Antibiotic-producing polyketide synthases (PKSs) are enzymes responsible for the biosynthesis in Streptomyces and related filamentous bacteria of a remarkably broad range of bioactive metabolites, including antitumour aromatic compounds such as mithramycin and macrolide antibiotics such as erythromycin. The molecular basis for the selection of the starter unit on aromatic PKSs is unknown. Here we show that a component of aromatic PKS, previously named 'chain-length factor', is a factor required for polyketide chain initiation and that this factor has decarboxylase activity towards malonyl-ACP (acyl carrier protein). We have re-examined the mechanism of initiation on modular PKSs and have identified as a specific initiation factor a domain of previously unknown function named KSQ, which operates like chain-length factor. Both KSQ and chain-length factor are similar to the ketosynthase domains that catalyse polyketide chain extension in modular multifunctional PKSs and in aromatic PKSs, respectively, except that the ketosynthase domain active-site cysteine residue is replaced by a highly conserved glutamine in KSQ and in chain-length factor. The glutamine residue is important both for decarboxylase activity and for polyketide synthesis.

The biosynthesis of complex reduced polyketides is catalysed in actinomycetes by large multifunctional enzymes, the modular Type I polyketide synthases (PKSs). Most of our current knowledge of such systems stems from the study of a restricted number of macrolide-synthesising enzymes. The sequencing of the genes for the biosynthesis of monensin A, a typical polyether ionophore polyketide, provided the first genetic evidence for the mechanism of oxidative cyclisation through which polyethers such as monensin are formed from the uncyclised products of the PKS. Two intriguing genes associated with the monensin PKS cluster code for proteins, which show strong homology with enzymes that trigger double bond migrations in steroid biosynthesis by generation of an extended enolate of an unsaturated ketone residue. A similar mechanism operating at the stage of an enoyl ester intermediate during chain extension on a PKS could allow isomerisation of an E double bond to the Z isomer. This process, together with epoxidations and cyclisations, form the basis of a revised proposal for monensin formation. The monensin PKS has also provided fresh insight into general features of catalysis by modular PKSs, in particular into the mechanism of chain initiation.

SummaryChain initiation on many modular polyketide synthases is mediated by acyl transfer from the CoA ester of a dicarboxylic acid, followed by decarboxylation in situ by KSQ, a ketosynthase-like decarboxylase domain. Consistent with this, the acyltransferase (AT) domains of all KSQ-containing loading modules are shown here to contain a key arginine residue at their active site. Site-specific replacement of this arginine residue in the oleandomycin (ole) loading AT domain effectively abolished AT activity, consistent with its importance for catalysis. Substitution of the ole PKS loading module, or of the tylosin PKS loading module, for the erythromycin (ery) loading module gave polyketide products almost wholly either acetate derived or propionate derived, respectively, instead of the mixture found normally. An authentic extension module AT domain, rap AT2 from the rapamycin PKS, functioned appropriately when engineered in the place of the ole loading AT

The immunologically dominant central portion of the circumsporozoite (CS) surface protein on the malaria parasite Plasmodium falciparum contains a large number of tandemly repeated NPNA tetrapeptide motifs. The preferred secondary structure of this repeat unit in aqueous solution has been investigated with the aid of the secondary structure-inducing amino acid (8-a-methylproline (PMe). \u27H-Nuclear magnetic resonance (Nh4R) and circular dichroism (CD) spectroscopy have been used to probe the structures of synthetic peptides containing one to three tetrapeptide NPMeNA units. The far-UV CD spectra of these peptides show more intense negative bands at 215 nm than do similar peptides based on the NPNA motif. This and the temperature dependence of the peptide amide chemical shifts, the pattern of NOE connectivities, and the magnitude of 3J coupling constants, derived from oneand two-dimensional NMR spectra of Ac(NPMeNA)3-OH, provide strong evidence for stable tumlike structures. From NOE distance and dihedral angle restraints, structures consistent with the NMR parameters were calculated. These reveal a stable hydrogen-bonded type-I p-tum conformation (most likely present at 70-80% population) within each NPMeNA motif, stabilized by the backbone C, methylation. Side chain to backbone hydrogen bonds involving the side chain amide groups of both asparagine residues also appear to impart stabilization to the turn conformation. No regular repeating conformations were detected in the linker regions connecting each NPMeNA unit. Polyclonal antisera raised in rabbits against (NPMeNA)3 recognized intact P. falciparum sporozoites in an immunofluorescence assay as efficiently as antisera raised against (NPNA)3. This indicates that the type-I ,\u26turn detected in the PMecontaining peptide is closely related to the immunologically dominant portion of the folded CS protein. An improved knowledge of the three-dimensional structure of this protein may be of value for the design of second-generation synthetic malaria vaccines

