title: Stereochemical, structural, and thermodynamic origins of stability differences between stereoisomeric benzo[a]pyrene diol epoxide deoxyadenosine adducts in a DNA mutational hot spot sequence.
abstract: Benzo[a]pyrene (BP), a prototype polycyclic aromatic hydrocarbon (PAH), can be metabolically activated to the enantiomeric benzo[a]pyrene diol epoxides (BPDEs), (+)-(7R,8S,9S,10R)-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and the (-)-(7S,8R,9R,10S) enantiomer. These can react with adenine residues in DNA, to produce the stereoisomeric 10S (+)- and 10R (-)-trans-anti-[BP]-N(6)-dA adducts. High-resolution NMR solution studies indicate that in DNA duplexes the 10R (-) adduct is intercalated on the 5'-side of the modified adenine, while the 10S (+) adduct is disordered, exhibits multiple adduct conformations, and is positioned on the 3'-side of the modified adenine. Duplexes containing the 10S (+) adduct positioned at A within codon 61 of the human N-ras sequence CAA are thermodynamically less stable and more easily excised by human DNA repair enzymes than those containing the 10R (-) adduct. However, the molecular origins of these differences are not understood and represent a fascinating opportunity for elucidating structure-function relationships. We have carried out a computational investigation to uncover the structural and thermodynamic origins of these effects in the 11-mer duplex sequence d(CGGACAAGAAG).d(CTTCTTGTCCG) by performing a 2-ns molecular dynamics simulation using NMR solution structures as the basis for the starting models. Then, we applied the MM-PBSA (molecular mechanics Poisson-Boltzmann surface area) method to compute free energy differences between the stereoisomeric adducts. The 10R (-) isomer is more stable by approximately 13 kcal/mol, of which approximately 10 kcal/mol is enthalpic, which agrees quite well with their observed differences in thermodynamic stability. The lower stability of the 10S (+) adduct is due to diminished stacking by the BP moiety in the intercalation pocket, more helix unwinding, and a diminished quality of Watson-Crick base pairing. The latter stems from conformational heterogeneity involving a syn-anti equilibrium of the glycosidic bond in the modified adenine residue. The lower stability and conformational heterogeneity of the 10S (+) adduct may play a role in its enhanced susceptibility to nucleotide excision repair.
authors.affiliation: Department of Chemistry, New York University, New York, New York 10003, USA.
authors.forename: S
authors.initials: S
authors.lastname: Yan
authors.forename: R
authors.initials: R
authors.lastname: Shapiro
authors.forename: N E
authors.initials: NE
authors.lastname: Geacintov
authors.forename: S
authors.initials: S
authors.lastname: Broyde
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000737:chemistry*; D000225:Adenine / Q000737:chemistry; D001483:Base Sequence; D002274:Carcinogens, Environmental / Q000737:chemistry; D018736:DNA Adducts / Q000737:chemistry*; D004355:Drug Stability; D006801:Humans; D007364:Intercalating Agents / Q000737:chemistry; D008958:Models, Molecular; D009153:Mutagens / Q000737:chemistry; D011084:Polycyclic Aromatic Hydrocarbons / Q000737:chemistry; D013237:Stereoisomerism; D013816:Thermodynamics
chemical_list: D002274:Carcinogens, Environmental; D018736:DNA Adducts; D007364:Intercalating Agents; D009153:Mutagens; D011084:Polycyclic Aromatic Hydrocarbons; C043931:benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide-DNA; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; D000225:Adenine