Anthelvencin B

A family of naturally occurring oligopeptides includes netropsin, distamycin, anthelvencin, kikumycin B, amidinomycin, and norformycin. Netropsin (I) and distamycin (II) express their biological activities by targeting specific sequences of chemical functionalities in the minor groove of DNA. Both netropsin and distamycin can be regarded as polyamide chains in which each alpha-carbon has been replaced by a five-membered pyrrole ring. The repeat distance in such an augmented polyamide chain is almost the same as the distance from one base pair to the next along the floor of a minor groove within beta-DNA. In this paper we report the synthesis of 16-21 cross-linked polyamides containing a thiazole heterocyclic ring bearing the active functionalites NH(2), NHCHO, or H. 16 and 17 were synthesized by DCC and HOBt catalyzed reaction of 5 with 14 and 15, while the formylation products 18 and 19 were obtained by coupling the formylated 4-methyl-thiazolated acid 6 with 14 and 15. The deaminated compounds 20 and 21 were obtained by the coupling of 5-trichloroacetyl-4-methylthiazole 7 synthesized from 4-methylthiazole. All the six cross-linked polyamides 16-21 were tested for their DNA gyrase inhibition. The studies have shown these polyamides have better sequence recognition and a greater percentage of inhibition than the corresponding monomers. The compound 17 shows complete inhibition of gyrase at 0.5 microM concentration as compared to the naturally occurring distamycin at 1.0 microM.

Ab initio calculations (Hartree-Fock) using the 6-31G basis set have been performed on two chiral oligopeptide antitumor antibiotics amidinomycin 5 and noformycin 6. The latter are DNA minor groove binding agents related to the A.T recognizing netropsin 4 and distamycin 3 but, unlike the latter, bear stereocenters (two for 5 and one for 6) that may be expected to affect binding to the B-DNA receptor. Geometry optimized conformations, energies and distribution of electrostatic charges within the molecules were derived. The rotational barrier for bond C3-C6 in 6 was calculated to be ca. 6 kcal.mole-1 and the dipole moment for 6 was 7.69D and for 5 was 5.58D. The ab initio derived parameters of the geometry optimized conformations of the different possible stereoisomeric forms of 5 and 6 were used to interpret their different interactions with the minor groove of DNA at both A.T and G.C sequences and the results were compared with molecular mechanics calculations. The order of binding of the four stereoisomers of 5 at the preferred (A.T)n sequences by both ab initio and molecular mechanics calculations is 1S,3R > RR > RS > SS. The predicted energy differences for complexation with DNA of the other stereoisomers from that of 1S,3R are: RR (4.2%); RS (6.7%) and SS (21.5%). In the case of noformycin the 4R structure binds more effectively than the enantiomer. Considerations of phasing in the computed distances between hydrogen bond donating sites in the DNA-bound antibiotics provide further insight into the binding processes. In the complexes of noformycin 6 the N-N1-N4 and N1-N5 distances (9.05 and 9.15 A respectively for 4R-6 and 9.23 and 9.26 A respectively for 4S-6) are close to the optimum value of 9.1 A for effective binding. In the case of amidinomycin 5 the best agreement with the optimum value occurs with the strongest binding diastereomer 1S,3R (N1-N3 = 8.91, N1-N4 = 9.41 A). The unexpected result, consistent in both ab initio and molecular mechanics treatments, is that, in contrast to the cases of kikumycin 1 and anthelvencin 2, the natural 3S configuration of 5 and 4S of 6 do not confer maximal binding efficiency. This suggests that biogenetic factors in the generation of the oligopeptide antibiotics lead to maximum DNA binding in the cases of kikumycin and anthelvencin but not in the cases of amidinomycin and noformycin.

Anthelvencins A and B are pyrrolamide metabolites produced by Streptomyces venezuelae ATCC 14583 and 14585. Isolated in 1965, they were reported to exhibit anthelmintic and moderate antibacterial activities. In this study, we revise the structure of anthelvencin A and identify a third anthelvencin metabolite, bearing two N-methylated pyrrole groups, which we named anthelvencin C. We sequenced the genome of S. venezuelae ATCC 14583 and identified a gene cluster predicted to direct the biosynthesis of anthelvencins. Functional analysis of this gene cluster confirmed its involvement in anthelvencin biosynthesis and allowed us to propose a biosynthetic pathway for anthelvencins. In addition to a nonribosomal peptide synthetase (NRPS), the assembly of anthelvencins involves an enzyme from the ATP-grasp ligase family, Ant23. We propose that Ant23 uses a PCP-loaded 4-aminopyrrole-2-carboxylate as substrate. As observed for the biosynthesis of the other pyrrolamides congocidine (produced by Streptomyces ambofaciens ATCC 25877) and distamycin (produced by Streptomyces netropsis DSM 40846), the NRPS assembling anthelvencins is composed of stand-alone domains only. Such NRPSs, sometimes called type II NRPSs, are less studied than the classical multimodular NRPSs. Yet, they constitute an interesting model to study protein-protein interactions in NRPSs and are good candidates for combinatorial biosynthesis approaches.