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

Cationic cyclizations are among the most demanding reactions that have been catalyzed by antibodies. These studies provided valuable mechanistic insig hts while opening up the possibility of formation of steroidal carbon frame works. However. they have involved substrates that contained an aryl sulfon ate group adjacent to a primary carbon center not observed in natural catio nic cyclization processes. This paper presents an extension of our earlier work, now focusing on substrates analogous to those seen in triterpene bios ynthesis. Three antibodies, 15D6, 20C7, and 25A10, have been generated by i mmunization with an 4-aza-steroid aminoxide hapten (termed HA8) that initia te the cationic cyclization of an oxidosqualene derivative and catalyze the formation of ring A of the lanosterol nucleus at neutral pH. Antibody HA8- 35A10 kinetically resolved its racemic substrates. Design of the substrate was based on a dual-anchor model for specific binding that consists of disp laying a functional group at the head (epoxide trigger) and the tail (amide functionality) of the otherwise hydrophobic poly-ene chain. The assay invo lved solubilization of the substrates with 0.2% surfactant. No ring formati on was detected in the absence of antibody catalyst. The uncatalyzed epoxid e hydrolysis was slow and did not deprive the antibody of substrate. Observ ations in the field of enzymic poly-ene cyclizations suggest that subtle ch anges in the substrate structure may well lead to multi-ring formation unde r the influence of the current catalyst.