David Mast earned the B.S. degree in chemistry in 2017 and is currently a PhD candidate in the group of Jonathan Sweedler in the Department of Chemistry, University of Illinois, Urbana-Champaign.

About the Seminar:

D-amino acid-containing peptides (DAACPs) result from an unusual post translational modification (PTM) where an all-L peptide is isomerized at a single amino acid residue. The biological relevance of DAACPs is not well understood because the prevalence of isomerization among animal neuropeptides is largely unknown. DAACPs are particularly difficult to identify and measure due to challenges differentiating DAACPs from all-L peptides in complex biological extracts using modern mass spectrometry (MS)-based approaches. Furthermore, stereochemistry is not routinely evaluated in “peptidomics” experiments. Here, non-targeted stereoselective enzymatic screening, liquid chromatography tandem mass spectrometry (LCMS/MS) and trapped ion mobility spectrometry (TIMS) were used to identify discover novel DAACPs in the central nervous system (CNS) of the Aplysia californica.

Five novel DAACPs were identified from the Aplysia Pleurin precursor: MdFYTKGSDSDYPRINH2, SdFYTTGNGNHYPRI-NH2, SFdYTTGNGNHYPRI-NH2, GdIFTQSAYGSYPRV-NH2, GIdFTQSAYGSYPRV-NH2. Enzymatic screening also identified YdLDHLGSSLV, YdLDGIASSLI and FdMRGF-NH2 from two uncharacterized proteins. The DAACPs identified indicate that an isomerase is present in cells producing these neuropeptides and has a specificity for aromatic amino acids (Phe and Tyr) as well as aliphatic amino acids (Ile, Leu, and Met) in the second position. These results also suggest an isomerase exhibits loose preference for the second versus third position. Multimodal MS-based approaches were successfully employed to identify eight novel DAACPs. The number of DAACPs found in the Aplysia CNS indicates isomerization is a more common PTM than previously thought and could be an overlooked PTM in other organisms.