Chemistry Reference
In-Depth Information
fermentation of
one molecule of glucose to lactate or ethanol yields just 2 ATP molecules, whereas full oxidation of glucose to
CO
2
and water yields between 36 and 38! So far we have only seen what the biochemist calls 'substrate-level'
phosphorylation
However, the overwhelming attraction of respiration is the greatly increased yield of ATP
e
ATP production in the course of metabolic processes. To achieve the yields of ATP production
we find in respiration, we need to harness the potential energy of the reducing equivalents
e
e
NADH and FADH
2
by
transferring their electrons to an electron acceptor with a much higher redox potential
e
and in the mitochondrion
this is dioxygen.
The standard redox potential E
0
o
(standard conditions for the biochemist are 1 M oxidant, 1 M reductant,
10
7
M[H
þ
], i.e., pH 7 and 25
C) for most biological redox couples are known. Remember that in this context E
0
o
refers to the partial reaction written as:
þ
e
/
Oxidant
Reductant
D
G
0
is related to the change in standard redox potential E
0
o
by
In addition, the standard free energy change
0
¼
nF
D
E
0
0
where n is the number of electrons transferred, F is the faraday, a constant equal to 96.48 kJ/mol/V, and
0
D
G
D
E
0
o
is the
difference between the two standard redox potentials in volts.
The driving force of oxidative phosphorylation is the difference between the electron transfer potential of
NADH or FADH
2
relative to that of O
2
. For the redox couple:
NAD
þ
/
H
þ
E
0
0
is
NADH
þ
0
:
315 V
;
while for the couple:
11
2H
þ
þ
2e
/
E
0
0
is
1
=
2
O
þ
H
2
O
þ
0
:
815 V
so that for the reaction:
H
þ
/
NAD
þ
D
E
0
0
is
1
=
2
O
þ
NADH
þ
H
2
O
þ
þ
1
:
130 V
D
G
0
¼ e
D
G
0
for ATP hydrolysis is
we can calculate that
31.4 kJ/mol, so
we should be able to make a few ATP molecules with this potential bonanza of energy. However, there are two
important conditions
220.1 KJ/mol. For comparison, the
e
first, we cannot simply dissipate all of the potential energy difference in one 'big bang',
but pass the electrons through a series of transporters which have progressively increasing redox potentials, and
second we must use a system coupled to electron transfer which will allow us to make ATP synthesis turn
e
e
and
that involves generating a proton gradient across the internal mitochondrial membrane.
The first condition is met by having a series of four protein complexes, inserted into the mitochondrial inner
membrane, each made up of a number of electron (and sometimes proton) acceptors of increasing redox potential.
Three of them (Complexes I, III, and IV) are presented in cartoon form in
Figure 5.18
.
Complex I, referred to more
prosaically as NADH-Coenzyme Q oxidoreductase, transfers electrons stepwise from NADH, through a flavo-
protein (containing FMN as cofactor) to a series of iron
sulfur clusters (of which more in Chapter 13) and
ultimately to coenzyme Q, a lipid-soluble quinone, which transfers its electrons to Complex III. The
e
D
E
0
o
for the
D
G
0
of
e
couple NADH/CoQ is 0.36 V, corresponding to a
69.5 kJ/mol, and in the process of electron transfer,
protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes).
Complex II (which is not shown in the Figure) contains succinate dehydrogenase, the FAD-dependent Krebs
cycle enzyme, and like complex I, transfers its electrons through iron
e
sulfur centres and a b-type cytochrome
11. It is clearly absurd to talk about 'half' oxygen molecules given the strength of the O
]
O double bond. However, for more pedestrian
reasons of considering the transfer of two electrons from NADH or FMNH
2
, through a long series of transporters all the way to the end of the
line at molecular oxygen, we would request our more chemically based readers to grant us this small indulgence.
