Antibodies mimic natural oxidosqualene-cyclase action in steroid ring a formation

Article abstract of DOI:10.1021/ja993054l

Cationic cyclizations are among the most demanding reactions that have been catalyzed by antibodies. These studies provided valuable mechanistic insights while opening up the possibility of formation of steroidal carbon frameworks. However, they have involved substrates that contained an aryl sulfonate group adjacent to a primary carbon center not observed in natural cationic cyclization processes. This paper presents an extension of our earlier work, now focusing on substrates analogous to those seen in triterpene biosynthesis. Three antibodies, 15D6, 20C7, and 25A10, have been generated by immunization with an 4-aza-steroid aminoxide hapten (termed HA8) that initiate the cationic cyclization of an oxidosqualene derivative and catalyze the formation of ring A of the lanosterol nucleus at neutral pH. Antibody HA8-25A10 kinetically resolved its racemic substrates. Design of the substrate was based on a dual-anchor model for specific binding that consists of displaying a functional group at the head (epoxide trigger) and the tail (amide functionality) of the otherwise hydrophobic poly-ene chain. The assay involved solubilization of the substrates with 0.2% surfactant. No ring formation was detected in the absence of antibody catalyst. The uncatalyzed epoxide hydrolysis was slow and did not deprive the antibody of substrate. Observations in the field of enzymic poly-ene cyclizations suggest that subtle changes in the substrate structure may well lead to multi-ring formation under the influence of the current catalyst.