The immunodominant central portion of the circumsporozoite (CS) surface protein of the malaria parasite Plasmodium falciparum contains a tetrapeptide motif, Asn-Pro-Asn-Ala (NPNA), tandemly repeated almost 40 times. The three-dimensional structure of the CS protein, including the central repeat region, is presently unknown. We have investigated an approach to stabilize β-turns in a single NPNA motif, by its incorporation into a template-bound cyclic peptide comprising the sequence ANPNAA. The template was designed to stabilize β-turns in the peptide loop and to allow its conjugation to T-cell epitopes in a multiple-antigen-peptide. NMR studies and MD simulations with time-averaged NOE-derived upper distance restraints support the formation of a stable β-I turn conformation in the NPNA motif of this template-bound antigen. Balb/c mice immunized with a multiple-antigen-peptide containing four copies of the template-bound loop conjugated to a single universal T-cell epitope produced antibodies that bound P. falciparum sporozoites in immunofluorescence assays. These results provide further support for the immunological relevance of a type-I β-turn conformation based on the NPNA cadence in the repeat region of the CS protein and illustrate the use of a novel template for the evaluation of conformationally constrained peptide immunogens.

In contrast with earlier results, KR1 selects only the (2S) isomer and reduces it stereospecifically to the (2S, 3R)-3-hydroxy-2-methyl acyl product. The KR domain of module 1 controls the stereochemical outcome at both methyl-and hydroxyl-bearing chiral centres in the hydroxy diketide intermediate. Earlier work showed that the normal enzyme-bound ketoester generated in module 2 is not epimerised, however. The stereochemistry at C-2 is therefore established by a condensation reaction that exclusively gives the (2R)-ketoester, and the stereo-chemistry at C-3 by reduction of the keto group. Two different mechanisms of stereochemical control, therefore, operate in modules 1 and 2 of the erythromycin PKS. These results should provide a more rational basis for designing hybrid PKSs to generate altered stereochemistry in polyketide products.

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O (2009Recent progress in the discovery of macrocyclic compounds as potential anti-infective therapeutics AbstractNovel therapeutic strategies are urgently needed for the treatment of serious diseases caused by viral, bacterial and parasitic infections, because currently used drugs are facing the problem of rapidly emerging resistance. There is also an urgent need for agents that act on novel pathogen-specific targets, in order to expand the repertoire of possible therapies. The high throughput screening of diverse small molecule compound libraries has provided only a limited number of new lead series, and the number of compounds acting on novel targets is even smaller. Natural product screening has traditionally been very successful in the anti-infective area. Several successful drugs on the market as well as other compounds in clinical development are derived from natural products. Amongst these, many are macrocyclic compounds in the 1-2 kDa size range. This review will describe recent advances and novel drug discovery approaches in the anti-infective area, focusing on synthetic and natural macrocyclic compounds for which in vivo proof of concept has been established. The review will also highlight the Protein Epitope Mimetics (PEM) technology as a novel tool in the drug discovery process. Here the structures of naturally occurring antimicrobial and antiviral peptides and proteins are used as starting points to generate novel macrocyclic mimetics, which can be produced and optimized efficiently by combinatorial synthetic methods. Several recent examples highlight the great potential of the PEM approach in the discovery of new anti-infective agents. Abstract:Novel therapeutic strategies are urgently needed for the treatment of serious