The Structure of Cyclic Neuropeptide Somatostatin and Octapeptide Octreotide in the Presence of Copper Ions: Insights from Transition Metal Ion FRET and Native Ion Mobility-Mass Spectrometry

The conformation and function of somatostatin (SST), a cyclic neuropeptide, was recently found to be altered in the presence of Cu(II) ions, which leads to self-aggregation and loss of biological function as a neurotransmitter. However, the impact of Cu(II) ions on the structure and function of SST is not fully understood. In this work, transition metal ion Fo''rster resonance energy transfer (tmFRET) and native ion mobility mass spectrometry (IM-MS) were utilized to study the structures of well-defined gas-phase ions of SST and of a smaller analogue, octreotide (OCT). The tmFRET results suggest two binding sites of Cu(II) ions in both native-like SST and OCT ions, either in close proximity to the disulfide bond or complexed by two aromatic residues, consistent with results obtained from collision-induced dissociation (CID). The former binding site was reported to initiate aggregation of SST, while the latter binding site could directly affect the essential motif for receptor binding and therefore impair the biological function of SST and OCT when bound to SST receptors. Our results demonstrate that tmFRET is capable of locating transition metal ion binding sites in neuropeptides. Furthermore, multiple distance constraints (tmFRET) and global shape (IM MS) provide additional structural insights of SST and OCT ions upon metal binding, which is related to the self-aggregation mechanisms and overall biological functions.

Automated summary

The recent research discovered that copper ions (Cu(II)) can alter the conformation and function of the cyclic neuropeptide somatostatin (SST), leading to self-aggregation and loss of biological function as a neurotransmitter. However, the precise impact of Cu(II) ions on the structure and function of SST remains unclear. In this study, transition metal ion Fo''rster resonance energy transfer (tmFRET) and native ion mobility mass spectrometry (IM-MS) were used to investigate the structures of well-defined gas-phase ions of SST and octreotide (OCT). The results indicate that Cu(II) ions bind to two specific sites in both native-like SST and OCT ions, either near the disulfide bond or complexed by two aromatic residues. These binding sites can initiate aggregation of SST and affect the essential motif for receptor binding, impairing the biological function of SST and OCT when bound to SST receptors. The study demonstrates the capability of tmFRET to locate transition metal ion binding sites in neuropeptides and provides structural insights into the self-aggregation mechanisms and overall biological functions.