Dihydroxyacetone Phosphate Aldolase Catalyzed Synthesis of Structurally Diverse Polyhydroxylated Pyrrolidine Derivatives and Evaluation of their Glycosidase Inhibitory Properties

Synthesis, α-mannosidase inhibition studies and molecular modeling of 1,4-imino-ᴅ-lyxitols and their C-5-altered N-arylalkyl derivatives

A synthesis of 1,4-imino-ᴅ-lyxitols and their N-arylalkyl derivatives altered at C-5 is reported. Their inhibitory activity and selectivity toward four GH38 α-mannosidases (two Golgi types: GMIIb from Drosophila melanogaster and AMAN-2 from Caenorhabditis elegans, and two lysosomal types: LManII from Drosophila melanogaster and JBMan from Canavalia ensiformis) were investigated. 6-Deoxy-DIM was found to be the most potent inhibitor of AMAN-2 (K _i = 0.19 μM), whose amino acid sequence and 3D structure of the active site are almost identical to the human α-mannosidase II (GMII). Although 6-deoxy-DIM was 3.5 times more potent toward AMAN-2 than DIM, their selectivity profiles were almost the same. N-Arylalkylation of 6-deoxy-DIM resulted only in a partial improvement as the selectivity was enhanced at the expense of potency. Structural and physicochemical properties of the corresponding inhibitor:enzyme complexes were analyzed by molecular modeling.

Synthesis and Glycosidase Inhibition of Broussonetine M and Its Analogues

Cross-metathesis (CM) and Keck asymmetric allylation, which allows access to defined stereochemistry of a remote side chain hydroxyl group, are the key steps in a versatile synthesis of broussonetine M (3) from the d-arabinose-derived cyclic nitrone 14. By a similar strategy, ent-broussonetine M (ent-3) and six other stereoisomers have been synthesized, respectively, starting from l-arabino-nitrone (ent-14), l-lyxo-nitrone (ent-3-epi-14), and l-xylo-nitrone (2-epi-14) in five steps, in 26%–31% overall yield. The natural product broussonetine M (3) and 10’-epi-3 were potent inhibitors of β-glucosidase (IC₅₀ = 6.3 μM and 0.8 μM, respectively) and β-galactosidase (IC₅₀ = 2.3 μM and 0.2 μM, respectively); while their enantiomers, ent-3 and ent-10’-epi-3, were selective and potent inhibitors of rice α-glucosidase (IC₅₀ = 1.2 μM and 1.3 μM, respectively) and rat intestinal maltase (IC₅₀ = 0.29 μM and 18 μM, respectively). Both the configuration of the polyhydroxylated pyrrolidine ring and C-10’ hydroxyl on the alkyl side chain affect the specificity and potency of glycosidase inhibition.

Aldolase-Catalyzed Asymmetric Synthesis of N-Heterocycles by Addition of Simple Aliphatic Nucleophiles to Aminoaldehydes

Nitrogen heterocycles are structural motifs found in many bioactive natural products and of utmost importance in pharmaceutical drug development. In this work, a stereoselective synthesis of functionalized N-heterocycles was accomplished in two steps, comprising the biocatalytic aldol addition of ethanal and simple aliphatic ketones such as propanone, butanone, 3-pentanone, cyclobutanone, and cyclopentanone to N-Cbz-protected aminoaldehydes using engineered variants of d-fructose-6-phosphate aldolase from Escherichia coli (FSA) or 2-deoxy-d-ribose-5-phosphate aldolase from Thermotoga maritima (DERA Tma ) as catalysts. FSA catalyzed most of the additions of ketones while DERA Tma was restricted to ethanal and propanone. Subsequent treatment with hydrogen in the presence of palladium over charcoal, yielded low-level oxygenated N-heterocyclic derivatives of piperidine, pyrrolidine and N-bicyclic structures bearing fused cyclobutane and cyclopentane rings, with stereoselectivities of 96-98 ee and 97:3 dr in isolated yields ranging from 35 to 79%.

The convergent synthesis and anticancer activity of broussonetinines related analogues

The convergent synthesis of broussonetinines related congeners 3 and 4 with the simple C₁₃ alkyl side chain and differently configured pyrrolidine skeleton has been achieved. Our approach relied on the [3,3]-sigmatropic rearrangements of chiral allylic substrates derived from d-xylose. Cross metathesis of the common oxazolidinone intermediates 7 and 8 with tridec-1-ene followed by alkylative cyclization completed the construction of both C-alkyl iminosugars. The targeted compounds 3 and 4 were screened for antiproliferative/cytotoxic activities against multiple cancer cell lines by MTT assay. Compound 3 exhibited very good in vitro potency on Caco-2 and Jurkat cell lines with IC₅₀ value of 5.1 μM and 5.8 μM, respectively.

