A New Model for Ligand Release

MOLECULAR SIMULATIONS OF NEOCARZINOSTATIN CHROMOPHORE RELEASE MECHANISM

Neocarzinostatin (NCS) is an antitumor chromophore carrier protein with many applications in clinical use such as drug delivery system; however, so far its chromophore-releasing mechanism remains unclear. In this contribution the process and pathway of the chromophore releasing from holoprotein are revealed by conventional molecular dynamics simulations and essential dynamics (ED) sampling method. The results are consistent with the model for ligand release proposed in [D. H. Chin et al., J Biol Chem281:16025, 2006]. The further analysis suggests that the conformational changes of loop 99–104 and motions of side-chain of residue Phe78 are important factors for chromophore release; the opening state of loop 99–104 is a precondition for the release of ligand.

Insight into the strong inhibitory action of salt on activity of neocarzinostatin

Enediyne anticancer drugs belong to one of the most potent category in inducing DNA damage. We report 85+/-5% inhibition on activity of neocarzinostatin by salt. As high sodium ion concentration is a known tumor cell feature, we explored the dynamic mechanism of inhibition. Using various analytical tools, we examined parameters involved in the four consecutive steps of the drug action, namely, drug releasing from carrier protein, drug-DNA binding, drug activating, and DNA damaging. Neither protein stability, nor drug release rate, was altered by salt. The salt inhibition level was similar in between the protein-bound and unbound enediyne chromophore. Salt did not quench the thiol-induced drug activation. The inhibition was independent of DNA lesion types and irrelevant with thiol structures. Collectively, no salt interaction was found in the releasing, activating, and DNA damaging step of the drug action. However, binding with DNA decreased linearly with salt and corresponded well with the salt-induced inhibition on the drug activity. Salt interference on the affinity of DNA binding was the main and sole cause of the severe salt inhibition. The inhibition factor should be carefully considered for all agents with similar DNA binding mode.

Binding capability of the enediyne-associated apoprotein to human tumors and constitution of a ligand oligopeptide-integrated protein

The molecule of lidamycin that belongs to the chromoprotein family of antitumor antibiotics is composed of an apoprotein (LDP) and an enediyne chromophore. The enediyne moiety of the molecule is responsible for the potent cytotoxicity; however, the biological function of the apoprotein moiety, particularly its interaction with cancer cells, remains unclear. In present study, the binding capability of LDP to human tumors was detected for the first time by tissue microarray. LDP bound to various human tumors with significant difference from the corresponding normal tissues. Positive correlation between binding activity and the overexpression of VEGF and EGFR was confirmed by lung carcinoma tissue microarray. A fusion protein LG-LDP that consists of LDP and a ligand oligopeptide to EGFR was constructed by DNA recombination. LG-LDP showed augmented binding to EGFR-overexpressing cancer cells. Furthermore, an energized fusion protein LG-LDP-AE was prepared by integrating the active enediyne (AE) into LG-LDP molecule. By MTT assay, LG-LDP-AE displayed extremely potent cytotoxicity to cancer cells with IC50 approximate to 0.01nM. The results indicate that LDP binds to various human tumors and it might serve as a delivery carrier by integration of ligand oligopeptide to manufacture motif-based, targeted fusion proteins for cancer.

A superior drug carrier--aponeocarzinostatin in partially unfolded state fully protects the labile antitumor enediyne

Background

Neocarzinostatin is a potent antitumor drug consisting of an enediyne chromophore and a protein carrier.

Methods

We characterized an intermediate in the equilibrium unfolding pathway of aponeocarzinostatin, using a variety of biophysical techniques including 1-anilino-8-napthalene sulfonate binding studies, size-exclusion fast protein liquid chromatography, intrinsic tryptophan fluorescence, circular dichroism, and ¹H-¹⁵N heteronuclear single quantum coherence spectroscopy.

Results

The partially unfolded protein is in molten globule-like state, in which ~60% and ~20% tertiary and secondary structure is disrupted respectively. Despite lacking a fully coordinated tertiary structure for assembling a functional binding cleft, the protein in molten globule-like state is still able to fully protect the labile chromophore. Titration of chromophore leads the partially denatured apoprotein to fold into its native state.

Conclusion

These findings bring insight into conserving mechanism of neocarzinostatin under harsh environment, where even the partially denatured apoprotein exhibits protective effect, confirming the superiority of the drug carrier.

Is association of labile enediyne chromophore a mutually assured protection for carrier protein?

Most conjugate proteins undergo both conformational and stability changes on ligand removal. When architecture remains unchanged in the protein holo and apo forms, it is uncertain whether the protein stability also remains unaltered in both of the forms. Neocarzinostatin (NCS), a chromoprotein possessing a potent enediyne chromophore stands for such an instance. Protein-chromophore interaction has not been thoroughly explored previously due to a lack of strategies to independently and simultaneously monitor changes in the NCS conjugates. Here we report a method by which one can detect the signal exclusively from only one of the NCS conjugates without the spectral interference from the other. Stability of the NCS protein is significantly correlated to the protein-bound chromophore, irrespective of denaturation by heat, pH, urea, or ethanol. Despite the similarity in protein backbone conformation, protein stability of the NCS holo form diminishes and equalizes to that of the apo form when the chromophore is released and degraded. Although the enediyne chromophore is highly unstable, it intriguingly protects the protein by which it is protected. Significant mutual reliance between the carrier protein and its naturally associated ligand unveils important information on the NCS drug stability.

Protein–Small Molecule Interactions in Neocarzinostatin, the Prototypical Enediyne Chromoprotein Antibiotic

The enediyne chromoproteins are a class of potent antitumour antibiotics comprising a 1:1 complex of a protein and a noncovalently bound chromophore. The protein is required to protect and transport the highly labile chromophore, which acts as the cytotoxic component by reacting with DNA leading to strand cleavage. A derivative of the best-studied member of this class, neocarzinostatin (NCS), is currently in use as a chemotherapeutic in Japan. The application of the chromoproteins as therapeutics along with their unique mode of action has prompted widespread interest in this area. Notable developments include the discovery of non-natural ligands for the apoproteins and the observation that multiple binding modes are available for these ligands in the binding site. Mutation studies on the apoproteins have revealed much about their stability and variability, and the application of an in vitro evolution method has conferred new binding specificity for unrelated ligands. These investigations hold great promise for the application of the apoproteins for drug-delivery, transport and stabilisation systems.