Association of aureolic acid antibiotic, chromomycin A3 with Cu2+ and its negative effect upon DNA binding property of the antibiotic

Abstract

Here we have examined the association of an aureolic acid antibiotic, chromomycin A3 (CHR), with Cu(2+). CHR forms a high affinity 2:1 (CHR:Cu(2+)) complex with dissociation constant of 0.08 × 10(-10) M(2) at 25°C, pH 8.0. The affinity of CHR for Cu(2+) is higher than those for Mg(2+) and Zn(2+) reported earlier from our laboratory. CHR binds preferentially to Cu(2+) in presence of equimolar amount of Zn(2+). Complex formation between CHR and Cu(2+) is an entropy driven endothermic process. Difference between calorimetric and van't Hoff enthalpies indicate the presence of multiple equilibria, supported from biphasic nature of the kinetics of association. Circular dichroism spectroscopy show that [(CHR)(2):Cu(2+)] complex assumes a structure different from either of the Mg(2+) and Zn(2+) complex reported earlier. Both [(CHR)(2):Mg(2+)] and [(CHR)(2):Zn(2+)] complexes are known to bind DNA. In contrast, [(CHR)(2):Cu(2+)] complex does not interact with double helical DNA, verified by means of Isothermal Titration Calorimetry of its association with calf thymus DNA and the double stranded decamer (5'-CCGGCGCCGG-3'). In order to interact with double helical DNA, the (antibiotic)(2) : metal (Mg(2+) and Zn(2+)) complexes require a isohelical conformation. Nuclear Magnetic Resonance spectroscopy shows that the Cu(2+) complex adopts a distorted octahedral structure, which cannot assume the required conformation to bind to the DNA. This report demonstrates the negative effect of a bivalent metal upon the DNA binding property of CHR, which otherwise binds to DNA in presence of metals like Mg(2+) and Zn(2+). The results also indicate that CHR has a potential for chelation therapy in Cu(2+) accumulation diseases. However cytotoxicity of the antibiotic might restrict the use.