Biology Reference
In-Depth Information
can cross biological membranes and remove hydrogen atoms from polyunsaturated fatty acids and
lipid hydroperoxides leading to autooxidation of lipids (Haliwell and Gutteridge, 1989). Though
H
2
O
2
is relatively less reactive, it is fairly long-lived molecule with a half-life of 1 ms and can diffuse
to some distances from its site of production. Inactivation of Calvin cycle and superoxide dismutase
enzymes by the oxidation of their thiol groups by H
2
O
2
has been reported (Charles and Haliwell,
1980; Bowler
et
al
., 1994). Superoxide anions are scavenged by SODs to produce H
2
O
2
and O
2
. H
2
O
2
forms the substrate for a number of decomposing reactions catalyzed by catalases and peroxidases.
Due to its high reactivity, H
2
O
2
gets reduced by metal ions leading to the formation of highly reactive
OH· radical. The OH· radical oxidizes organic molecules in two different ways. In the fi rst type of
reaction, the OH· radical is added to the organic molecule to form a hydroxylated compound or
the formation of organic radical plus water. The organic radicals can react with oxygen to generate
peroxyl radicals, which can in turn produce other organic radicals. In this way organic radicals are
produced one after another in a chain reaction. Although H
2
O
2
and O
2
•-
do not directly target
DNA
but can
still be considered as potential mutagens due to their ability to cause the formation of OH·
radicals that can cause extensive lesions in DNA. In order to protect the cells from ROS there are
certain natural antioxidants that remove these radicals directly. Some of these are L-ascorbic acid
(vitamin C), glutathione (GSH; a tripeptide of Glu-Cys-Gly) and α-tocopherol (vitamin E). Oxidative
stress causes extensive damage and its effects are refl ected in many biochemical pathways leading
to slow degeneration and death of cells (Vranová
et al
., 2002; Imlay, 2003; Latifi
et al
., 2009).
4) Osmoregulation:
Water is the universal solvent and dissolved in it are other cosolvents (solutes)
that signifi cantly infl uence the behaviour of water as a solvent. The composition of cosolvents, ionic
strength, osmotic pressure (OP) and pH contribute to the characteristics of a solvent. Each cosolvent
also contributes to the osmolality of the solution. Osmolality is defi ned as the OP of a solution at
a particular temperature expressed as moles of solute per kg of solvent. In contrast, osmolarity is
an approximation for osmolality expressed as moles of solute per litre of solution (Wood, 1999).
Osmolarity is calculated as the sum of concentration of osmotically active solutes in a solution. OP
of the extracellular environment may increase (osmotic upshift) or decrease (osmotic downshift). An
OP differential between the inside and outside puts pressure on the cell membrane. Furthermore,
at high external OP, the cell would lose water if it did not increase its internal OP. Thus OP must
be regulated. However, enzymes need an environment where the ionic strength does not vary too
drastically. So a particular OP range for the bacterial media that supports growth can be identifi ed (as
osmotolerant) and the cells try to adapt to changes in the cosolvent concentration by adjusting their
internal cosolvent levels to balance the outside. However cells need 100-150 mM K
+
and about the
same concentration of metabolic intermediates so they can not reduce their internal cosolvent levels
below a minimum of approximately 250 mM. Conversely, an increase of the internal salt concentration
to too high levels inhibits many enzymes. Very high concentrations of metal ions damage proteins
by altering their structural conformations. During osmotic upshift, water loss results in a decrease
of cell volume and the turgor is lost. Turgor pressure (ΔP) is a hydrostatic difference which balances
the OP difference between cell interior and exterior. It is the turgor pressure that renders the chemical
potentials of intracellular and extracellular water equal at equilibrium. An osmoregulatory response
is one in which the cell exhibits a physiological response that mitigates passive adjustments in cell
structure caused by changes in the extracellular osmolality. Enzymes are not generally affected
by OP per se, but it is the ionic environment that affects their structure. The secret of adapting to
osmotic upshift is the use of “compatible solutes”. These are special solutes which, even in high
concentrations, do not inhibit enzyme function. The most common are: glutamate (L-α-glutamate
