Chemistry Reference
In-Depth Information
permissible ring conformations for each monosaccharide in the chain and the potential for
chemical group modification, and it becomes clear that oligo- and polysaccharides can
generate incredible molecular structural diversity for recognition purposes.
In the previous section, oligosaccharide-modified proteins, or glycoproteins, were
introduced. The attachment of oligosaccharide chains to a protein is usually via an
O
−
glycosidic linkage with either serine
−
OH or threonine
−
OH sidechain groups, or, a
N
glycosidic linkage with the amide nitrogen of asparagine. Most glycoproteins are
either O- or N-linked, but there are some which feature both attachment types. The
addition of oligosaccharide chains to a protein conveys an additional layer of molecular
recognition capabilities, in that the oligosaccharide chains can be recognized by other
proteins, and, they themselves may recognize and bind to other macromolecules, charged
groups or ions, and mineral interfaces.
Polysaccharides which are involved in the formation of mineralized tissues belong to
the family of structural polysaccharides. Structural polysaccharides typically feature
linear chains with very little branching, and, possess
−
glycosidic bonds which allow
individual polysaccharide chains to adopt an open structure and form hydrogen bonds
with other linear polysaccharide chains. Two structural polysaccharides are worth noting:
β
β−
O
−
-chitin, a linear chain polymer of N-acetylglucosamine monosaccharides found in the
extracellular matrices of invertebrates, and glycosaminoglycans (GAGs), linear chain
polymers comprised of amino disaccharide repeat units that comprise proteoglycans of
vertebrate connective tissues (Lowenstam and Weiner 1989).
Membrane assemblies
Lipids, consisting of two fatty acid chains, a glycerol group, and a phosphate group,
will assemble into a monolayer or bilayer structure in aqueous solutions, in which the
amphipathic or polar/hydrophobic nature of each lipid drives the assembly process, i.e.,
polar headgroups are oriented towards the aqueous environment, and the hydrophobic
fatty acid chains are oriented away from the solvent and towards each other in the interior
of the assemblage (Fig. 7). The simplest assembly is the micelle, a spherical monolayer
assembly where all fatty acid chains are oriented inwards. This assembly is also adopted
by detergents and long chain alcohols in water. Small assemblies of lipids can form
continuous bilayer structures called liposomes or vesicles, compartmentalized structures
which contain an aqueous interior. Micelles, liposomes, and vesicles can fuse together or
with other membrane bilayer assemblies.
Because of the amphipathic nature of membrane bilayers, other amphipathic
molecules, such as cholesterol, proteins, or detergents, can insert within the bilayer
structure and become part of the overall assembly. However, polar molecules cannot
easily transfer across the bilayer, and require the assistance of polypeptide-forming
channels within the bilayer, or induction of small ruptures of the bilayer via chemical
methods, to traverse the bilayer. The fact that the polar headgroups of the micelle
(exterior) or bilayer (both interior and exterior) are available for interaction with metal
ions and anions permits the use of membrane surfaces to nucleate inorganic solids
(Lowenstam and Weiner 1989). In addition, membrane vesicles known as matrix vesicles
(see Fig. 1) are produced by biomineral-competent cells to synthesize and deposit
inorganic solids in the extracellular matrix (Lowenstam and Weiner 1989), and,
magnetotactic bacteria use intracellular vesicles to compartmentalize magnetite deposits
within the cell for magnetic field detection (Lowenstam and Weiner 1989).
