Insulin is responsible for the regulation of glucose metabolism and energy storage in the human body.[1] It is the primary treatment option for diabetes mellitus,[2] a disease affecting millions worldwide. Aside from its pharmacological appeal, insulin has piqued interests in the fields of chemistry and structural biology owing to the difficulty of its synthesis[3] and the elusiveness of its mechanism of action,[4] respectively. Methodologies developed by our group for the on-resin ring-closing metathesis (RCM) of peptidomimetics[5] have allowed the synthesis of [A6-11-dicarba]-insulin, a derivative of insulin whereby an unsaturated all-carbon bridge is substituted for its intrachain cystine moiety.
The RCM methodology affording [A6-11-dicarba]-insulin results in two isomeric products with distinct RP-HPLC retention times. These isomers were assigned as cis and trans geometric isomers using a hybrid synthetic/analytical approach. The isomers were found to have disparate biological activities despite differing only in olefin geometry. The cis isomer was found to possess biological activity comparable to that of the native peptide whereas the trans isomer was found to be inactive by comparison. This startling disparity indicates that the orientation of the insulin A6-11 cystine is crucial for binding and activation of the insulin receptor.
Efforts have since focussed on producing derivatives that incorporate changes to the crucial A6-11 dicarba linkage. Four chiral mutants of the [A6-11-dicarba]-insulin isomers, substituting D-allylglycine in the A6 and A6+11 positions prior to RCM, have been synthesised and assayed for biological activity. Attempted synthesis of two additional mutants, incorporating D-allylglycine at the A11 position, has shed light on the importance of this region not only for biological activity but also for correct folding during synthesis. This work contributes to a burgeoning understanding of the biochemical mechanism underpinning insulin’s activity as well as the structure-activity relationship of its receptor. This knowledge is crucial for the development of improved treatments for diabetes mellitus.