Effect of Extracellular Tyrosinase on the Expression Level of P450, Fpr, and Fdx and Ortho-hydroxylation of Daidzein in Streptomyces avermitilis.

We have reported that the expression of CYP105D7 in Streptomyces avermitilis produces 112.5 mg L-1 of 7,3',4'-trihydroxyisoflavone (3'ODI) in 15 h of the reaction time, when 7,4'-dihydroxyisoflavone (daidzein) is used as a substrate. Although production is significant, rapid degradation of 3'ODI after 15 h was observed in a whole-cell biotransformation system, suggesting the further modification of 3'ODI by endogenous enzymes. In this present study, the effect of deletion of extracellular tyrosinase (melC2) in S. avermitilis for 3'ODI production as well as the expressions of CYP105D7, ferredoxin (Fdx), and ferredoxin reductase (Fpr) were investigated. The result revealed that daidzein hydroxylation activity in the ∆melC2 mutant decreased by 40% compared with wild-type S. avermitilis. Further, melC2 deletion significantly affects the messenger RNA (mRNA) expression profile of CYP105D7 and its electron transfer counterparts. Real-time PCR analysis of 9 Fdx, 6 Fpr, and CYP105D7 revealed a significant decrease in mRNA expression level compared to wild-type S. avermitilis. The result clearly shows that the decrease in daidzein hydroxylation activity is due to the lower expression level of CYP105D7 and its electron transfer counterpart in the ∆melC2 mutant. Furthermore, melC2 deletion prevents the degradation of 3'ODI.

Heterologous expression of tyrosinase (MelC2) from Streptomyces avermitilis MA4680 in E. coli and its application for ortho-hydroxylation of resveratrol to produce piceatannol.

Recombinant tyrosinase from Streptomyces avermitilis MA4680, MelC2 (gi:499291317), was heterologously expressed in Escherichia coli BL21 (DE3). The expression level of active MelC2 was increased by the codon-optimized MelC1 caddie protein (KP198295.1). By performing saturation mutagenesis of the Y91 residue of MelC1, it was found that aromatic residues such as Y, F, and W at the 91st position help produce a correctly folded conformation of MelC2. The recombinant MelC2 was utilized as a biocatalyst to convert trans-resveratrol into piceatannol. In order to improve the product yield through suppression of the formation of melanin, a by-product, an increase in the ratio of monooxygenation (k 1) to dioxygenation (k 2) of MelC2 is desirable. This was achieved by a combination of protein engineering and regeneration of NADH with glucose dehydrogenase (GDH). Saturation mutagenesis was performed at 15 residues within 8-Å radius from copper ions of MelC2. A total of 2760 mutants were examined (99.7 % probability for NNK codon) and I41Y, a mutant, was screened. The ratio of k 1 to k 2 of the mutant increased sevenfold on tyrosine and fivefold on resveratrol, when compared to wild-type MelC2. As a result, the overall product yield from 500 µM resveratrol in 50-mL reaction was 15.4 % (77.4 µM piceatannol), 1.7 times higher than wild type. When I41Y was incorporated with the NADH regeneration system, the total product yield was 58.0 %, an eightfold increase (290.2 µM of piceatannol).

Identification of the specific electron transfer proteins, ferredoxin, and ferredoxin reductase, for CYP105D7 in Streptomyces avermitilis MA4680.

It was previously proposed that regiospecific hydroxylation of daidzein at 3'-position is mediated by cytochrome P450 hydroxylase (CYP105D7) in the presence of putidaredoxin (CamB) and putidaredoxin reductase (CamA) as electron transfer proteins from Pseudomonas putida. The genome sequence of Streptomyces avermitilis MA4680 revealed 33 P450 (CYPs) with 6 ferredoxin reductases (Fprs) and 9 ferredoxins (Fdxs) as their putative electron transfer partner proteins. To identify right endogenous electron transfer proteins for CYP105D7 activity, in vitro reconstitution, gene disruption, and quantitative reverse transcription polymerase chain reaction (qRT-PCR) mRNA expression profile analysis were examined. The most effective electron transfer proteins for CYP105D7 appear to be FdxH (SAV7470), which is located downstream to CYP105D7 as a cluster, and FprD (SAV5675). Throughout our overall analysis, we proposed that the primary electron transfer pathway for CYP105D7 follows as such NAD(P)H→FdxH→FprD→CYP105D7.

Protein engineering of α2,3/2,6-sialyltransferase to improve the yield and productivity of in vitro sialyllactose synthesis.

