Tailoring lumazine synthase assemblies for bionanotechnology.

Nanoscale compartments formed by hierarchical protein self-assembly are valuable platforms for nanotechnology development. The well-defined structure and broad chemical functionality of protein cages, as well as their amenability to genetic and chemical modification, have enabled their repurposing for diverse applications. In this review, we summarize progress in the engineering of the cage-forming enzyme lumazine synthase. This bacterial nanocompartment has proven to be a malleable scaffold. The natural protein has been diversified to afford a family of unique proteinaceous capsules that have been modified, evolved and assembled with other components to produce nanoreactors, artificial organelles, delivery vehicles and virus mimics.

Chemical approaches for the construction of multi-enzyme reaction systems.

Inspired by nature, researchers aim at bringing together different types of enzymes by the generation of multi-enzymatic structures. Amongst others, chemical methods have been exploited enabling the covalent linkage of a set of enzymes to the same macromolecular scaffold or direct cross-linking. Control over the relative position of enzymes in the system has been realized by sequential immobilization in microchannels and by positional co-localization on DNA nanostructures. So far, site-specific conjugation reactions such as the azide-alkyne cycloaddition, N-terminal transamination and enzyme-mediated cross-linking, have been applied to a limited extent only. These methods are expected to allow for co-immobilization of less robust enzymes, hence, an expansion in the diversity of immobilized biocatalytic cascades. In addition, the combination of multiple bioconjugation methods will provide control over the composition in scaffold-free multi-enzyme complexes.

Self-assembly of synthetic metabolons through synthetic protein scaffolds: one-step purification, co-immobilization, and substrate channeling.

One-step purification of a multi-enzyme complex was developed based on a mixture of cell extracts containing three dockerin-containing enzymes and one family 3 cellulose-binding module (CBM3)-containing scaffoldin through high-affinity adsorption on low-cost solid regenerated amorphous cellulose (RAC). The three-enzyme complex, called synthetic metabolon, was self-assembled through the high-affinity interaction between the dockerin in each enzyme and three cohesins in the synthetic scaffoldin. The metabolons were either immobilized on the external surface of RAC or free when the scaffoldin contained an intein between the CBM3 and three cohesins. The immobilized and free metabolons containing triosephosphate isomerase, aldolase, and fructose 1,6-biphosphatase exhibited initial reaction rates 48 and 38 times, respectively, that of the non-complexed three-enzyme mixture at the same enzyme loading. Such reaction rate enhancements indicated strong substrate channeling among synthetic metabolons due to the close spatial organization among cascade enzymes. These results suggested that the construction of synthetic metabolons by using cohesins, dockerins, and cellulose-binding modules from cellulosomes not only decreased protein purification labor and cost for in vitro synthetic biology projects but also accelerated reaction rates by 1 order of magnitude compared to non-complexed enzymes. Synthetic metabolons would be an important biocatalytic module for in vitro and in vivo synthetic biology projects.

[Progress in the ligands and their complex structures of farnesoid X receptor].

Farnesoid X receptor (FXR) belongs to the nuclear receptor superfamily. It is highly related to the formation of metabolic syndrome and the glucose homeostasis, and therefore represents an important drug target against metabolic diseases and diabetes. In recent years, great progress has been made in the agonists, antagonists, and crystal structures of FXR. The diverse FXR ligands and their structure-activity relationship are reviewed in this article. The advances in the crystal structures of FXR in complex with different ligands are also introduced.

In vivo encapsulation of nucleic acids using an engineered nonviral protein capsid.

