Understanding Magnetite Biomineralisation: The Effect of Short Amino Acid Sequences on the {100} and the {111} Surface
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Understanding Magnetite Biomineralisation: The Effect of Short Amino Acid Sequences on the {100} and the {111} Surface Amy E. Monnington and David J. Cooke Department of Chemical & Biological Sciences, University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK ABSTRACT Magnetite (Fe3O4) formation within Magnetospirillum magneticum strain AMB-1 occurs under the influence of the Mms6 protein. It is hypothesised that if key iron binding sites within the C-terminus of the Mms6 protein are substituted for alanine, the protein’s overall iron binding ability is diminished. In this study, an atomistic model of Mms6-driven magnetite formation was developed and the attachment of series amino acid repeats (alanine-alanine, alanine-glutamic acid & glutamic acid-glutamic acid) to the {100} & {111} magnetite surfaces were investigated. Our results suggest the substitution of glutamic acid for alanine residues significantly reduces iron binding affinity of the system, thus confirming the hypothesis. In addition, it is shown that the surface of preferable attachment is the {111} magnetite surface. INTRODUCTION Biomineralisation is the process by which living organisms form minerals. The earliest known example of biomineralisation is that of the biosynthesis of magnetite (Fe3O4) which can be traced back ~2000 Ma [1]. A major step forward in understanding the principles of magnetite biomineralisation occurred with the discovery of Magnetotactic bacteria (MTB) in 1975 [2]. These bacteria use specific proteins to aid the uptake of iron ions from the surrounding aqueous environment, and subsequently produce intracellular membrane-bound single nano-crystals of pure magnetite, called magnetosomes. They do this via a largely uncharacterised process known as biologically controlled mineralisation (BCM) [3]. The resulting magnetic nanoparticles (MNPs) align in a chain, imparting a magnetic dipole to the bacterial cell, allowing the bacteria to orient along the Earth’s magnetic field lines in a process known as magnetotaxis [4]. Whilst the size and morphology of the MNPs formed is uniform, the exact composition observed is often species, or strain, dependant, suggesting an element of high biological control [5]. Magnetospirillum species-derived MNPs are generally of cubooctahedral shape, based on a combination of the {100} & {111} crystallographic faces. One such strain of this species is Magnetospirillum magneticum AMB-1, which possesses magnetosomes that are elongated along the {111} axis. Arakaki et al. identified a class of proteins within M. magneticum AMB-1 that are tightly associated with the magnetite crystal surface showing common features in their amino acid sequences [6]. Particular interest has been shown in the Mms6 protein, whose amino acid sequence is amphiphilic; possessing a membrane-spanning hydrophobic N-terminus, and a highly acidic hydrophilic C-terminal region which contains dense carboxyl and hydroxyl groups that are able to bind iron ions [7]. Numerous commercial applications for bacterial MNPs have been suggest
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