In the large-quantity production of α2,3- and α2,6-sialyllactose (Neu5Ac(α2,3)Galß1,4Glc (3'-SL) and Neu5Ac(α2,6)Galß1,4Glc (6'-SL)) using sialyltransferases (STs), there are major hurdles to overcome for further improvement in yield and productivity of the enzyme reactions. Specifically, Pasteurella multocida α2,3-sialyltransferase (α2,3PST) forms a by-product to a certain extent, owing to its multifunctional activity at pH below 7.0, and Photobacterium damselae α2,6-sialyltransferase (α2,6PdST) shows relatively low ST activity. In this study, α2,3PST and α2,6PdST were successfully engineered using a hybrid approach that combines rational design with site-saturation mutagenesis. Narrowly focused on the substrate-binding pocket of the STs, putative functional residues were selected by multiple sequence alignment and alanine scanning, and subsequently subjected to site-saturation mutagenesis. In the case of α2,3PST, R313N single mutation improved its activity slightly (by a factor of 1.5), and further improvement was obtained by making the double mutants (R313N/T265S and R313H/T265S) resulting in an overall 2-fold improvement in its specific α2,3 ST activity, which is mainly caused by the increase in kcat. It was revealed that the R313 mutations to N, D, Y, H or T greatly reduced the α2,6 ST side-reaction activity of α2,3PST at below pH 7.0. In the case of α2,6PdST, single-mutation L433S/T and double-mutation I411T/L433T exhibited 3- and 5-fold enhancement of the α2,6 ST-specific activity compared with the wild-type, respectively, via increase in kcat values. Our results show a very good model system for enhancing ST activity and demonstrate that the generated mutants could be used efficiently for the mass production of 3'-SL and 6'-SL with enhanced productivity and yield.

Integrating cell-free biosyntheses of heme prosthetic group and apoenzyme for the synthesis of functional P450 monooxygenase.

Harnessing the isolated protein synthesis machinery, cell-free protein synthesis reproduces the cellular process of decoding genetic information in artificially controlled environments. More often than not, however, generation of functional proteins requires more than simple translation of genetic sequences. For instance, many of the industrially important enzymes require non-protein prosthetic groups for biological activity. Herein, we report the complete cell-free biogenesis of a heme prosthetic group and its integration with concurrent apoenzyme synthesis for the production of functional P450 monooxygenase. Step reactions required for the syntheses of apoenzyme and the prosthetic group have been designed so that these two separate pathways take place in the same reaction mixture, being insulated from each other. Combined pathways for the synthesis of functional P450 monooxygenase were then further integrated with in situ assay reactions to enable real-time measurement of enzymatic activity during its synthesis.

Rational design of modular allosteric aptamer sensor for label-free protein detection.

An aptamer can be redesigned to new functional molecules by conjugating with other oligonucleotides. However, it requires experimental trials to optimize the conjugating module with the sensitivity and selectivity toward a target. To reduce these efforts, we report rationally-designed modular allosteric aptamer sensor (MAAS), which is composed of coupled two aptamers and the regulator. For label-free protein detection, the protein-aptamer was conjugated with the malachite green (MG) aptamer for signaling. The MAAS additionally has the regulator domain which is designed to hybridize to a protein binding domain. The regulator makes MAAS to be inactive by destructing the original structure of the two aptamers. However, its conformation becomes active by dissociating the hybridization from the protein recognition signal, thereby inducing the binding of MG emitting the enhanced fluorescence. The design of regulator is based on the thermodynamic energy difference by the RNA conformational change and protein-aptamer affinity. Here we first demonstrated the MAAS for hepatitis C helicase and replicase. The target proteins were detected up to 250nM with minimized blank signals and displayed high specificities 10-fold greater than in non-specific proteins. The MAAS provides valuable tools that can be adapted to a wide range of configurations in bioanalytical applications.

Regioselective hydroxylation of trans-resveratrol via inhibition of tyrosinase from Streptomyces avermitilis MA4680.

Secreted tyrosinase from melanin-forming Streptomyces avermitilis MA4680 was involved in both ortho-hydroxylation and further oxidation of trans-resveratrol, leading to the formation of melanin. This finding was confirmed by constructing deletion mutants of melC(2) and melD(2) encoding extracellular and intracellular tyrosinase, respectively; the melC2 deletion mutant did not produce piceatannol as well as melanin, whereas the melD2 deletion mutant oxidized resveratrol and synthesized melanin with the same yields, suggesting that MelC2 is responsible for ortho-hydroxylation of resveratrol. Extracellular tyrosinase (MelC2) efficiently converted trans-resveratrol into piceatannol in the presence of either tyrosinase inhibitors or reducing agents such as catechol, NADH, and ascorbic acid. Reducing agents slow down the dioxygenase reaction of tyrosinase. In the presence of catechol, the regio-specific hydroxylation of trans-resveratrol was successfully performed by whole cell biotransformation, and further oxidation of trans-resveratrol was efficiently blocked. The yield of this ortho-hydroxylation of trans-resveratrol was dependent upon inhibitor concentration. Using 1.8 mg of wild-type Streptomyces avermitilis cells, the conversion yield of 100 µM trans-resveratrol to piceatannol was 78% in 3 h in the presence of 1 mM catechol, indicating 14 µM piceatannol h(-1) DCW mg(-1) specific productivity, which was a 14-fold increase in conversion yield compared to that without catechol, which is a remarkably higher reaction rate than that of P450 bioconversion. This method could be generally applied to biocatalysis of various dioxygenases.