In Nature, protein capsids function as molecular containers for a wide variety of molecular cargoes. Such containers have great potential for applications in nanotechnology, which often require encapsulation of non-native guest molecules. Charge complementarity represents a potentially powerful strategy for engineering novel encapsulation systems. In an effort to explore the generality of this approach, we engineered a nonviral, 60-subunit capsid, lumazine synthase from Aquifex aeolicus (AaLS), to act as a container for nucleic acid. Four mutations were introduced per subunit to increase the positive charge at the inner surface of the capsid. Characterization of the mutant (AaLS-pos) revealed that the positive charges lead to the uptake of cellular RNA during production and assembly of the capsid in vivo. Surprisingly, AaLS-pos capsids were found to be enriched with RNA molecules approximately 200-350 bases in length, suggesting that this simple charge complementarity approach to RNA encapsulation leads to both high affinity and a degree of selectivity. The ability to control loading of RNA by tuning the charge at the inner surface of a protein capsid could illuminate aspects of genome recognition by viruses and pave the way for the development of improved RNA delivery systems.

Estabilidad de un suero control multienzimático utilizado como material de referencia en los laboratorios clínicos / Stability of a multienzyme control serum used as reference material in clinical laboratories

Una de las vías fundamentales para garantizar la calidad de los ensayos realizados en los laboratorios clínicos es mediante el uso de materiales de referencia. Una problemática a la que nos enfrentamos es la escasez de estos productos en el mercado nacional dado su alto costo. Objetivo: evaluar la estabilidad de un suero bovino adulto enriquecido con las enzimas alanina aminotransferasa (ALAT/TGP), aspartato aminotransferasa (ASAT/TGP), fosfatasa alcalina (FA) y amilasa. Métodos: se evaluó la estabilidad a tiempo real de la matriz enriquecida con las diferentes enzimas durante 12 meses a 2 temperaturas (refrigeración y congelación). Se evaluó el efecto del glicerol sobre la actividad enzimática de los extractos, así como el efecto de los preservantes propilenglicol y etilenglicol en la estabilidad de las enzimas. Resultados: los extractos enzimáticos obtenidos comenzaron a perder la actividad biológica a partir de los 15 días, independientemente de la temperatura de almacenamiento y de la presencia o no de glicerol. Los resultados del ensayo a tiempo real realizados a la matriz enriquecida, mostraron que la estabilidad varió con el tiempo y con el tipo de enzima, independientemente del preservante ensayado, disminuyendo por debajo de los límites aceptables de actividad enzimática luego de 3 meses de almacenamiento del producto a 4 ºC. Conclusiones: se logró un material de referencia multienzimático estable por un período de 3 meses

A fundamental method to assure the quality of clinical laboratory tests is the use of reference materials. A problem we are faced with is the scarcity of these products in the domestic market, due their high cost. Objective: Evaluate the stability of an adult bovine serum enriched with the enzymes alanine aminotransferase (ALT, GPT), aspartate aminotransferase (AST, GPT), alkaline phosphatase (AP) and amylase. Methods: This enzyme-enriched matrix underwent real-time stability assessment during 12 months at two temperatures (refrigerated and frozen). An evaluation was made of the effect of glycerol on the enzymatic activity of extracts, as well as the effect of the preservatives propylene glycol and ethylene glycol on enzymatic stability. Results: The enzyme extracts obtained began to lose their biological activity at 15 days, irrespective of the storage temperature and the presence or absence of glycerol. The real time assessment of the enriched matrix showed that stability varied with time and enzyme type, irrespective of the preservative tested, and fell below acceptable limits of enzymatic activity after three months of storage at 4 ºC. Conclusions: A multienzyme reference material was obtained which was stable for a period of 3 months

Estabilidad de un suero control multienzimático utilizado como material de referencia en los laboratorios clínicos / Stability of a multienzyme control serum used as reference material in clinical laboratories

