Lecture 2: Biomineralisation

8/3/2019 Lecture 2: Biomineralisation

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Advanced Bioinorganic and Medicinal Inorganic chemistry

Lecture 2: Biomineralisation

Biomineralisation is the synthesis of minerals from simple compoundsby organisms.

This can be for a variety of purposes, including:

to lend mechanical strength to bone (calcium phosphates) to orient magnetotactic bacteria (magnetite particles-

Fe3O4)

for storage of iron (ferritin)

Biominerals also include shells, teeth and many different types ofskeleton.

These compounds can be regarded as the true inorganics in life asthey have no organic component c.f. metalloenzymes where mostof the molecule is organic.This is not completely true- as will be discussed.

One particular area of interest is the molecular control mechanisms

that biological systems use to form the well-defined inorganic solidstate materials.

In fact both inorganic and organic materials are used in exo- and endoskeletons.Sharks and invertebrates use chitin (polysaccharide) structures that are largelyorganic.

In their bones, vertebrates use biomineral constructions that are composite organic/inorganic consisting of calcium hydroxyapatite and an organic matrix.

Why is this the case? the inorganic component brings hardness and pressure resistance without

which larger land living animals could not exist the organic matrix consists of collagen, glycoproteins and polysaccharides to

give elasticity and tensile strength.

Much of modern materials science is concerned with hybrid materials leading to someoverlap in the techniques used in these investigations.


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Chemical

composition

Mineral form Function/ examples

Calcium carbonate

CaCO3 CalciteAragoniteVateriteAmorphous

Exoskeletons in corals,egg shells, molluscshellsGravity sensor

Calcium phosphatesCa10(OH)2(PO4)6 hydroxyapaptite Endoskeletons (human

and other vertebratesbones and teeth

Calcium oxalateCaC

2O

4.nH

2O

Whewelliteweddelite

Calcium storage anddefense of plants

Metal sulfatesCaSO4.2H2OSrSO4BaSO4

GypsumCelestitebaryte

Gravity sensors orexoskeletons

Amorphous silicaSiO2.nH2O amorphous Valves of diatoms and

defence mechanisms inplants

Iron oxidesFe3O4,-Fe(O)OH.5Fe2O3.9H2O

MagnetiteGoethite,lepidocrocite,ferrihydrite

Magnetic sensorsTeeth of chitonsIron storage

One property of biominerals must be low solubility under physiological conditions.

Formation can take place intracellularly, at the cell surface or in the extracellularspace.

Important biomineral functions: use as part of mechanically robust instrument and weapons e.g. teeth sensor components e.g. magnetotactic bacteria passive mechanical protection of animal e.g. shells or spikes

Both hardness and morphology contributeto these functions.

Organic components can have a templatefunction, leading to formation of different

phases from the unrestrained system viacatalysis and nucleation control.


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Biominerals have to be formed and dissolved on much shorter timescales thangeological minerals.

Calcium carbonate deposition is general controlled by an equilibrium shift due to theconsumption of CO2 but there are many processes with greater control for the

formation of other biominerals.

Nucleation and crystal growth: careful control of these processes can beachieved via highly regulated active transportmechanisms and via specific modulation ofsurface reactivity.

Often biominerals do not coincide with the regularly encountered forms of theinorganic mineral. This can be achieved by spatial limitations or by other chemicals inthe growth medium.

EXAMPLES

Calcium phosphate

Vertebrate bones: 30% elastic fibrous proteins (mainly collagen)55% inorganic components embedded in glycoproteins

(hydroxyapatite)15% calcium carbonate, silica, magnesium carbonate, citrate,

other metal ions.

Bone can be studied with 31P NMR to determine mineral forms.

Phosphoproteins are arranged at regular intervals on the collagen fibres and thespacing is correct to allow calcium binding at distances corresponding to thecrystalline inorganic phase.

Non-biological uses of bones: fertilisers (apatite/ inorganic material.Collagen glue (organic component)

Continuous formation of bone occurs in a peripheral zone with an outer and innerlayer of connective tissue containing osteoblast cells. These cells are graduallyincorporated into the hardening structure and turn into bone cells.

There is continuous exchange of the calcium in bones. This can lead to substitution byother metal ions, which may be present by poisoning, altering the mechanical

properties of the bone.

Ca2+ + 2HCO3- CaCO3(s) + CO2(g) + H2O

photosynthesis


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Calcium carbonate

The CaCO3 crystals formed in egg and mollusc shells grow in amatrix of proteins and polysaccharides. Significant amounts ofcarbonic anhydrase are present to produce the required HCO3

-. Marine organisms such as corals or molluscs form the mineral in

large amounts due to photosynthetic activity which assimilates CO2,increasing the pH of the surrounding medium and shifting theequilibrium towards precipitation.

Amorphous silica

It is mainly utilised in unicellular organisms, sponges and several plants. Insome plants it occurs in the cell membranes as a passive deterrent. The brittletips of the stinging hairs of some nettle plants are made of amorpous silica.

Strontium and Barium sulfates

Acantharia is a unicellular plankton which features an exoskeleton with asymmetry determined by the inherent crystal structure of the SrSO4 that formsit. The exoskeleton consists of exactly twenty single crystals which canassume complex shapes, and nearly perfect D4h symmetry in some cases.

EXAMPLE: Ferritin

When iron is released into the cell from transport proteins it musteither be incorporated into a functional role or stored.

Ferritin serves as an iron storage site in some animals and plants,with about 13% of the iron in the human body present in this form.

Apoferritin is the iron-free form of the protein and has beenstructurally characterised. It is highly symmetric and roughlyspherical within an inner diameter of 80 and an outer diameter of120 . The inside of the protein is lined with hydrophilic residues asare the three fold symmetric channels- suggesting a route of entry for

the metal.

It can accommodate up to 4,500 iron atoms with a typicalfilling of approx. 1,200.

Ferritin has been studied with a number of spectroscopic techniquesincluding EXAFS, Mossbauer and optical spectroscopy.

It consists of octahedrally coordinated Fe(III) ions joined by bridging oxideand/or hydroxide ions.

The most consistent structure with the spatial parameters is a close packedarray of oxygen atoms with iron atoms partially occupying the octahedralholes.

There is insufficient data to define the structure unequivocally.

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Lecture 2: Biomineralisation