Fructosyl Amino Acids Research Articles

Molecular two-point recognition of fructosyl valine and fructosyl glycyl histidine in water by fluorescent Zn(II)-terpyridine complexes bearing boronic acids.

Selective recognition of fructosyl amino acids in water by arylboronic acid-based receptors is a central field of modern supramolecular chemistry that impacts biological and medicinal chemistry. Fructosyl valine (FV) and fructosyl glycyl histidine (FGH) occur as N-terminal moieties of human glycated hemoglobin; therefore, the molecular design of biomimetic receptors is an attractive, but very challenging goal. Herein, we report three novel cationic Zn-terpyridine complexes bearing a fluorescent N-quinolinium nucleus covalently linked to three different isomers of strongly acidified phenylboronic acids (ortho-, 2Zn; meta-, 3Zn and para-, 4Zn) for the optical recognition of FV, FGH and comparative analytes (D-fructose, Gly, Val and His) in pure water at physiological pH. The complexes were designed to act as fluorescent receptors using a cooperative action of boric acid and a metal chelate. Complex 3Zn was found to display the most acidic -B(OH)2 group (pKa = 6.98) and exceptionally tight affinity for FV (K = 1.43 × 105 M-1) with a strong quenching analytical response in the micromolar concentration range. The addition of fructose and the other amino acids only induced moderate optical changes. On the basis of several spectroscopic tools (1H, 11B NMR, UV-Vis, and fluorescence titrations), ESI mass spectrometry, X-ray crystal structure, and DFT calculations, the interaction mode between 3Zn and FV is proposed in a 1 : 1 model through a cooperative two-point recognition involving a sp3 boronate-diol esterification with simultaneous coordination bonding of the carboxylate group of Val to the Zn atom. Fluorescence quenching is attributed to a static complexation photoinduced electron transfer mechanism as evidenced by lifetime experiments. The addition of FGH to 3Zn notably enhanced its emission intensity with micromolar affinity, but with a lower apparent binding constant than that observed for FV. FGH interacts with 3Zn through boronate-diol complexation and coordination of the imidazole ring of His. DFT-optimized structures of complexes 3Zn-FV and 3Zn-FGH show a picture of binding which shows that the Zn-complex has a suitable (B⋯Zn) distance to the two-point recognition with these analytes. Molecular recognition of fructosyl amino acids by transition-metal-based receptors has not been explored until now.

Lac-L-TTA, a novel lactose-based amino acid-sugar conjugate for anti-metastatic applications.

Here we describe the synthesis, chromatographic purification, MS and NMR characterization of a new lactosyl-derivative, i.e. a lactosyl thiophenyl-substituted triazolyl-thione L-alanine (Lac-L-TTA). This amino acid-sugar conjugate was prepared by solution synthesis in analogy to the natural fructosyl-amino acids. Furthermore, we investigated the inhibition of PC-3 prostate cancer cell colony formation by this lactose derivative in comparison with the less polar fructose-based derivative, Fru-L-TTA. This let us to compare the properties of the artificial derivative, object of the present work, with the monosaccharide-based counterpart and to obtain a preliminary information on the influence of polarity on such biological activity. A significantly higher anticancer effect of Lac-L-TTA with respect to the fructose analogue emerged from our study suggesting that the anti-metastatic potential of fructosyl-amino acids can be enhanced by increasing the polarity of the compounds, for example by introducing disaccharide moieties in place of fructose.

Synthesis and biological evaluation of a novel Amadori compound.

Here, we report the synthesis, purification, ESI MS and NMR characterization, as well as the SEM analysis of a fructosyl thiophenyl-substituted triazolyl-thione L-alanine (denominated Fru-L-TTA). This novel fructosyl derivative was obtained by solution synthesis using the Amadori reaction, in analogy to other natural fructosyl-amino acids, and fully characterized. In particular, we report an accurate NMR/MS/SEM characterization of Fru-L-TTA alongside some biological properties, and investigated to compare the properties of the artificial derivative of this work with the natural counterparts. In particular, Fru-L-TTA shares with natural fructosyl-amino acids the possibility to inhibit the colony formation of prostate cancer cells and additionally decreases their migration.

