Zinc accumulation in the <i>jaw of Nereis aibuhitensis</i>

Zn accumulates in the jaws of green worms, Nereis aibuhitensis, a phylum of annelid worms, to enhance the mechanical properties of the jaws for predation and migration. In this study, we precisely mapped the localization of zinc and identified the matrix proteins responsible for its binding in the jaws. X-ray diffraction (XRD) analysis revealed no distinct crystalline peaks in powdered jaw samples, indicating that zinc exists predominantly in a non-crystalline form. X-ray absorption fine structure (XAFS) spectra further demonstrated that zinc coordinates with organic molecules containing imidazole groups, implicating histidine (His) residues in zinc binding. Elemental analyses by Inductively coupled plasma mass spectrometry (ICP-MS) and particle-induced X-ray emission (PIXE) showed zinc concentrated on the inner side of the jaw tip, while halogens were mainly localized on the outer surfaces of the jaw. A comparison of protein extracts from the zinc-rich jaw tip and the zinc-poor bottom showed a specific protein band at the tip region, identified as a His-rich protein (Nai11527). These findings reveal a previously uncharacterized mechanism whereby histidine-rich proteins bind zinc to reinforce jaw structure. This study advances our understanding of biomineralization and offers a promising blueprint for the design of novel bio-inspired materials with enhanced mechanical properties.