Recent advances in the synthesis of rare sugars using DHAP-dependent aldolases

The occurrence rates of non-communicable diseases like obesity, diabetes and hyperlipidemia have increased remarkably due to excessive consumption of a high-energy diet. Rare sugars therefore have become increasingly attractive owing to their unique nutritional properties. In the past two decades, various rare sugars have been successfully prepared guided by the "Izumoring strategy". As a valuable complement to the Izumoring approach, the controllable dihydroxyacetone phosphate (DHAP)-dependent aldolases have generally predictable regio- and stereoselectivity, which makes them powerful tools in C-C bond construction and rare sugar production. However, the main disadvantage for this group of aldolases is their strict substrate specificity toward the donor molecule DHAP, a very expensive and relatively unstable compound. Among the current methods involving DHAP, the one that couples DHAP production from inexpensive starting materials (for instance, glycerol, DL-glycerol 3-phosphate, dihydroxyacetone, and glucose) with aldol condensation appears to be the most promising. This review thus focuses on recent advances in the application of L-rhamnulose-1-phosphate aldolase (RhaD), L-fuculose-1-phosphate aldolase (FucA), and D-fructose-1,6-bisphosphate aldolase (FruA) for rare sugar synthesis in vitro and in vivo, while illustrating strategies for supplying DHAP in efficient and economical ways.

γ-Aminoalcohol rearrangement applied to pentahydroxylated azepanes provides pyrrolidines epimeric to homoDMDP

A series of pentahydroxylated pyrrolidines, displaying five contiguous stereogenic centres and epimeric to α-glucosidase inhibitor homoDMDP, have been synthesized. The key step involves a γ-aminoalcohol rearrangement applied to polyhydroxylated azepanes. These five-membered iminosugars demonstrate micromolar inhibition of glycosidases.

The role of biocatalysis in the asymmetric synthesis of alkaloids

Alkaloids are not only one of the most intensively studied classes of natural products, their wide spectrum of pharmacological activities also makes them indispensable drug ingredients in both traditional and modern medicine. Among the methods for their production, biotechnological approaches are gaining importance, and biocatalysis has emerged as an essential tool in this context. A number of chemo-enzymatic strategies for alkaloid synthesis have been developed over the years, in which the biotransformations nowadays take an increasingly 'central' role. This review summarises different applications of biocatalysis in the asymmetric synthesis of alkaloids and discusses how recent developments and novel enzymes render innovative and efficient chemo-enzymatic production routes possible.

From amino alcohol to aminopolyol: one-pot multienzyme oxidation and aldol addition

In this work, the successful coupling of enzymatic oxidation and aldol addition reactions for the synthesis of a Cbz-aminopolyol from a Cbz-amino alcohol was achieved for the first time in a multienzymatic one-pot system. The two-step cascade reaction consisted of the oxidation of Cbz-ethanolamine to Cbz-glycinal catalyzed by chloroperoxidase from the fungus Caldariomyces fumago and aldol addition of dihydroxyacetone phosphate to Cbz-glycinal catalyzed by rhamnulose-1-phosphate aldolase expressed as a recombinant enzyme in Escherichia coli, yielding (3R,4S)-5-{[(benzyloxy)carbonyl]amino}-5-deoxy-1-O-phosphonopent-2-ulose. Tools of enzymatic immobilization, reactor configurations, and modification of the reaction medium were applied to highly increase the production of the target compound. While the use of soluble enzymes yielded only 23.6 % of Cbz-aminopolyol due to rapid enzyme inactivation, the use of immobilized ones permitted an almost complete consumption of Cbz-ethanolamine, reaching Cbz-aminopolyol yields of 69.1 and 71.9 % in the stirred-tank and packed-bed reactor, respectively. Furthermore, the reaction production was 18-fold improved when it was catalyzed by immobilized enzymes in the presence of 5 % (v/v) dioxane, reaching a value of 86.6 mM of Cbz-aminopoliol (31 g/L).

Chemo-enzymatic synthesis and glycosidase inhibitory properties of DAB and LAB derivatives

A chemo-enzymatic strategy for the preparation of 2-aminomethyl derivatives of (2R,3R,4R)-2-(hydroxymethyl)pyrrolidine-3,4-diol (also called 1,4-dideoxy-1,4-imino-D-arabinitol, DAB) and its enantiomer LAB is presented. The synthesis is based on the enzymatic preparation of DAB and LAB followed by the chemical modification of their hydroxymethyl functionality to afford diverse 2-aminomethyl derivatives. This strategy leads to novel aromatic, aminoalcohol and 2-oxopiperazine DAB and LAB derivatives. The compounds were preliminarily explored as inhibitors of a panel of commercial glycosidases, rat intestinal disaccharidases and against Mycobacterium tuberculosis, the causative agent of tuberculosis. It was found that the inhibitory profile of the new products differed considerably from the parent DAB and LAB. Furthermore, some of them were active inhibiting the growth of M. tuberculosis.