Crystal structure of the deglycating enzyme Amadoriase I in its free form and substrate-bound complex.

Amadoriases, also known as fructosyl amine oxidases (FAOX), are enzymes that catalyze the de-glycosylation of fructosyl amino acids. As such, they are excellent candidates for the development of enzyme-based diagnostic and therapeutic tools against age- and diabetes-induced protein glycation. However, mostly because of the lack of a complete structural characterization of the different members of the family, the molecular bases of their substrate specificity have yet to be fully understood. The high resolution crystal structures of the free and the substrate-bound form of Amadoriase I shown herein allow for the identification of key structural features that account for the diverse substrate specificity shown by this class of enzymes. This is of particular importance in the context of the rather limited and partially incomplete structural information that has so far been available in the literature on the members of the FAOX family. Moreover, using molecular dynamics simulations, we describe the tunnel conformation and the free energy profile experienced by the ligand in going from bulk water to the catalytic cavity, showing the presence of four gating helices/loops, followed by an "L-shaped" narrow cavity. In summary, the tridimensional architecture of Amadoriase I presented herein provides a reference structural framework for the design of novel enzymes for diabetes monitoring and protein deglycation. Proteins 2016; 84:744-758. © 2016 Wiley Periodicals, Inc.

Synthesis and inhibitory activity of substrate-analog fructosyl peptide oxidase inhibitors

Fructosyl peptide oxidases (FPOXs) play a crucial role in the diagnosis of diabetes. Their main function is to cleave fructosyl amino acids or fructosyl peptides into glucosone and the corresponding amino acids/dipeptides. In this study, the substrate-analog FPOX inhibitors 1a-c were successfully designed and synthesized. These inhibitors mimic N(α)-fructosyl-L-valine (Fru-Val), [N(α)-fructosyl-L-valyl]-L-histidine (Fru-ValHis), and N(ε)-fructosyl-L-lysine (εFru-Lys), respectively. The secondary nitrogen atom in the natural substrates, linking fructose and amino acid or dipeptide moieties, was substituted in 1a-c with a sulfur atom to avoid enzymatic cleavage. Kinetic studies revealed that 1a-c act as competitive inhibitors against an FPOX obtained from Coniochaeta sp., and Ki values of 11.1, 66.8, and 782 μM were obtained for 1a-c, respectively.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of EtFPOX fromEupenicillium terrenumsp.

The flavoenzyme fructosyl peptide oxidase (FPOX) catalyses the oxidative deglycation of fructosyl amino acids or fructosyl dipeptides to produce amino acids, glucosone and hydrogen peroxide. In this study, FPOX protein from Eupenicillium terrenum sp. (EtFPOX) was expressed in Escherichia coli and purified by Ni-affinity and gel-filtration chromatography. EtFPOX crystals were obtained using the sitting-drop vapour-diffusion method with polyethylene glycol 3350 as precipitant. X-ray diffraction data were collected to 1.90 Å resolution using a synchrotron-radiation source. The crystals belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 65.6, b = 80.0, c = 83.4 Å, and contained one molecule in the asymmetric unit. The calculated Matthews coefficient and solvent content were 2.22 Å(3) Da(-1) and 44.62%, respectively.

Cloning and Characterization of Fructosamine-6-Kinase from Arthrobacter aurescens

