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Cell Signalling Biology Michael J. Berridge Module 2 Cell Signalling Pathways 2 75

NADPH oxidase Redox balance
The NADPH oxidase (NOX/DUOX) family (Module 2: The redox balance of the cell is maintained by energy meta-
Table redox signalling components) consists of a number bolism, primarily the pentose phosphate shunt that feeds
of enzymes with different cellular locations. reducing equivalents into the cell in the form of NADPH.
NOX2, also known as gp91 phox , has been described best The latter is then used to maintain redox buffers such as
for phagocytes, where it is made up of a number of sub- glutathione (GSH) (Module 2: Figure GSH/GSSG couple)
units. The catalytic component is cytochrome b 558 ,which and thioredoxin (Trx) in their reduced forms. The concen-
is a heterodimer formed from gp91 phox and p22 phox .In tration of GSH in the cytoplasm lies within the 1--10 mM
addition to this heterodimer, which is located in the mem- range, with over 99% existing as the reduced GSH form.
brane, there are cytoplasmic components (e.g. p47 phox , The 2GSH/GSSG redox couple is thus a measure of the
p67 phox , Rap1A and Rac) that play a role in regulating redox balance in the cell. A similar balance exists for the
enzyme activity. The DUOX (dual oxidase) enzymes are oxidized and reduced forms of TRX. The reduced GSH
sensitive to Ca 2 + and play an important role in interacting and Trx are used for a number of reductive processes such
with the Ca 2 + signalling pathway (Module 2: Figure ROS as the metabolism of hydrogen peroxide by glutathione
effects on Ca 2 + signalling). peroxidase (GPx) (Module 2: Figure H 2 O 2 metabolism)
There is a strong relationship between redox signalling or as a source of reducing equivalents for the glutaredoxin
and schizophrenia during which the induction of NOX2 system (Module 2: Figure recovery of protein oxidation).
may play an important role in triggering the formation The GSSG (the oxidized form of glutathione) is converted
of the peroxynitrite that acts to inhibit NMDA receptor back into GSH by glutathione reductase.
function (Module 12: Figure schizophrenia). Similar redox control enzymes and buffers are located in
both the mitochondria and within the lumen of the endo-
plasmic reticulum (ER). Like the cytoplasm, the mitochon-
drial matrix maintains a reducing environment and uses
Mitochondrial reactive oxygen species
similar enzymatic mechanisms to control the ROS eman-
(ROS) formation
ating from the electron transport chain. The ER, however,
Most of the electrons that enter the electron transport chain
is somewhat different in that the GSH/GSSG ratio is close
are transferred to oxygen in an orderly manner, but there is
to 1 and this more oxidizing environment is necessary for
always a 1--2% leakage during which an electron is trans-
the formation of the disulphide bonds that are an integral
−•
ferred directly to oxygen to form superoxide (O 2 ) and
this is the source of mitochondrial reactive oxygen species component of the extracellular proteins that are processed
(ROS) (Module 5: Figure mitochondrial Ca 2 + signalling). and packaged within the ER.
This orderly electron transfer to oxygen occurs during en- Since the cytoplasm and the ER lumen have different
ergy metabolism, where oxygen is reduced to water by redox potentials, there is a redox potential gradient across
accepting four electrons (e ) from cytochrome c oxidase: the ER membrane and this might be used to modulate
−
Ca 2 + signalling by altering the activity of the ion channels
that release Ca 2 + .
O 2 + 4e − + 4H + → 2H 2 O
Reactive oxygen species (ROS) metabolism
Mitochondrial energy metabolism is inherently dan- Like all other intracellular signalling molecules, ROS are
gerous, because the electron transport chain is somewhat metabolized rapidly. Superoxide dismutase (SOD) rapidly
leaky in that some of the molecular oxygen is diverted into converts superoxide radical (O 2 −• ) into hydrogen per-
−• ), which is oxide (H 2 O 2 ), which is then metabolized by a number of
the formation of the superoxide radical (O 2
then converted into hydrogen peroxide (H 2 O 2 ) and the enzyme systems including catalase, glutathione peroxidase
hydroxyl radical (OH ). These mitochondrial ROS may (GPx) and peroxiredoxin (Prx) (Module 2: Figure H 2 O 2
•
play an important role in apoptosis by acting synergist- metabolism).
ically with Ca 2 + to stimulate the formation of the mi- With so many enzyme systems co-operating to meta-
tochondrial permeability transition pore (MTP) (Module bolize H 2 O 2 , it is likely that this messenger will have a
5: Figure mitochondrial Ca 2 + signalling). The formation highly restricted sphere of influence localized to its site of
of mitochondrial ROS appears to be highly localized in production either at the plasma membrane or within the
that small superoxide flashes have been recorded in single mitochondrion.
mitochondria. This is another example of reactive oxygen
species (ROS) microdomains.
The generation of ROS by mitochondria might be a Catalase
regenerative process in that a local release of ROS in car- Catalase is a haem-containing protein that decomposes
diac myocytes causes a rapid mitochondrial depolarization hydrogen peroxide (H 2 O 2 ) to water and oxygen (Module
due to formation of a MTP and a concomitant increase in 2: Figure H 2 O 2 metabolism).
intrinsic ROS production, i.e. a process of ROS-induced Most of the catalase in cells is found in peroxisomes,
ROS release (RIRR). This RIRR often occurs synchron- thus restricting the role of the enzyme in dealing with the
ously and reversibly among long chains of adjacent mito- H 2 O 2 generated during the redox signalling mechanism at
chondria, suggesting a co-operative mechanism. the plasma membrane.

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