Chemical Interactions between Mercurial Species and Surface Biomolecules
163
green non-sulphur bacteria, green sulphur bacteria, Spirochetes, Planctomyces,
Cytophagas, Gram-positive bacteria, Purple bacteria, Cyanobacteria, Flavobacteria, Thermotoga and Aquifex; (2) Domain Archaea: extreme halophiles, Methanosarcina, Methanobacterium, Methanococcus, Thermococcus, Thermoproteus
and Pyrodictium; and (3) Domain Eukarya: Entamoebae, slime moulds, animals,
fungi, plants, ciliates, flagellates, trichomonads, micro sporidia and diplomonads.
The phylogenetic tree, considering these three domains, revealed that since the
appearance of the "universal ancestor", two different lines of evolution have,
evolved one in the direction of the Domain Bacteria, and another in the direction
of the Domain Eukarya. However, this evolutionary tendency changed abruptly,
giving rise to another branch from the ongoing line to Domain Eukarya, and the
Domain Archaea appeared.
Irrespective of considerations about these evolutionary lines, it is not possible
to describe the surface structures for all living organisms, even if we consider the
updated phylogeny that considers only three domains. However, based on the
available information on the chemistry of some well-studied biological surfaces,
some groups of organisms can be selected for study and, based on the analysis of
their surfaces, it is possible to associate their biochemistry with the uptake of
mercury or any other heavy metal, giving an indication of the reaction surface
sites in certain groups of organisms.
5
The Cytoplasmic Membrane and its Potential Reaction Sites
The cytoplasmic membrane completely surrounds the cells, acting as a highly
selective barrier. It is a phospholipid bilayer whose hydrophobic groups are
directed inward, while the cytoplasm and the hydrophilic groups are arranged
outwards. This selective barrier also presents several embedded proteins, being
similar in overall structure for both prokaryotes and eukaryotic cells. Nevertheless, even though the cytoplasmic membrane is not the outermost layer in the
cells, its basic structure provides some potential binding sites for mercury and
other heavy metals. As the cell wall, located outside the cytoplasmic membrane,
does not act as a selective barrier, heavy metal ions can be transported into the
cytoplasmic membrane. Thus, its overall structure can be discussed indicating
potential binding sites.
Basically, the cytoplasmic membrane can be considered as constituted of lipids
and proteins, mainly. Both components of this membrane contain potential
binding sites for the uptake of mercury. A schematic representation of a
phospholipid molecule is represented in Fig. 1.
It can be observed that the non-polar head of the phospholipid is of a
hydrophobic nature, not presenting any sort of binding sites for the uptake
of mercury or any other heavy metal. However, in the polar head of the molecule
there is a phosphate group, negatively charged, acting as a potential mercury ligand. As previously mentioned, the cytoplasmic membrane is a bilipid
163
green non-sulphur bacteria, green sulphur bacteria, Spirochetes, Planctomyces,
Cytophagas, Gram-positive bacteria, Purple bacteria, Cyanobacteria, Flavobacteria, Thermotoga and Aquifex; (2) Domain Archaea: extreme halophiles, Methanosarcina, Methanobacterium, Methanococcus, Thermococcus, Thermoproteus
and Pyrodictium; and (3) Domain Eukarya: Entamoebae, slime moulds, animals,
fungi, plants, ciliates, flagellates, trichomonads, micro sporidia and diplomonads.
The phylogenetic tree, considering these three domains, revealed that since the
appearance of the "universal ancestor", two different lines of evolution have,
evolved one in the direction of the Domain Bacteria, and another in the direction
of the Domain Eukarya. However, this evolutionary tendency changed abruptly,
giving rise to another branch from the ongoing line to Domain Eukarya, and the
Domain Archaea appeared.
Irrespective of considerations about these evolutionary lines, it is not possible
to describe the surface structures for all living organisms, even if we consider the
updated phylogeny that considers only three domains. However, based on the
available information on the chemistry of some well-studied biological surfaces,
some groups of organisms can be selected for study and, based on the analysis of
their surfaces, it is possible to associate their biochemistry with the uptake of
mercury or any other heavy metal, giving an indication of the reaction surface
sites in certain groups of organisms.
5
The Cytoplasmic Membrane and its Potential Reaction Sites
The cytoplasmic membrane completely surrounds the cells, acting as a highly
selective barrier. It is a phospholipid bilayer whose hydrophobic groups are
directed inward, while the cytoplasm and the hydrophilic groups are arranged
outwards. This selective barrier also presents several embedded proteins, being
similar in overall structure for both prokaryotes and eukaryotic cells. Nevertheless, even though the cytoplasmic membrane is not the outermost layer in the
cells, its basic structure provides some potential binding sites for mercury and
other heavy metals. As the cell wall, located outside the cytoplasmic membrane,
does not act as a selective barrier, heavy metal ions can be transported into the
cytoplasmic membrane. Thus, its overall structure can be discussed indicating
potential binding sites.
Basically, the cytoplasmic membrane can be considered as constituted of lipids
and proteins, mainly. Both components of this membrane contain potential
binding sites for the uptake of mercury. A schematic representation of a
phospholipid molecule is represented in Fig. 1.
It can be observed that the non-polar head of the phospholipid is of a
hydrophobic nature, not presenting any sort of binding sites for the uptake
of mercury or any other heavy metal. However, in the polar head of the molecule
there is a phosphate group, negatively charged, acting as a potential mercury ligand. As previously mentioned, the cytoplasmic membrane is a bilipid