Fructosamine-6-kinases (FN6Ks) that catalyze phosphorylation of glycated amino acids, i.e., fructosyl amino acids (FAs), have been shown as a potential recognition element for glycated protein detection. However, there are only two available FN6Ks: those from Escherichia coli which is specific for ε-fructosyl lysine (ε-FK) and Bacillus subtilis which recognizes both ε-FK and α-FA as substrates. In this study, we characterized an FN6K homologue isolated from Arthrobacter, some of whose species are reported to assimilate FA. The BLAST searches of Arthrobacter genomic database, using the bacterial FN6K primary structure information, revealed the presence of an FN6K homologue in Arthrobacter aurescens TC1 strain. Indeed, enzymatic assays confirmed that the putative FN6K from A. aurescens is an FN6K that is specific for ε-FK, although the primary sequence alignments showed similarity of A. aurescens FN6Ks with FN6Ks from B. subtilis and E. coli at the same level. In this study, we describe for the first time the presence of FN6K in Arthrobacter spp. and ε-FK-specific degradation pathway from Gram-positive bacteria, providing important information for the development of FA-recognizing molecules as well as for the FA assimilation system in bacteria.

Identification and functional analysis of fructosyl amino acid-binding protein from Gram-positive bacteriumArthrobactersp.

Fructosyl amino acid-binding protein (FABP) is a substrate-binding protein (SBP), which recognizes fructosyl amino acids (FAs) as its ligands. Although FABP has been shown as a molecular recognition tool of biosensing for glycated proteins, the availability of FABP is still limited and no FABP was reported from Gram-positive bacteria. In this study, a novel FABP from Gram-positive bacteria, Arthrobacter spp., was reported. BLAST analysis revealed that FABP homologues exist in some of Arthrobacter species genomes. An FABP homologue cloned from Arthrobacter sp. FV1-1, FvcA, contained a putative lipoprotein signal sequence, suggesting that it is a lipoprotein anchored to the bacterial cytoplasmic membrane, which is a typical characteristic for SBPs from Gram-positive bacteria. In contrast, FvcA also exhibits high amino acid sequence similarity to a known Gram-negative bacterial FABP, which exists as a free periplasmic protein. FvcA, without the N-terminal anchoring region, was then recombinantly produced as soluble protein and was found to exhibit Nα-FA-specific binding activity by intrinsic fluorescent measurement. This study identified a novel FABP from a Gram-positive bacterium, Arthrobacter sp., which exhibited Nα-FA-specific binding ability. This is the first report concerning an FABP from a Gram-positive bacterium, suggesting that FABP-dependent FA catabolism system is also present in Gram-positive bacteria. The novel FABP exhibits the ability to specifically bind to Nα-FA with a high affinity. This selectivity is beneficial for applying FABP in HbA1c sensing. The successful preparation of water-soluble, functionally expressed Gram-negative bacterial FABP may make way for future applications for a variety of SBPs from Gram-positive bacteria employing the same expression strategy. The results obtained here enhance our understanding of bacterial FA catabolism and contribute to the improved development of FABP as Nα-FA-sensing molecules.

Loop engineering of amadoriase II and mutational cooperativity

Amadori compounds and their cross-linked products have been implicated in diabetic complications and some age-related diseases. Fructosyl amine oxidases (FAOXs) are a family of enzymes that can cleave the amadori compounds. However, the natural enzymes are only active on small substrates (fructosyl amino acids or dipeptides), which limits the therapeutic and diagnostic applications of these enzymes. In this study, amadoriase II, a member of the FAOX family from Aspergillus fumigatus was engineered to broaden its substrate range using a modified combinatorial active site saturation testing approach. The two loops at the entrance of the substrate channel were targeted. Saturation mutagenesis was carried out to search for hot-spot sites, followed by pairwise mutagenesis and subsequent combination of active mutations. Five sites on the loops were found to be critical for accessibility for two model bulky substrates, fructosyl adamantanamine and fructosyl-polylysine (3-13 lysines). Two best mutants (with three and five mutations, respectively) were obtained, with a specific activity toward the model substrates 20.6-fold and 16.8-fold that of the wild-type, respectively. Deconvolution experiments revealed the cooperativity of the mutations.