Una de las vías fundamentales para garantizar la calidad de los ensayos realizados en los laboratorios clínicos es mediante el uso de materiales de referencia. Una problemática a la que nos enfrentamos es la escasez de estos productos en el mercado nacional dado su alto costo. Objetivo: evaluar la estabilidad de un suero bovino adulto enriquecido con las enzimas alanina aminotransferasa (ALAT/TGP), aspartato aminotransferasa (ASAT/TGP), fosfatasa alcalina (FA) y amilasa. Métodos: se evaluó la estabilidad a tiempo real de la matriz enriquecida con las diferentes enzimas durante 12 meses a 2 temperaturas (refrigeración y congelación). Se evaluó el efecto del glicerol sobre la actividad enzimática de los extractos, así como el efecto de los preservantes propilenglicol y etilenglicol en la estabilidad de las enzimas. Resultados: los extractos enzimáticos obtenidos comenzaron a perder la actividad biológica a partir de los 15 días, independientemente de la temperatura de almacenamiento y de la presencia o no de glicerol. Los resultados del ensayo a tiempo real realizados a la matriz enriquecida, mostraron que la estabilidad varió con el tiempo y con el tipo de enzima, independientemente del preservante ensayado, disminuyendo por debajo de los límites aceptables de actividad enzimática luego de 3 meses de almacenamiento del producto a 4 ºC. Conclusiones: se logró un material de referencia multienzimático estable por un período de 3 meses(AU)

A fundamental method to assure the quality of clinical laboratory tests is the use of reference materials. A problem we are faced with is the scarcity of these products in the domestic market, due their high cost. Objective: Evaluate the stability of an adult bovine serum enriched with the enzymes alanine aminotransferase (ALT, GPT), aspartate aminotransferase (AST, GPT), alkaline phosphatase (AP) and amylase. Methods: This enzyme-enriched matrix underwent real-time stability assessment during 12 months at two temperatures (refrigerated and frozen). An evaluation was made of the effect of glycerol on the enzymatic activity of extracts, as well as the effect of the preservatives propylene glycol and ethylene glycol on enzymatic stability. Results: The enzyme extracts obtained began to lose their biological activity at 15 days, irrespective of the storage temperature and the presence or absence of glycerol. The real time assessment of the enriched matrix showed that stability varied with time and enzyme type, irrespective of the preservative tested, and fell below acceptable limits of enzymatic activity after three months of storage at 4 ºC. Conclusions: A multienzyme reference material was obtained which was stable for a period of 3 months(AU)

Synthesis of functionalized cinnamaldehyde derivatives by an oxidative Heck reaction and their use as starting materials for preparation of Mycobacterium tuberculosis 1-deoxy-D-xylulose-5-phosphate reductoisomerase inhibitors.

Cinnamaldehyde derivatives were synthesized in good to excellent yields in one step by a mild and selective, base-free palladium(II)-catalyzed oxidative Heck reaction starting from acrolein and various arylboronic acids. Prepared α,ß-unsaturated aldehydes were used for synthesis of novel α-aryl substituted fosmidomycin analogues, which were evaluated for their inhibition of Mycobacterium tuberculosis 1-deoxy-D-xylulose 5-phosphate reductoisomerase. IC(50) values between 0.8 and 27.3 µM were measured. The best compound showed activity comparable to that of the most potent previously reported α-aryl substituted fosmidomycin-class inhibitor.

DNA-directed assembly of artificial multienzyme complexes.

This study aims to establish model systems for the exploration of proximity effects, occurring in natural multienzyme complexes. DNA-directed assembly of covalent conjugates of DNA oligonucleotides and Glucose Oxidase (GOX) or Horseradish peroxidase (HRP) was used to generate supramolecular complexes, in which the two enzymes were arranged with defined spatial orientation. Electrophoretic studies indicated that the assembly efficiency significantly depends on positional and sterical factors of the two DNA-enzyme conjugates. Kinetic rate measurements of the coupled reaction of glucose oxidation and Amplex Red peroxidation were carried out with microplate-immobilized DNA-GOX-HRP complexes, and the influence of Catalase on this reaction was determined. The kinetic measurements revealed a significant increase in the reactivity of the complexes, in which GOX and HRP were immobilized in direct proximity on a complementary DNA carrier.

Redesigning the PheA domain of gramicidin synthetase leads to a new understanding of the enzyme's mechanism and selectivity.

