Chemistry Reference
In-Depth Information
polysaccharides (>30 monosaccharides/chain) are created intracellularly by polymerizing
enzymes known as transferases. Some important polysaccharides which play a role in the
biomineralization process include
-chitin—a scaffolding polysaccharide found in
exoskeleton and in invertebrate mineralized tissues—and glycosaminoglycans, sugar units
which comprise proteoglycan complexes found in bone, cartilage, and tooth dentine
(Lowenstam and Weiner 1989). The formation of sugar polymers takes place in both the
ER and in the Golgi apparatus, where the extracellular sugar polymers are packaged into
secretory vesicles and exported outside the cell.
Lipids and membrane assemblies
. Lipids are macromolecules consisting of a
glycerol backbone, two fatty acids which are linked to
β
OH groups of the glycerol
molecule via ester bonds, and, and polar headgroup (i.e., phosphate, or phosphate +
organic molecule) which is covalently linked to the remaining
−
OH group of the glycerol
molecule (Fig. 7). The synthesis of lipids takes place on the surface of the ER membrane
via a set of enzymes which links the individual components together. Eventually, the
synthesized lipids assemble together and form membrane fragments which fuse with the
ER membrane. In turn, the ER membrane buds off transport vesicles which travel to the
Golgi apparatus. However, there are some intracellular vesicles which also form (e.g.,
peroxisomes, lysosomes, etc.), and these are earmarked for specific functions within the
cell. Eventually, the fusion of secretory vesicles with the cytoplasmic membrane allow
newly synthesized lipids to regenerate the cytoplasmic membrane.
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MOLECULAR PERFORMANCE
Now that we understand how proteins are produced from the genome, we will now
explore macromolecular structure and function. There are three categories of
macromolecules that have been implicated in the biomineralization process: proteins,
polysaccharides, and membrane assemblies. These will be discussed in the following
sections.
Protein structure
Essentially, the amino acid sequence of every protein dictates the structure of each
protein, its shape, and its function. A protein is basically a polymer comprised of amino
acids linked together by chemical bonds (Fig. 5). There are twenty naturally-occurring
amino acids in Nature, and, there are also artificial or non-natural amino acids that are
made in the laboratory, plus natural modifications (phosphorylation, sulfation,
hydroxylation, etc.) that occur to amino acids during post-translational processing. Each
amino acid has an a central carbon atom (
α−
carbon) to which four substituents are
attached: (1)
hydrogen atom, (4)
sidechain chemical group (R). The polypeptide chain amide bond, also referred to as the
peptide bond, consists of a head-to-tail condensation of the
α−
amino group, (2)
α−
carboxylate group, (3)
α−
α−
carboxylate group with the
α−
amino group of another amino acid, with loss of a water molecule. The most important
feature of the amino acids are the 20 natural sidechain groups, which can be categorized
into hydrophobic, anionic, cationic, and polar. The reader is urged to consult standard
biochemistry textbooks for a complete listing of amino acid sidechains and their
chemistry. Thus, depending on the amino acid composition of a polypeptide, it may
possess a net charge based upon the summation of all sidechain charges at a given pH,
including the free amino and carboxylate termini. The linear ordering of a polypeptide
chain is referred to as the
primary sequence
or
primary structure
, and it is directly derived
from the linear codon usage in the DNA which codes for the protein. Moreover, it is the
amino acid sequence that allows us to define similarities and differences in structure and
function between different proteins.
