In this lecture, we began by analyzing and describing the biominerals found in nature. These biomaterials correspond mainly to the very abundant mineral sources found on Earth: calcium carbonates, phosphates, oxalates and sulfates, silica and silicates and iron-based compounds. Biomineralization is a biological phenomenon which, in a living organism, enables the transition from a solubilized state of metallic elements (ionic or molecular) to a mineralized solid state. During these processes, four main components are systematically involved: the extracellular organic macromolecular framework or template, specialized cells, small organic molecules, enzymes... and mineral constituents. Using calcium carbonate-based biominerals such as coccoliths and mother-of-pearl as models, we take stock of current knowledge and the mechanisms involved in these processes, which share a number of common elements:
- The cell controls its immediate environment by creating a specific physico-chemical environment in which an organic matrix of biomacromolecules is constructed.
- This organic matrix serves as a template, a space in which mineralization will take place. This organic template defines the structure, texture and shape of the mineralization space, as well as its functionalities. It is often made up of compartments or interfibrillar spaces built up through the formation of lyotropic liquid crystal assemblies based on chitin or collagen. In intermediate stages, the formation of gels, porous or globular matrices is often observed.
- This space is activated by the introduction of biomolecules that interact with the organic matrix and/or mineral precursors to nucleate the biominerals (phosphoproteins in the case of dentin; aspartate-rich proteins for mollusc shells, sulfated proteoglycan in the case of eggshells, etc.).
- The introduction of mineral precursors in the form of cations, or oligomers, takes place by diffusion and transport via vesicles, siderophores, complexing proteins or enzymes, or biovalves.
- Control of the size and shape of the "crystallites" formed, which can be addressed by genes, takes place by confinement and/or inhibition. This control is very often kinetic and can lead in a first stage to the formation of amorphous mineral phases, and to unconventional growth nucleation processes such as those described under the term mesocrystallization (see lessons 5 and 6 from 2013-2014[1]).
[1] Abstract available in theAnnuaire du Collège de France 2013-2014.
