Characterization of GdFFD, a d-Amino Acid-containing Neuropeptide That Functions as an Extrinsic Modulator of the Aplysia Feeding Circuit

Abstract

Background: L-to-D conversion of an amino acid in a neuropeptide can be required for bioactivity. Results: A new D-amino acid-containing peptide (DAACP), GdFFD, shows stereospecific bioactivity in the feeding circuit. Conclusion: Our findings broaden the importance of this unusual post-translational modification, providing new methods to accelerate DAACP discovery. Significance: GdFFD is the first DAACP showing bioactivity in a well defined circuit.

A d-Amino Acid-Containing Neuropeptide Discovery Funnel

“…5,6 In neuropeptides, this involves the enzymatic conversion of one residue near the N-terminus from an l -amino acid to a d -amino acid. 5,7−13 …”

“…6 One way to overcome this difficulty is via the use of MS/MS-based techniques and comparing peptides with only l -amino acids to their DAACP epimers. 7,14−18 Recently, MALDI-TOF/TOF was used for the discovery of d -Leu in the peptide GH-2. 19 This study also established a stereosensitive fragmentation effect of d -amino acids from various amino acid classes on different peptide sequences, which can aid in the discovery of DAACPs through MALDI-TOF/TOF by observing the fragmentation ratios of specific peptides.…”

“…Another way to facilitate the discovery is to use bioinformatics to find neuropeptides that are homologous to known DAACPs in related species. 7 What is needed is a set of measurement techniques capable of identifying DAACPs in a nontargeted manner, rather than targeting a suspected DAACP based on homology.…”

The stereoselective separation of serine containing peptides by zwitterionic ion exchanger type chiral stationary phases and the study of serine racemization mechanisms by isotope exchange and tandem mass spectrometry

“…The enantioseparation can then be carried out directly, by so-called enantioselective chromatography [27], or indirectly via the generation of diastereomers through derivatization with enantiomerically pure reagents followed by achiral chromatography [6]. Furthermore, Bai et al reported a technique of directly separating diastereomeric DAACPs by applying MALDI TOF/TOF measurements, yielding significantly different fragmentation patterns [23,28].…”

“…Even more specifically, DAACPs have been shown to represent a very specific form of posttranslational modification and to play an important role in cell signaling pathways [20][21][22][23]. The partial isomerization of certain AAs in all l-AA containing peptides has been shown to drastically alter the peptides' biological activities [22].…”