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QH2

QH2 is a shorthand designation used in redox chemistry and biology for the hydroquinone form of a quinone-based electron carrier. In many contexts, QH2 refers specifically to ubiquinol, the reduced form of ubiquinone (coenzyme Q), in mitochondria, or to plastoquinol in photosynthetic organisms. The pair Q (quinone) and QH2 (hydroquinone) constitutes a redox couple by which two electrons and two protons are transferred in biological membranes.

In the mitochondrial electron transport chain, ubiquinone (CoQ) is reduced to ubiquinol (CoQH2) by Complex I

Properties of QH2 depend on the specific quinone it is derived from; ubiquinol is lipid-soluble and mobile

See also: ubiquinone, ubiquinol, plastoquinone, plastoquinol, Q cycle.

and
Complex
II,
using
protons
from
the
mitochondrial
matrix
to
form
CoQH2.
CoQH2
diffuses
within
the
inner
membrane
to
donate
electrons
to
Complex
III
(cytochrome
bc1)
where
it
is
oxidized
back
to
CoQ,
releasing
protons
to
the
intermembrane
space
in
the
process.
This
redox
reaction
contributes
to
the
proton
motive
force
that
drives
ATP
synthesis.
In
photosynthetic
organisms,
plastoquinone
is
reduced
to
plastoquinol
(PQH2)
at
photosystem
II
and
subsequently
oxidized
as
electrons
are
transferred
along
the
electron
transport
chain.
within
membranes.
The
exact
redox
potential
of
the
Q/QH2
couple
varies
with
pH
and
the
chemical
substituents
of
the
quinone.
In
broadened
usage,
QH2
also
denotes
the
hydroquinone
form
of
other
quinones
beyond
ubiquinone,
and
the
term
is
used
in
discussions
of
redox
biochemistry
and
energy
metabolism.