MODEL OF CRYSTALLIZATION IN A CONFINED SPACE APPLIED TO IRON UPTAKE AND RELEASE BY FERRITIN

Many living organisms Store iron in solid form, Fe(lll), as a crystal in the inner cavity of the ferritin molecule. When iron is needed for biosynthesis, a reducing agent reduces Fe(lll) into the soluble form F e2+ released by ferritin. Crystallization and release processes are re versible, and their rates evolve in an identical way as a function of the number n of iron atoms in the molecule. The rate increases with n, showing a maximum value when n is approximately 1,300, and then stabi lizes for the highest values of n, which can reach 4,500. On the other hand plotting the amount of released iron as a function of time gives curves with a sigmoid shape. The proposed model was based on the theo retical description of different steps involved in crystal growth insi de the protein shell: several independent crystals grow freely at the inner protein wall, and then a distribution function takes into accoun t possible overlapping of different crystallite clusters, whose furthe r growth is limited by diminution of the available space inside the ca vity. The kinetics derived was then used to calculate the release curv e as a function of time. Solving the system of differential mass-balan ce equations was simplified by describing the ferritin population as a large discrete distribution of species. The model fully fitted and ex plained the variation in the crystallization rate with n, and the sigm oid shape of the release curve as a function of time obtained experime ntally in a thin-layer electrochemical cell.