Novel iron-sulfur containing NADPH-reductase from Nocardia farcinica IFM10152 and fusion construction with CYP51 lanosterol demethylase.

CYP51, a sterol 14α-demethylase, is one of the key enzymes involved in sterol biosynthesis and requires electrons transferred from its redox partners. A unique CYP51 from Nocardia farcinica IFM10152 forms a distinct cluster with iron-sulfur containing NADPH-P450 reductase (FprD) downstream of CYP51. Previously, sequence alignment of nine reductases from N. farcinica revealed that FprC, FprD, and FprH have an additional sequence at their N-termini that has very high identity with iron-sulfur clustered ferredoxin G (FdxG). To construct an artificial self-sufficient cytochrome P450 monooxygenase (CYP) with only FprD, CYP51, and iron-sulfur containing FprD were fused together with designed linker sequences. CYP51-FprD fusion enzymes showed distinct spectral properties of both flavoprotein and CYP. CYP51-FprD F1 and F2 in recombinant Escherichia coli BL21(DE3) catalyzed demethylation of lanosterol more efficiently, with k(cat) /K(m) values of 96.91 and 105.79 nmol/min/nmol, respectively, which are about 35-fold higher compared to those of CYP51 and FprD alone.

Screening of bacterial cytochrome P450s responsible for regiospecific hydroxylation of (iso)flavonoids.

Screening of cytochrome P450 monoxygenases responsible for the regiospecific hydroxylation of flavones, isoflavones and chalcones was attempted using a P450 library constructed from Streptomyces avermitilis MA4680, Bacillus and Nocardia farcinica IFM10152 strains. As electron transfer redox partners with the P450s in Escherichia coli system, putidaredoxin reductase (PdR) and putidaredoxin (Pdx) from Pseudomonas putida were used. Among the 50 soluble P450s in the library screened, three cytochrome P450s, i.e. CYP107Y1, CYP125A2 and CYP107P2 from S. avermitilis MA4680 showed good hydroxylation activities towards flavones and isoflavones. However, low product yields prevented us from identifying complete structure of the products. By using S. avermitilis MA4680 as their expression host, further analysis identified that CYP107Y1(SAV2377), CYP125A2(SAV5841) and CYP107P2(SAV4539) showed good regiospecific hydroxylation activities towards genistein (4',5,7-trihydroxyisoflavone), chrysin (5,7-dihydroxyisoflavone) and apigenin (4',5,7-dihydroxyisoflavone) to produce 3',4',5,7,-tetrahydroxyisoflavone, B-ring hydroxylated 5,7-dihydroxyflavone and 3',4',5,7,-tetrahydroxyflavone, respectively. Analyses of the reaction products were performed using HPLC, ESI-MS-MS and GC-MS and 1H NMR.

Bioconjugation of L-3,4-dihydroxyphenylalanine containing protein with a polysaccharide.

We describe the simple bioconjugation strategy in combination of periodate chemistry and unnatural amino acid incorporation. The residue specific incorporation of 3,4-dihydroxy-l-phenylalanine can alter the properties of protein to conjugate into the polymers. The homogeneously modified protein will yield quinone residues that are covalently conjugated to nucleophilic groups of the amino polysaccharide. This novel approach holds great promise for widespread use to prepare protein conjugates and synthetic biology applications.

Regioselective hydroxylation of daidzein using P450 (CYP105D7) from Streptomyces avermitilis MA4680.

Regiospecific 3'-hydroxylation reaction of daidzein was performed with CYP105D7 from Streptomyces avermitilis MA4680 expressed in Escherichia coli. The apparent K(m) and k(cat) values of CYP105D7 for daidzein were 21.83 +/- 6.3 microM and 15.01 +/- 0.6 min(-1) in the presence of 1 microM of CYP105D7, putidaredoxin (CamB) and putidaredoxin reductase (CamA), respectively. When CYP105D7 was expressed in S. avermitilis MA4680, its cytochrome P450 activity was confirmed by the CO-difference spectra at 450 nm using the whole cell extract. When the whole-cell reaction for the 3'-hydroxylation reaction of daidzein was carried out with 100 microM of daidzein in 100 mM of phosphate buffer (pH 7.5), the recombinant S. avermitilis grown in R2YE media overexpressing CYP105D7 and ferredoxin FdxH (SAV7470) showed a 3.6-fold higher conversion yield (24%) than the corresponding wild type cell (6.7%). In a 7 L (working volume 3 L) jar fermentor, the recombinants S. avermitilis grown in R2YE media produced 112.5 mg of 7,3',4'-trihydroxyisoflavone (i.e., 29.5% conversion yield) from 381 mg of daidzein in 15 h.