Tuning Fructosyl Peptidyl Oxidase into Dehydrogenase and Its Application for the Construction of an Enzyme Electrode

Fructosyl valyl-histidine, which is released from protease digestion of the major diabetes indicator HbA1c, is recognized as a target molecule for HbA1c sensors. The flavoenzyme fructosyl amino acid oxidase (FAOD) is an enzyme catalyzing the oxidative deglycation of fructosyl amino acids, with those also possessing high activity towards fructosyl dipeptides being referred to as fructosyl peptidyl oxidases (FPOXs). The high oxygen reactivity of FAODs and FPOXs limits their potential utility in amperometric enzyme sensors employing artificial electron mediators. We successfully modified the proton relay system of Pichia N1-1 FAOD, significantly decreasing its catalytic ability to employ oxygen as the electron acceptor, while having little effect on the dye-mediated dehydrogenase activity employing artificial electron acceptors instead of oxygen. We expect that similar engineering of a recently discovered FPOX will produce an enzyme that is ideally suited for HbA1c sensing.

Engineering of dye-mediated dehydrogenase property of fructosyl amino acid oxidases by site-directed mutagenesis studies of its putative proton relay system

The flavoenzyme fructosyl amino acid oxidase (FAOD) catalyzes the oxidative deglycation of fructosyl amino acids, model compounds of glycated proteins. The high oxygen reactivity of FAODs limits their potential utility in amperometric enzyme sensors employing artificial electron mediators. To alter their electron acceptor availability, site-directed mutagenesis was carried out on conserved residues predicted to be involved in the proton relay system (PRS) of two eukaryotic FAODs, the FAOD from the marine yeast Pichia sp. N1-1 and amadoriase II from the fungus Aspergillus fumigatus. The substitution of a single conserved Asn residue in the putative PRS, Asn47Ala of N1-1 FAOD and Asn52Ala of amadoriase II, resulted in significant loss in the catalytic ability to employ O(2) as the electron acceptor, while having little effect on the dye-mediated dehydrogenase activity employing artificial electron acceptors instead of O(2).

Engineered amadoriase II exhibiting expanded substrate range

Amadori compounds are ubiquitous in vivo as well as in food and have been implicated in diabetic complications and aging. In recent years, fructosyl amine oxidases (FAOXs) which cleave Amadori products are gaining increasing attention. Until now, however, all FAOXs can only react with small glycated substrates (such as fructosyl amino acids or dipeptides), which has hindered the applications of this new class of enzymes in diagnosis, therapeutics, and detergents. In this study, Aspergillus fumigatus amadoriase II was engineered with the aim to expand its substrate range, using a heat-inducible autolytic vector and fructosyl-polylysine (3-13 lysines) as an intermediate-sized model substrate. After two rounds of directed evolution, a mutant (SII-82) was obtained that showed an 8.78-fold increase in the activity toward fructosyl-polylysine and which also performed several fold better than the wild-type on real gravy stains at concentrations of 10-100 microg/ml (parts per million). Mutational analyses revealed useful clues for altering the substrate-binding pocket. This study suggests that it is possible to manipulate fructosyl amine oxidases to accommodate larger substrates, and that mutant SII-82 might serve as a template for further engineering.

Alteration of substrate specificity of fructosyl-amino acid oxidase from Fusarium oxysporum

Fructosyl-amino acid oxidase (FOD-F) from Fusarium oxysporum f. sp. raphani (NBRC 9972) is the enzyme catalyzing the oxidative deglycation of fructosyl-amino acids such as N(epsilon)-fructosyl N(alpha)-benzyloxycarbonyl-lysine (FZK) and fructosyl valine (FV), which are model compounds of the glycated proteins in blood. Wild-type FOD-F has high activities toward both substrates. We obtained a mutant FOD-F, which reacts with FZK but not with FV by random mutagenesis. One amino-acid substitution (K373R) occurred in the mutant FOD-F. In addition to K373R, K373W, K373M, K373T, and K373V, which were selected for optimization of the substitution at position K373, were purified and characterized. Kinetic analysis showed that the catalytic turnover for FV greatly decreased, whereas that for FZK did not. In consequence, the specificities toward FZK were increased in the mutant FOD-Fs. The relation between the substrate specificity of the mutant FOD-Fs and the position of the carboxyl group of the substrates was demonstrated using a series of the substrates having the carboxyl group at the different position. The mutant FOD-Fs are attractive candidates for developing an enzymatic measurement method for glycated proteins such as glycated albumin in serum. This study will be helpful to establish an easier and rapid clinical assay system of glycated albumin.

