Three-dimensional solution structure of apo-neocarzinostatin from Streptomyces carzinostaticus determined by NMR spectroscopy.

The three-dimensional solution structure of apo-neocarzinostatin has been resolved from nuclear magnetic resonance spectroscopy data. Up to 1034 constraints were used to generate an initial set of 45 structures using a distance geometry algorithm (DSPACE). From this set, ten structures were subjected to refinement by restrained energy minimization and molecular dynamics. The average atomic root mean square deviations between the final ten structures and the mean structure obtained by averaging their coordinates run from 0.085 nm for the best defined beta-sheet regions of the protein to 0.227 nm for the side chains of the most flexible loops. The solution structure of apo-neocarzinostatin is closely similar to that of the related proteins, macromomycin and actinoxanthin. It contains a seven-stranded antiparallel beta-barrel which forms, together with two external loops, a deep cavity that is the chromophore binding site. It is noteworthy that aromatic side chains extend into the binding cleft. They may be responsible for the stabilization of the holo-protein complex and for the chromophore specificity within the antitumoral family.