A-ring ortho-specific monohydroxylation of daidzein by cytochrome P450s of Nocardia farcinica IFM10152.

The bioconversion of the isoflavonoid daidzein using whole cell Nocardia farcinica IFM10152 showed two kinds of major metabolic modifications, i.e. mono-hydroxylation and subsequent O-methylation. The major hydroxylated products of daidzein prior to the O-methylation reaction were 3',4',7-trihydroxyisoflavone (3'-ODI), 4',6,7-trihydroxyisoflavone (6-ODI) and 4',7,8-trihydroxyisoflavone (8-ODI), which are mono-hydroxylated at the ortho position of each hydroxyl group of daidzein. To identify monooxygenases playing a key role in the monohydroxylation of the A-ring of daidzein, all genes of 27 cytochrome P450s from N. farcinica IFM10152 were cloned and transformed into a E. coli BL21 (DE3) host system. By this enzymatic reaction using the mutants and the genome sequence analysis of N. farcinica IFM10152, it was revealed that nfa12130 and nfa33880 P450 genes clustered with their own ferredoxins and ferredoxin reductases (nfa12140+nfa12150 and nfa338870+nfa33860, respectively) are responsible for the hydroxylation of the A-ring of daidzein, and their major reaction products were 6-ODI and 8-ODI, respectively.

Biomolecular detection with a thin membrane transducer.

We present a thin membrane transducer (TMT) that can detect nucleic acid based biomolecular reactions including DNA hybridization and protein recognition by aptamers. Specific molecular interactions on an extremely thin and flexible membrane surface cause the deflection of the membrane due to surface stress change which can be measured by a compact capacitive circuit. A gold-coated thin PDMS membrane assembled with metal patterned glass substrate is used to realize the capacitive detection. It is demonstrated that perfect match and mismatch hybridizations can be sharply discriminated with a 16-mer DNA oligonucleotide immobilized on the gold-coated surface. While the mismatched sample caused little capacitance change, the perfectly matched sample caused a well-defined capacitance decrease vs. time due to an upward deformation of the membrane by a compressive surface stress. Additionally, the TMT demonstrated the single nucleotide polymorphism (SNP) capabilities which enabled a detection of mismatching base pairs in the middle of the sequence. It is intriguing that the increase of capacitance, therefore a downward deflection due to tensile stress, was observed with the internal double mismatch hybridization. We further present the detection of thrombin protein through ligand-receptor type recognition with 15-mer thrombin aptamer as a receptor. Key aspects of this detection such as the effect of concentration variation are investigated. This capacitive thin membrane transducer presents a completely new approach for detecting biomolecular reactions with high sensitivity and specificity without molecular labelling and optical measurement.

A novel sensor platform based on aptamer-conjugated polypyrrole nanotubes for label-free electrochemical protein detection.

We first present a simple yet versatile strategy for the functionalization of polymer nanotubes in a controlled fashion. Carboxylic-acid-functionalized polypyrrole (CPPy) nanotubes were fabricated by using cylindrical micelle templates in a water-in-oil emulsion system, and the functional carboxyl groups were effectively incorporated into the polymer backbone during the polymerization by using pyrrole-3-carboxylic acid (P3CA) as a co-monomer without a sophisticated functionalization process. It was noteworthy that the chemical functionality of CPPy nanotubes was readily controlled in both qualitative and quantitative aspects. On the basis of the controlled functionality of CPPy nanotubes, a field-effect transistor (FET) sensor platform was constructed to detect specific biological entities by using a buffer solution as a liquid-ion gate. The CPPy nanotubes were covalently immobilized onto the microelectrode substrate to make a good electrical contact with the metal electrodes, and thrombin aptamers were bonded to the nanotube surface via covalent linkages as the molecular recognition element. The selective recognition ability of thrombin aptamers combined with the charge transport property of CPPy nanotubes enabled the direct and label-free electrical detection of thrombin proteins. Upon exposure to thrombin, the CPPy nanotube FET sensors showed a decrease in current flow, which was probably attributed to the dipole-dipole or dipole-charge interaction between thrombin proteins and the aptamer-conjugated polymer chains. Importantly, the sensor response was tuned by adjusting the chemical functionality of CPPy nanotubes. The efficacy of CPPy nanotube FET sensors was also demonstrated in human blood serum; this suggests that they may be used for practical diagnosis applications after further optimization.