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An ab initio quantum mechanics calculation that correlates with ligand orientation and DNA cleavage site selectivity in camptothecin-DNA-topoisomerase I ternary cleavage complexes.

ABSTRACT: Camptothecin (CPT), a cytotoxic natural alkaloid isolated from Camptotheca acuminata, and its derivatives represent an important class of cancer chemotherapeutic drugs that act by inhibiting topoisomerase I (top1). The mechanism of top1 inhibition by CPT has been determined by X-ray crystallography. Biochemical studies carried out both in vitro and in vivo indicated that CPT has strict DNA sequence preference for -1 T and strong preference for +1 G at the cleavage site. To understand the molecular determinants for the CPT binding orientation and DNA sequence selectivity, we present a quantum mechanics calculation where only pi-pi stacking interactions were included to shed some light on the mechanism of this sequence selectivity. This ab initio calculation can not only reproduce the experimental binding orientation of CPT at the cleavage site but also shows very good correlation between the binding energy for different sequences and the observed frequency of CPT-stabilized sites in the SV40 viral genome. Therefore, it can be concluded that hydrogen bonding of the ligand to the surrounding amino acid residues of the protein is of minor significance. The present method should be applicable to other polycyclic top1 inhibitors.

SUBMITTER: Xiao X

PROVIDER: S-EPMC2526349 | biostudies-literature | 2005 Jul

REPOSITORIES: biostudies-literature

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An ab initio quantum mechanics calculation that correlates with ligand orientation and DNA cleavage site selectivity in camptothecin-DNA-topoisomerase I ternary cleavage complexes.

Xiao Xiangshu X Cushman Mark M

Journal of the American Chemical Society 20050701 28

Camptothecin (CPT), a cytotoxic natural alkaloid isolated from Camptotheca acuminata, and its derivatives represent an important class of cancer chemotherapeutic drugs that act by inhibiting topoisomerase I (top1). The mechanism of top1 inhibition by CPT has been determined by X-ray crystallography. Biochemical studies carried out both in vitro and in vivo indicated that CPT has strict DNA sequence preference for -1 T and strong preference for +1 G at the cleavage site. To understand the molecul ...[more]

PMID: 16011334

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Project description:Reliable studies of enzymatic reactions by combined quantum mechanics/molecular mechanics (QM/MM) approaches, with an ab initio description of the quantum region, presents a major challenge to computational chemists. The main problem is the need for a very large computer time for the evaluation of the QM energy, which in turn makes it extremely challenging to perform proper configurational sampling. A seemingly reasonable alternative is to perform energy minimization studies of the type used in gas-phase ab initio studies. However, it is hard to see why such an approach should give reliable results in protein active sites. To examine the problems with energy minimization QM/MM approaches, we chose the hypothetical reaction of a metaphosphate ion with water in the Ras.GAP complex. This hypothetical reaction served as a simple benchmark reaction. The possible problems with the QM/MM minimization were explored by generating several protein configurations from long MD simulations and using energy minimization and scanning of the reaction coordinates to evaluate the corresponding potential energy surfaces of the reaction for each of these different protein configurations. Comparing these potential energy surfaces, we found major variations of the corresponding minima. Furthermore, the reaction energies and activation energies also varied significantly even for similar protein configurations. The specific coordination of a magnesium ion, present in the active center of the protein complex, turned out to influence the energetics of the reaction in a major way, where a direct coordination to the reactant leads to an increase of the activation energy by 17 kcal/mol. Apparently, using energy minimization to generate potential surfaces for an enzymatic reaction, while starting from a single protein structure, could lead to major errors in calculations of activation free energies and binding free energies. Thus we believe that extensive samplings of the configurational space of the protein are essential for meaningful determination of the energetics of enzymatic reactions. The possible relevance of our conclusion with regard to a recent study of the RasGAP reaction is discussed.

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Dataset Information

An ab initio quantum mechanics calculation that correlates with ligand orientation and DNA cleavage site selectivity in camptothecin-DNA-topoisomerase I ternary cleavage complexes.

Publications

An ab initio quantum mechanics calculation that correlates with ligand orientation and DNA cleavage site selectivity in camptothecin-DNA-topoisomerase I ternary cleavage complexes.

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