Role of actinomycin pentapeptides in actinomycin-deoxyribonucleic acid binding and kinetics.

Results are reported on equilibrium and kinetic experiments probing the DNA binding properties of a series of actinomycin analogues differing at the 3'-amino acid position. While the parent compound, actinomycin D, contains proline at this position on both pentapeptide lactone rings, the analogues under consideration here contain either azetidine-2-carboxylic acid, pipecolic acid, or 4-ketoproline on one or both pentapeptide rings. This study extends our earlier results on doubly substituted analogues [Shafer, R.H., Burnett, R. R., & Mirau, P.A. (1980) Nucleic Acids Res. 8, 1121]. DNA binding constants were determined from Scatchard plots constructed from visible absorption data and covered the range of (0.3-9) X 10(6) M-1 for the whole series of analogues. The thermal denaturation temperature of calf-thymus DNA was increased by 3-17 degrees C. DNA dissociation kinetics, along with enthalpies and entropies of activation, were also determined. The time constant for the slowest dissociation process ranged from 278 to 10 900 s. The strongest DNA binding analogue, in terms of the largest binding constant, the largest increase in DNA thermal denaturation temperature, and the slowest DNA dissociation rate, was actinomycin V, which has 4-ketoproline in the beta peptide ring, while the weakest DNA binding analogue has pipecolic acid on both peptide rings. Evidence is presented for one peptide ring exerting a greater influence than the other in the interaction with DNA. Also, the possible role of cis-trans isomerization about one or two peptide bonds in determining the slow DNA binding kinetics is discussed.