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The dynamics of matrix protein self-assembly and mineralization
James DeYoreo - Northwest National Laboratory
Monday, September 29, 2014, 4:00-5:00 pm Calendar
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Abstract

 

Self-assembly of organic matrices and subsequent directed nucleation of the mineral constituents is a widespread paradigm in biomineral formation.  The architecture of the underlying matrix imposes order on the nucleating mineral species.  For example, in bone collagen monomers form into triple helices, which then self assemble into well-organized fibrils.  Within these fibrils highly-oriented hydroxyapatite crystals nucleate and grow with a specific crystal face in contact with the collagen fibrils. In order to understand the underlying physical controls governing both matrix self-assembly and biomolecule-directed crystallization, we are using in situ AFM, TEM and FTIR, combined with molecular dynamics (MD) to investigate these processes.

 

Investigations into the assembly of extended protein structures formed from both collagen and microbial S-layer membrane proteins reveal the key role that is played by conformational transformations in controlling the pathways and kinetics of assembly.  The large barriers associated with these transformation renders them rate-limiting in forming the ordered structures.  Consequently, before the ordered state can emerge, these systems must be driven to condense into metastable, liquid-like clusters in which protein-protein contact times are large.  The emergence of order within these clusters catalyzes the further transformation and attachment of the monomeric proteins.  In addition, the pathway to the final ordered state can pass through transient, less-ordered conformational states.  Thus the concept of a folding funnel with kinetic traps often used to describe folding of individual proteins is also useful in considering protein assembly.

 

Studies of mineral nucleation dynamics on alkyl thiol SAMs and collagen matrices are then used to show that these scaffolds promote nucleation through a reduction in interfacial energy. However, nucleation of the amorphous phase in the calcium phosphate-on-collagen system is observed at supersaturations that are too low to be explained by classical nucleation theory (CNT).  The existence of muti-ion complexes is shown to provide a low-barrier pathway to crystallization that circumvents the large barriers predicted by CNT.  Finally, both AFM and TEM studies of matrix mineralization illustrate the strong control that ion-matrix binding has in defining the location of nucleation, both through its direct influence on interfacial energy and through sequestration of ions to build up the local concentration.  Taken together, these results provide new insights into the mechanisms controlling biological crystallization, from formation of the initial matrix to the maturation of final crystalline structures

 

 

 

 

 

This talk is organized by Star Jackson