The PheA domain of gramicidin synthetase A, a non-ribosomal peptide synthetase, selectively binds phenylalanine along with ATP and Mg2+ and catalyzes the formation of an aminoacyl adenylate. In this study, we have used a novel protein redesign algorithm, K*, to predict mutations in PheA that should exhibit improved binding for tyrosine. Interestingly, the introduction of two predicted mutations to PheA did not significantly improve KD, as measured by equilibrium fluorescence quenching. However, the mutations improved the specificity of the enzyme for tyrosine (as measured by kcat/KM), primarily driven by a 56-fold improvement in KM, although the improvement did not make tyrosine the preferred substrate over phenylalanine. Using stopped-flow fluorometry, we examined binding of different amino acid substrates to the wild-type and mutant enzymes in the pre-steady state in order to understand the improvement in KM. Through these investigations, it became evident that substrate binding to the wild-type enzyme is more complex than previously described. These experiments show that the wild-type enzyme binds phenylalanine in a kinetically selective manner; no other amino acids tested appeared to bind the enzyme in the early time frame examined (500 ms). Furthermore, experiments with PheA, phenylalanine, and ATP reveal a two-step binding process, suggesting that the PheA-ATP-phenylalanine complex may undergo a conformational change toward a catalytically relevant intermediate on the pathway to adenylation; experiments with PheA, phenylalanine, and other nucleotides exhibit only a one-step binding process. The improvement in KM for the mutant enzyme toward tyrosine, as predicted by K*, may indicate that redesigning the side-chain binding pocket allows the substrate backbone to adopt productive conformations for catalysis but that further improvements may be afforded by modeling an enzyme:ATP:substrate complex, which is capable of undergoing conformational change.

On [Fe4S4]2+ -(mu2-SR)-M II bridge formation in the synthesis of an A-cluster analogue of carbon monoxide dehydrogenase/acetylcoenzyme A synthase.

The construction of a synthetic analogue of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme synthase, the site of acetylcoenzyme A formation, requires as a final step the formation of an unsupported [Fe(4)S(4)]-(mu(2)-SR)-Ni(II) bridge to a preformed cluster. Our previous results (Rao, P. V.; Bhaduri, S.; Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 5833) and the work of others have addressed synthesis of dinuclear complexes relevant to the A-cluster. This investigation concentrates on reactions pertinent to bridge formation by examining systems containing dinuclear and mononuclear Ni(II) complexes and the 3:1 site-differentiated clusters [Fe(4)S(4)(LS(3))L'](2-) (L' = TfO(-) (14), SEt (15)). The system 14/[{Ni(L(O)-S(2)N(2))}M(SCH(2)CH(2)PPh(2))](+) results in cleavage of the dinuclear complex and formation of [{Ni(L(O)-S(2)N(2))}Fe(4)S(4)(LS(3))]- (18), in which the Ni(II) complex binds at the unique cluster site with formation of a Ni(mu(2)-SR)(2)Fe bridge rhomb. Cluster 18 and the related species [{Ni(phma)}Fe(4)S(4)(LS(3))](3)- (19) are obtainable by direct reaction of the corresponding cis-planar Ni(II)-S(2)N(2) complexes with 14. The mononuclear complexes [M(pdmt)(SEt)]- (M = Ni(II), Pd(II)) with 14 in acetonitrile or Me(2)SO solution react by thiolate transfer to give 15 and [M(2)(pdmt)(2)]. However, in dichloromethane the Ni(II) reaction product is interpreted as [{Ni(pdmt)(mu(2)-SEt)}Fe(4)S(4)(LS(3))](2-) (20). Reaction of Et(3)NH(+) and 15 affords the double cubane [{Fe(4)S(4)(LS(3))}(2)(mu(2)-SEt)](3-) (21). Cluster 18 contains two mutually supportive Fe-(mu(2)-SR)-Ni(II) bridges, 19 exhibits one strong and one weaker bridge, 20 has one unsupported bridge (inferred from the (1)H NMR spectrum), and 21 has one unsupported Fe-(mu(2)-SR)-Fe bridge. Bridges in 18, 19, and 21 were established by X-ray structures. This work demonstrates that a bridge of the type found in the enzyme A-clusters is achievable by synthesis and implies that more stable, unsupported single thiolate bridges may require reinforcement by an additional covalent linkage between the Fe(4)S(4) and nickel-containing components. (LS(3) = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3-); L(O)-S(2)N(2) = N,N'-diethyl-3,7-diazanonane-1,9-dithiolate(2-); pdmt = pyridine-2,6-methanedithiolate(2-); phma = N,N'-1,2-phenylenebis(2-acetylthio)acetamidate(4-); TfO = triflate.).

