Functional metabolite assemblies-a review

Abstract

Metabolites are essential for the normal operation of cells and fulfill various physiological functions. It was recently found that in several metabolic disorders, the associated metabolites could self-assemble to generate amyloid-like structures, similar to canonical protein amyloids that have a role in neurodegenerative disorders. Yet, assemblies with typical amyloid characteristics are also known to have physiological function. In addition, many non-natural proteins and peptides presenting amyloidal properties have been used for the fabrication of functional nanomaterials. Similarly, functional metabolite assemblies are also found in nature, demonstrating various physiological roles. A notable example is the structural color formed by guanine crystals or fluorescent crystals in feline eyes responsible for enhanced night vision. Moreover, some metabolites have been used for the in vitro fabrication of functional materials, such as glycine crystals presenting remarkable piezoelectric properties or indigo films used to assemble organic semiconductive electronic devices. Therefore, we believe that the study of metabolite assemblies is not only important in order to understand their role in normal physiology and in pathology, but also paves a new route in exploring the fabrication of organic, bio-compatible materials.

Keywords: Functional amyloids; Metabolites; Nanostructures; Photonic crystals; Self-assembly; Supramolecular structures.

Fig. 1

The rearrangement of guanine-based photonic crystals in the panther chameleon serves as the molecular mechanism for controlled color change. Upper panel: Panther chameleon in a relaxed (a) and an excited (b) state. TEM micrograph of the guanine nanocrystals lattice from panther chameleon iridophores in a relaxed (c) and an excited (d) state, scale bar 200 nm (adapted from Teyssier et al. 2015)

Fig. 2

The anatomy and ultrastructure of the multilayer mirror of scallop eyes. a An image of five eyes of the scallop. b–d Cryo-SEM micrographs of high-pressure-frozen, freeze-fractured cross sections through the eye of P. maximus. b The mirror viewed perpendicular to the eye axis. White arrow indicates direction of on-axis incident light. c The tiled mirror viewed from above. d Crystals in adjacent layers, stacked directly on top of each other, viewed in a fracture through the mirror. e TEM micrograph of a single, regular square crystal extracted from the eye (adapted from Palmer et al. 2017b with permission)

Fig. 3

Enhanced iridescent of the cuticle. a–d The visual appearances of different colored jewel Chrysina scarab beetles (adapted from Vargas et al. 2016)

Fig. 4

Indigo field effect transistors. a Photograph of ambipolar indigo transistors on shellac. b Detailed composition of the OFET composed of completely natural and biodegradable compounds (adapted from Irimia-Vladu et al. 2012 with permission)

Fig. 5

Piezoelectric properties of glycine crystals polymorphs. a Open-circuit voltage response of a layer of γ-glycine seed crystals, orientated along the crystallo-graphic 2-axis. Manual force is applied along the crystallographic 2-axis over time. b A layer of γ-glycine seed crystals on a square copper 18 mm × 18 mm electrode insulated with varnish. c Manual compression of a γ-glycine seed crystal layer. Full compression of the layer as shown averaged a force of 0.172 N (adapted from Guerin et al. 2018 with permission)