(28d) Kinetics and Thermodynamics of Peptide Binding and Peptide Release from Oxyntomodulin and Aib2-Oxyntomodulin Nano-Fibrils

Obesity and its implication on type-2 diabetes have reached epidemic proportions worldwide. The urgent need to find a safe and effective therapies for treatment of these chronic disorders has gained tremendous attention in scientific community. Recent studies have shown that peptide hormones such as Oxyntomodulin (Oxm) and its analogue 2-aminoisobutyric acid oxyntomodulin (Aib2-Oxm) are extremely safe and effective for body weight loss as well as regulating the blood glucose level for treating type-2 diabetes patients. Despite their promising potential therapeutic effects they exhibit a short in vivo bioactivity in serum due to the enzymatic degradation. In order to address this challenge, it was reported that when these peptides are packed in fibrillar structures they are much less exposed to any degradation and hence their bioactivity is significantly improved. Therefore, it is essential for developing a systematic approach for explore strategies that control the formation of such fibrils and their subsequent controlled dissociation in vivo once injected in patients. An understanding of the fibrillation and dissociation processes of such peptides requires a quantitative knowledge of the kinetics and thermodynamics of fibril formation and dis-assembly; such measurements that to date have remained elusive for these peptides. Here, we report for the first time a through thermodynamic and kinetic study of fibril formation and fibril dissociation using quartz crystal microbalance with dissociation (QCM-D) and bulk experiments for Oxm and Aib2-Oxm. We investigated the speed of peptide attachment and detachment as well as the related thermodynamics processes via the Gibbs free energy change for the fibril formation as well as fibril dissociation. Our study showed that fibril formation of Oxm and Aib2-Oxm are both exothermic (i.e., spontaneous process) as the Gibbs free energy change associated with them is negative due to the peptide attachment whilst Oxm fibril are more stable than Aib2-Oxm fibril due to higher Gibbs free energy loss which is attributed to the larger α-helix contents in the secondary structure of Aib2-Oxm fibrils compared to Oxm fibrils. Moreover, the speed at which Oxm peptides bind to Oxm seeds towards fibrillation is much higher than the speed of Aib2-Oxm fibrillation which is related to the larger amount of required Gibbs free energy of activation for Aib2-Oxm peptide unfolding towards nucleation compared to Oxm peptide. These findings from the kinetics and the thermodynamics of fibril formation indicated the higher tendency of Oxm peptide for fibril elongation than Aib2-Oxm. Moreover, it was found that Aib2-Oxm fibrils are thermodynamically less stable than Oxm fibrils which makes Aib2-Oxm fibrils more feasible and faster to dissociate in vivo relative to Oxm fibrils which shows Aib2-Oxm exhibits much higher bioactivity in serum relative to Oxm due to the fact that Aib2-Oxm fibrils might send more free peptides in to serum in a given time compared to Oxm fibrils.