Enabling multienzyme biocatalysis using nanoporous materials.

Multistep reactions catalyzed by a covalently immobilized enzyme-cofactor-enzyme system were achieved. Lactate dehydrogenase (LDH), glucose dehydrogenase (GDH), and cofactor NADH were incorporated into two porous silica glass supports. One of the glass supports had pores of 30 nm in diameter, while the other was of 100-nm pore size. Effective shuttling of the covalently bound NADH between LDH and GDH was achieved, such that regeneration cycles of NADH/NAD(+) were observed. The glass of 30-nm pore size afforded enzyme activities that were about twice those observed for the glass of 100-nm pore size, indicating the former provided better enzyme-cofactor integration. The effect of the size of spacers was also examined. The use of longer spacers increased the reaction rates by approximately 18 times as compared to those achieved with glutaraldehyde linkage. It appeared that the concave configuration of the nanopores played an important role in enabling the multistep reactions. The same multienzyme system immobilized on nonporous polystyrene particles of 500-nm diameter was only approximately 2% active as the glass-supported system. It is believed that the nanoporous structure of the glass supports enhances the molecular interactions among the immobilized enzymes and cofactor, thus improving the catalytic efficiency of the system.

Rapid and flexible synthesis of 1-deoxy-D-xylulose-5-phosphate, the substrate for 1-deoxy-D-xylulose-5-phosphate reductoisomerase.

1-Deoxy-D-xylulose-5-phosphate (DXP) is a key intermediate in the non-mevalonate pathway to terpenoids in bacteria, and it is the substrate for the enzyme 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXP-R). In order to study the mechanism of DXP-R, we required a flexible synthesis of the substrate which would allow the incorporation of isotopic labels, and the variation of the two stereocentres. Thus 1,4-dihydroxypent-2-yne was selectively reduced to give the E-olefin, and selective phosphorylation of the primary alcohol followed by oxidation of the secondary alcohol gave a substrate suitable for dihydroxylation. Dihydroxylation using stoichiometric OsO4 in the presence of chiral ligands gave protected DXP in high ee. Final hydrogenolysis gave DXP in quantitative yield and high purity. DXP-R was produced by rapid cloning of the dxr gene from Escherichia coli through controlled expression and ion exchange chromatography. The synthetic DXP was fully active in enzyme assays catalysed by recombinant DXP-R.

Transglutaminase-catalyzed synthesis of trypsin-cyclodextrin conjugates: kinetics and stability properties.

Bovine pancreatic trypsin was modified by the mono-6-amino-6-deoxy derivatives of alpha-, beta-, and gamma-cyclodextrin through a transglutaminase-catalyzed reaction. The trypsin-cyclodextrin conjugates, containing about 3 mol of oligosaccharide per mole of protein, were tested for their catalytic and stability properties. The specific esterolytic activity and the kinetics constants of trypsin were significantly improved following the transglutaminase-induced structural modifications. Trypsin-cyclodextrin conjugates were also found markedly (sixfold) more resistant to autolytic degradation at alkaline pH, and their thermal stability profile was improved by about 16 degrees C. Moreover, they were particularly resistant to heat inactivation when treated at different temperatures ranging from 45 degrees C to 70 degrees C for different periods of time.

Skipping in a hybrid polyketide synthase. Evidence for ACP-to-ACP chain transfer.

A tetraketide synthase containing a loading module (LM), the extension modules erythromycin module 1, rapamycin module 2, and erythromycin module 2 (LM-Ery1-Rap2-Ery2-TE), when expressed in Saccharopolyspora erythraea strain JC2, produced as previously reported a mixture of tetraketide lactones (minor products) and triketide lactones (major products). Several alternative plausible mechanisms by which this "skipping" phenomenon might occur may be proposed. Site-directed mutagenesis of the ketosynthase (KS) and acylcarrier protein (ACP) domains in the interpolated module has shown that skipping within the hybrid PKS involves passage of the growing polyketide through the interpolated module, by direct ACP-to-ACP transfer of the polyketide chain.