Novel fluorescent sensing system for α-fructosyl amino acids based on engineered fructosyl amino acid binding protein

A novel fluorescent sensing system for α-glycated amino acids was created based on fructosyl amino acid binding protein (FABP) from Agrobacterium tumefaciens. The protein was found to bind specifically to the α-glycated amino acids fructosyl glutamine (Fru-Gln) and fructosyl valine (Fru-Val) while not binding to ɛ-fructosyl lysine. An Ile166Cys mutant of FABP was created by genetic engineering and modified with the environmentally sensitive fluorophore acrylodan. The acrylodan-conjugated mutant FABP showed eight-fold greater sensitivity to Fru-Val than the unconjugated protein and could detect concentrations as low as 17 nM, making it over 100-fold more sensitive than enzyme-based detection systems. Its high sensitivity and specificity for α-substituted fructosyl amino acids makes the new sensing system ideally suited for the measurement of hemoglobin A1c (HbA1c), a major marker of diabetes.

Determination of Fructosyl Amino Acids and Fructosyl Peptides in Protease-digested Blood Sample by a Flow-Injection System with an Enzyme Reactor

A flow-injection system with an enzyme reactor was proposed for the measurement of fructosyl amino acids and fructosyl peptides in protease-digested blood samples. A fructosyl-amino acid oxidase (FAOX-TE) and two fructosyl-peptide oxidases (FPOX-CE and FPOX-CET) were covalently immobilized onto an inert support. They were used as the enzyme reactor in a FIA system with a hydrogen peroxide electrode. In particular, the FPOX-CET reactor possessed high selectivity for the detection of fructosyl valine (FV) and fructosyl valyl histidine (FVH) and an excellent operational stability. The proposed FIA system with the FPOX-CET reactor responded linearly to the concentration of FV over the dynamic range of 7.8 x 10(-6) to 5.8 x 10(-4) M. The present method could be successfully applied to the assay of FV and FVH in the protease-digested blood samples.

Functional analysis of fructosyl-amino acid oxidases of Aspergillus oryzae.

Three active fractions of fructosyl-amino acid oxidase (FAOD-Ao1, -Ao2a, and -Ao2b) were isolated from Aspergillus oryzae strain RIB40. N-terminal and internal amino acid sequences of FAOD-Ao2a corresponded to those of FAOD-Ao2b, suggesting that these two isozymes were derived from the same protein. FAOD-Ao1 and -Ao2 were different in substrate specificity and subunit assembly; FAOD-Ao2 was active toward N(epsilon)-fructosyl N(alpha)-Z-lysine and fructosyl valine (Fru-Val), whereas FAOD-Ao1 was not active toward Fru-Val. The genes encoding the FAOD isozymes (i.e., FAOAo1 and FAOAo2) were cloned by PCR with an FAOD-specific primer set. The deduced amino acid sequences revealed that FAOD-Ao1 was 50% identical to FAOD-Ao2, and each isozyme had a peroxisome-targeting signal-1, indicating their localization in peroxisomes. The genes was expressed in Escherichia coli and rFaoAo2 showed the same characteristics as FAOD-Ao2, whereas rFaoAo1 was not active. FAOAo2 disruptant was obtained by using ptrA as a selective marker. Wild-type strain grew on the medium containing Fru-Val as the sole carbon and nitrogen sources, but strain Delta faoAo2 did not grow. Addition of glucose or (NH(4))(2)SO(4) to the Fru-Val medium did not affect the assimilation of Fru-Val by wild-type, indicating glucose and ammonium repressions did not occur in the expression of the FAOAo2 gene. Furthermore, conidia of the wild-type strain did not germinate on the medium containing Fru-Val and NaNO(2) as the sole carbon and nitrogen sources, respectively, suggesting that Fru-Val may also repress gene expression of nitrite reductase. These results indicated that FAOD is needed for utilization of fructosyl-amino acids as nitrogen sources in A. oryzae.