Rational approaches for engineering novel functionalities in carbon-carbon bond forming enzymes

Enzymes that catalyze carbon-carbon bond formation can be exploited as biocatalyst for synthetic organic chemistry. However, natural enzymes frequently do not possess the required properties or specificities to catalyze industrially useful transformations. This mini-review describes recent work using knowledge-guided site-specific mutagenesis of key active site residues to alter substrate specificity, stereospecificity and reaction specificity of these enzymes. In addition, examples of de novo designed enzymes that catalyze C-C bond reactions not found in nature will be discussed.

Structure-guided minimalist redesign of the L-fuculose-1-phosphate aldolase active site: expedient synthesis of novel polyhydroxylated pyrrolizidines and their inhibitory properties against glycosidases and intestinal disaccharidases

A minimalist active site redesign of the L-fuculose-1-phosphate aldolase from E. coli FucA was envisaged, to extend its tolerance towards bulky and conformationally restricted N-Cbz-amino aldehyde acceptor substrates (Cbz=benzyloxycarbonyl). Various mutants at the active site of the FucA wild type were obtained and screened with seven sterically demanding N-Cbz-amino aldehydes including N-Cbz-prolinal derivatives. FucA F131A showed an aldol activity of 62 μmol h(-1) mg(-1) with (R)-N-Cbz-prolinal, whereas no detectable activity was observed with the FucA wild type. For the other substrates, the F131A mutant gave aldol activities from 4 to about 25 times higher than those observed with the FucA wild type. With regard to the stereochemistry of the reactions, the (R)-amino aldehydes gave exclusively the anti configured aldol adducts whereas their S counterparts gave variable ratios of anti/syn diastereoisomers. Interestingly, the F131A mutant was highly stereoselective both with (R)- and with (S)-N-Cbz-prolinal, exclusively producing the anti and syn aldol adducts, respectively. Molecular models suggest that this improved activity towards bulky and more rigid substrates, such as N-Cbz-prolinal, could arise from a better fit of the substrate into the hydrophobic pocket created by the F131A mutation, due to an additional π-cation interaction with the residue K205' and to efficient contact between the substrate and the mechanistically important Y113' and Y209' residues. An expedient synthesis of novel polyhydroxylated pyrrolizidines related to the hyacinthacine and alexine types was accomplished through aldol additions of dihydroxyacetone phosphate (DHAP) to hydroxyprolinal derivatives with the hyperactive FucA F131A as catalyst. The iminocyclitols obtained were fully characterised and found to be moderate to weak inhibitors (relative to 1,4-dideoxy-1,4-imino-L-arabinitol (LAB) and 1,4-dideoxy-1,4-imino-D-arabinitol (DAB)) against glycosidases and rat intestinal saccharidases.

The synthesis and biological evaluation of 1-C-alkyl-L-arabinoiminofuranoses, a novel class of α-glucosidase inhibitors

The asymmetric synthesis of 1-C-alkyl-l-arabinoiminofuranoses 1 was achieved by asymmetric allylic alkylation (AAA), ring closing metathesis (RCM), and Negishi cross coupling as key reactions. Some of the prepared compounds showed potent inhibitory activities towards intestinal maltase, with IC(50) values comparable to those of commercial drugs such as acarbose, voglibose, and miglitol, which are used in the treatment of type 2 diabetes. Among them, the inhibitory activity (IC(50)=0.032μM) towards intestinal sucrase of 1c was quite strong compared to the above commercial drugs.

Redesigning the active site of transaldolase TalB from Escherichia coli: new variants with improved affinity towards nonphosphorylated substrates

Recently, we reported on a transaldolase B variant (TalB F178Y) that is able to use dihydroxyacetone (DHA) as donor in aldol reactions. In a second round of protein engineering, we aimed at improving the affinity of this variant towards nonphosphorylated acceptor aldehydes, that is, glyceraldehyde (GA). The anion binding site was identified in the X-ray structure of TalB F178Y where a sulfate ion from the buffer was bound in the active site. Therefore, we performed site-directed saturation mutagenesis at three residues forming the putative phosphate binding site, Arg181, Ser226 and Arg228. The focused libraries were screened for the formation of D-fructose from DHA and d,l-GA by using an adjusted colour assay. The best results with respect to the synthesis of D-fructose were achieved with the TalB F178Y/R181E variant, which exhibited an at least fivefold increase in affinity towards d,l-GA (K(M)=24 mM). We demonstrated that this double mutant can use D-GA, glycolaldehyde and the L-isomer, L-GA, as acceptor substrates. This resulted in preparative synthesis of D-fructose, D-xylulose and L-sorbose when DHA was used as donor. Hence, we engineered a DHA-dependent aldolase that can synthesise the formation of polyhydroxylated compounds from simple and cheap substrates at preparative scale.