Production and characterization of bifunctional enzymes. Domain swapping to produce new bifunctional enzymes in the aspartate pathway.

The bifunctional enzyme aspartokinase-homoserine dehydrogenase I from Escherichia coli catalyzes non-consecutive reactions in the aspartate pathway of amino acid biosynthesis. Both catalytic activities are subject to allosteric regulation by the end product amino acid L-threonine. To examine the kinetics and regulation of the enzymes in this pathway, each of these catalytic domains were separately expressed and purified. The separated catalytic domains remain active, with each of their catalytic activities enhanced in comparison to the native enzyme. The allosteric regulation of the kinase activity is lost, and regulation of the dehydrogenase activity is dramatically decreased in these separate domains. To create a new bifunctional enzyme that can catalyze consecutive metabolic reactions, the aspartokinase I domain was fused to the enzyme that catalyzes the intervening reaction in the pathway, aspartate semialdehyde dehydrogenase. A hybrid bifunctional enzyme was also created between the native monofunctional aspartokinase III, an allosteric enzyme regulated by lysine, and the catalytic domain of homoserine dehydrogenase I with its regulatory interface domain still attached. In this hybrid the kinase activity remains sensitive to lysine, while the dehydrogenase activity is now regulated by both threonine and lysine. The dehydrogenase domain is less thermally stable than the kinase domain and becomes further destabilized upon removal of the regulatory domain. The more stable aspartokinase III is further stabilized against thermal denaturation in the hybrid bifunctional enzyme and was found to retain some catalytic activity even at temperatures approaching 100 degrees C.

Production and characterization of bifunctional enzymes. Substrate channeling in the aspartate pathway.

The direct channeling of an intermediate between enzymes that catalyze consecutive reactions in a pathway offers the possibility of an efficient, exclusive, and protected means of metabolite delivery. Aspartokinase-homoserine dehydrogenase I (AK-HDH I) from Escherichia coli is an unusual bifunctional enzyme in that it does not catalyze consecutive reactions. The potential channeling of the intermediate beta-aspartyl phosphate between the aspartokinase of this bifunctional enzyme and aspartate semialdehyde dehydrogenase (ASADH), the enzyme that catalyzes the intervening reaction, has been examined. The introduction of increasing levels of inactivated ASADH has been shown to compete against enzyme-enzyme interactions and direct intermediate channeling, leading to a decrease in the overall reaction flux through these consecutive enzymes. These same results are obtained whether these experiments are conducted with aspartokinase III, a naturally occurring monofunctional isozyme, with an artificially produced monofunctional aspartokinase I, or with a fusion construct of AK I-ASADH. These results provide definitive evidence for the channeling of beta-aspartyl phosphate between aspartokinase and aspartate semialdehyde dehydrogenase in E. coli and suggest that ASADH may provide a bridge to channel the intermediates between the non-consecutive reactions of AK-HDH I.

Helix-loop-helix peptides as scaffolds for the construction of bridged metal assemblies in proteins: the spectroscopic A-cluster structure in carbon monoxide dehydrogenase.

