has no dehydrase activity, but can act as a PLP-dependent transaminase. However, the reaction is not catalytic since PLP cannot be regenerated during turnover. Reported herein are the results of this investigation and the implications for the role of His220 in the catalytic mechanism of E1.

Synthesis of globo- and isoglobotriosides bearing a cinnamoylphenyl tag as novel electrophilic thiol-specific carbohydrate reagents

The galactosyl donor, 4,6-di-O-acetyl-2,3-di-O-benzyl-D-galactopyranosyl trichloroacetimidate, was efficiently coupled with regioselectively benzylated lactoside acceptors under standard conditions to stereoselectively afford the corresponding globotrioside and isoglobotrioside derivatives in very good yields. These glycosides were smoothly functionalized with a 6-(p-cinnamoylphenoxy)-hexyl tether tag as novel electrophilic thiol-specific carbohydrate reagents. Immobilization of the globotrioside conjugate to Thiopropyl Sepharose 6B for purification of B-subunit of Shiga toxin (StxB) and coupling of a model cysteine-containing protein (StxB-Z(n)-Cys) to the isoglobotrioside conjugate were both performed with high efficiency.

A new synthesis of the oligosaccharide domain of acarbose

Synthesis of the oligosaccharide domain of acarbose was reinvestigated and was optimally performed using a maltosidic acceptor, already bearing a alpha-D-Glc-(1-->4)-D-Glc bond, and a new D-fucopyranosyl donor. The crucial glycosylation step was improved by varying three different parameters and notably by focusing on the C-4 protecting group of the fucosyl residue, solvent and promoter. The resulting trisaccharide was further transformed into an electrophilic species in order to open further derivatization perspectives for designing new acarbose analogues. Substitution reactions were efficiently carried out with azide and thiocyanate anions. Two other potentially interesting trisaccharidic compounds were also synthesized, i.e. the C-4III amine and the corresponding isothiocyanate.

The enzymology of combinatorial biosynthesis

Combinatorial biosynthesis involves the genetic manipulation of natural product biosynthetic enzymes to produce potential new drug candidates that would otherwise be difficult to obtain. In either a theoretical or practical sense, the number of combinations possible from different types of natural product pathways ranges widely. Enzymes that have been the most amenable to this technology synthesize the polyketides, nonribosomal peptides, and hybrids of the two. The number of polyketide or peptide natural products theoretically possible is huge, but considerable work remains before these large numbers can be realized. Nevertheless, many analogs have been created by this technology, providing useful structure-activity relationship data and leading to a few compounds that may reach the clinic in the next few years. In this review the focus is on recent advances in our understanding of how different enzymes for natural product biosynthesis can be used successfully in this technology.

Biosynthetic studies on the α-glucosidase inhibitor acarbose: the chemical synthesis of isotopically labeled 2-epi-5-epi-valiolone analogs

2-epi-5-epi-valiolone is a cyclization product of the C(7) sugar phosphate, sedoheptulose 7-phosphate, involved in the biosynthesis of the aminocyclitol moieties of acarbose, validamycin, and pyralomicin. As part of our investigation into the pathway from 2-epi-5-epi-valiolone to the valienamine moiety of acarbose, we prepared 1-epi-5-epi-(6-(2)H(2))valiolol [(6-(2)H(2))-6], 5-epi-(6-(2)H(2))valiolol [(6-(2)H(2))-17], 1-epi-2-epi-5-epi-(6-(2)H(2))valiolol [(6-(2)H(2))-12] and 2-epi-5-epi-(6-(2)H(2))valiolamine [(6-(2)H(2))-11]. Compounds (6-(2)H(2))-6 and (6-(2)H(2))-17 were synthesized from 2,3,4,6-tetra-O-benzyl-D-glucopyranose in 10 and seven steps, respectively, whereas (6-(2)H(2))-12 and (6-(2)H(2))-11 were synthesized from 2,3,4,6-tetra-O-benzyl-D-mannopyranose in eight and 10 steps, respectively.