Role of Steric Effects in Protein-Directed Enediyne Cycloaromatization of Neocarzinostatin

• Published in: Chemistry : a European journal 2011-02, Vol.17 (5), p.1493-1506
• Main Authors: Chi, Hung-Wen, Chien, Yi-Chih, Liu , Cheng-Yun, Tseng, Chin-Jui, Lee, Yan-Jiun, Chan, Ja-Lin, Chu, Yu-Ru, Chin, Der-Hang
• Format: Article
• Language: English
• Subjects: antibiotics; Antibiotics, Antineoplastic - chemistry; Carrier Proteins - chemistry; Carrier Proteins - metabolism; Chemistry; cyclization; Cysteine - chemistry; Cysteine - metabolism; Disulfides; Enediynes; Enediynes - chemistry; Holes; Ligands; Magnetic Resonance Spectroscopy; Molecular Structure; Mutation; Mutations; Pathways; Protein Binding; Protein Conformation; protein modifications; protein structure; Proteins; Rigidity; Rupture; structure-activity relationships; Sulfhydryl Compounds - chemistry; Zinostatin - chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201002330

The antibiotic neocarzinostatin comprises a carrier protein with a well‐defined cavity for accommodating an active enediyne chromophore. The protein has two disulfides, one (Cys37–Cys47) lies on the cavity bottom and the other (Cys88–Cys93) in a constrained short loop. When the chromophore is not bound to the protein, a thiol‐induced cycloaromatization of the enediyne into a tetrahydroindacene derivative is responsible for the potent antitumor activity. When it is protein‐bound, the protein diverts the cycloaromatization pathway to form a distinct hydroxyisochromene‐type product. How the protein directs the enediyne chemistry is an interesting puzzle, and various suggestions have been proposed in the past. We screened more than fifty thiols and manipulated conditions to locate reaction features and search for factors that could influence the protein directing strength. Thiol‐ and oxygen‐concentration‐dependence studies suggested that disulfides, which maintain the steric rigidity of the protein, could play a key role in diverting the cycloaromatization pathway. For direct proofs, we made mutations at each of the two disulfides by replacing sulfur atoms with oxygen. Circular dichroism and two‐dimensional NMR spectroscopy studies suggested that the mutations changed neither the protein conformation nor the ligand interactions. Analyses of the thiol‐induced cycloaromatization revealed that rupture of Cys37–Cys47 made the protein almost completely lose its chemical directing ability, whereas rupture of Cys88–Cys93 had only a minor influence. The results demonstrated that the steric rigidity of the binding cavity, but not necessary the whole protein, played an important role in the protein‐directed mechanism. Protein directing chemistry: The enediyne cycloaromatization pathway depends on the steric rigidity of the binding cleft (see figure). Wild‐type protein and protein without the Cys88–Cys93 linkage directed the cyclization to produce 2, whereas protein with the Cys37–Cys47 link ruptured could only produce 1.