Four helix-loop-helix 63mer peptides were designed and synthesized in order to assess the utility of peptides as scaffolds for the stabilization of complex metal sites in proteins. Bridged assembly [Ni(II)-(mu(2)-S.Cys)-Fe(4)S(4)], consistent with spectroscopic information on the A-cluster of carbon monoxide dehydrogenase, was chosen as the target assembly. The peptides consist of two helices with approximately 20 residues connected by a flexible loop containing the ferredoxin consensus sequence Cys-Ile-Ala-Cys-Gly-Ala-Cys to bind the Fe(4)S(4) cluster. A fourth cysteine was positioned to serve as the bridging ligand between the cluster and Ni(II). Three other binding residues were incorporated in appropriate positions to constitute a binding site for Ni(II). One of the peptides was designed with an N(3)S (His(3)Cys) site, and each of the other three with N(2)S(2) (His(2)Cys(2)) sites. A detailed account of the synthesis and characterization of the peptides and their metalloderivatives is presented. The four peptides were synthesized using an Fmoc/t-Bu-based solid-phase strategy, purified by reversed-phase HPLC, and characterized by ES-MS. On the basis of size-exclusion chromatography and circular dichroism spectropolarimetry, these peptides appear to dimerize in solution to form four-helix bundles of high helical contents. Reactions of the peptides with preformed cluster [Fe(4)S(4)(SCH(2)CH(2)OH)(4)](2)(-) and subsequent purification by column chromatography yield a product consistent with the incorporation of one [Fe(4)S(4)](2+) cluster per 63mer, as judged from absorption and Mössbauer spectra. Addition of a Ni(II) salt to the [Fe(4)S(4)]-peptides results in an apparent equilibrium between free Ni(II) and a peptide-bound nickel form, as established by column chromatography studies. Nickel EXAFS data (Musgrave, K. B.; Laplaza, C. E.; Holm, R. H.; Hedman, B.; Hodgson, K. O. Results to be published.) provide strong evidence that the peptide-bound nickel binds in the desired site in two of the metallopeptides. This work represents the first exploration of peptides as scaffolds for the support of biologically relevant bridged assemblies containing iron-sulfur clusters.

Epothilone biosynthesis: assembly of the methylthiazolylcarboxy starter unit on the EpoB subunit.

BACKGROUND: Polyketides (PKs) and non-ribosomal peptides (NRPs) are therapeutically important natural products biosynthesized by multimodular protein assembly lines, termed the PK synthases (PKSs) and NRP synthetases (NRPSs), via a similar thiotemplate-mediated mechanism. The potential for productive interaction between these two parallel enzymatic systems has recently been demonstrated, with the discovery that PK/NRP hybrid natural products can be of great therapeutic importance. One newly discovered PK/NRP product, epothilone D from Sorangium cellulosum, has shown great potential as an anti-tumor agent. RESULTS: The chain-initiating methylthiazole ring of epothilone has been generated in vitro as an acyl-S-enzyme intermediate, using five domains from two modules of the polymodular epothilone synthetase. The acyl carrier protein (ACP) domain, excised from the EpoA gene, was expressed in Escherichia coli, purified as an apo protein, and then post-translationally primed with acetyl-CoA using the phosphopantetheinyl transferase enzyme Sfp. The four-domain 150-kDa EpoB subunit (cyclization-adenylation-oxidase-peptidyl carrier protein domains: Cy-A-Ox-PCP) was also expressed and purified in soluble form from E. coli. Post-translational modification with Sfp and CoASH introduced the HS-pantP prosthetic group to the apo-PCP, enabling subsequent loading with L-cysteine to generate the Cys-S-PCP acyl enzyme intermediate. When acetyl-S-ACP (EpoA) and cysteinyl-S-EpoB were mixed, the Cy domain of EpoB catalyzed acetyl transfer from EpoA to the amino group of the Cys-S-EpoB, generating a transient N-Ac-Cys-S-EpoB intermediate that is cyclized and dehydrated to the five-membered ring methylthiazolinyl-S-EpoB. Finally, the FMN-containing Ox domain of EpoB oxidized the dihydro heterocyclic thiazolinyl ring to the heteroaromatic oxidation state, the methylthiazolylcarboxy-S-EpoB. When other acyl-CoAs were substituted for acetyl-CoA in the Sfp-based priming of the apo-CP domain, additional alkylthiazolylcarboxy-S-EpoB acyl enzymes were produced. CONCLUSIONS: These experiments establish chain transfer across a PKS and NRPS interface. Transfer of the acetyl group from the ACP domain of EpoA to EpoB reconstitutes the start of the epothilone synthetase assembly line, and installs and converts a cysteine group into a methyl-substituted heterocycle during this natural product chain growth.