Kinetic studies, mechanism and substrate specificity of amadoriase I from Aspergillus sp.

Amadoriase is a flavoenzyme that catalyzes the oxidative deglycation of Amadori products (fructosyl amino acids or aliphatic amines) to yield free amine, glucosone, and hydrogen peroxide. The mechanism of action of amadoriase I from Aspergillus sp. has been investigated by stopped-flow kinetic studies using fructosyl propylamine and O2 as substrates in 10 mM Tris HCl, pH 7.9, 4 °C. Using both substrate analogues and fast kinetic techniques, the active configuration of the substrate was found to be the β-pyranose form. Stopped-flow studies showed that the reductive half-reaction is triphasic and generates intermediates that absorb at long wavelengths and is consistent either with (i) the reaction of the substrate with the flavin followed by iminium deprotonation or hydrolysis and then product release or with (ii) the formation of flavin reduction intermediates (carbanion equivalents or adducts), followed by product release. The rate of product release after flavin reduction is lower than the aerobic turnov...

Kinetic Studies, Mechanism, and Substrate Specificity of Amadoriase I from Aspergillus sp.

Amadoriase is a flavoenzyme that catalyzes the oxidative deglycation of Amadori products (fructosyl amino acids or aliphatic amines) to yield free amine, glucosone, and hydrogen peroxide. The mechanism of action of amadoriase I from Aspergillus sp. has been investigated by stopped-flow kinetic studies using fructosyl propylamine and O(2) as substrates in 10 mM Tris HCl, pH 7.9, 4 degrees C. Using both substrate analogues and fast kinetic techniques, the active configuration of the substrate was found to be the beta-pyranose form. Stopped-flow studies showed that the reductive half-reaction is triphasic and generates intermediates that absorb at long wavelengths and is consistent either with (i) the reaction of the substrate with the flavin followed by iminium deprotonation or hydrolysis and then product release or with (ii) the formation of flavin reduction intermediates (carbanion equivalents or adducts), followed by product release. The rate of product release after flavin reduction is lower than the aerobic turnover rate, 14.4 s(-1), suggesting that it is not involved in the catalytic cycle and that reoxidation of the reduced enzyme occurs in the E(red)-product complex. In the oxidative half-reaction, the reduced flavin is oxidized by O(2) in a single phase. The observed rate constant has a linear dependence on oxygen concentration, giving a bimolecular rate constant of 4.9 x 10(4) M(-1) s(-1) in the absence of product, and 3.6 x 10(4) M(-1) s(-1) when the product is bound. The redox potentials of amadoriase have been measured at pH 7.0, 25 degrees, giving values of +48 and -52 mV for the oxidized enzyme/anionic semiquinone and anionic semiquinone/reduced enzyme couples, respectively.

Subcellular localization of fructosyl amino acid oxidases in peroxisomes of Aspergillus terreus and Penicillium janthinellum

Fructosyl amino acid oxidase (FAOD) is the enzyme catalyzing the oxidative deglycation of Amadori compounds, such as fructosyl amino acids, yielding the corresponding amino acids, glucosone, and H 2O 2. In a previous report, we determined the primary structures of cDNAs coding for FAODs from two fungal strains Aspergillus terreus AP1 and Penicillium janthinellum and we found that both fungal FAODs included the putative peroxisome targeting signal 1 (PTS1) at the carboxyl terminal (Yoshida, N. et al., Eur. J. Biochem., 242, 499–505, 1996). In this study, we determined the intracellular localization of FAODs in these two fungi. Subcellular fractionation experiments and immuno-electronmicroscopic observations, together with the previous findings indicated that the FAODs were localized in peroxisomes of A. terreus AP1 and P. janthinellum. These FAODs were also found to belong to a new member of “peroxisomal sarcosine oxidase family protein” in eucaryotic cells.