section h 1
Mitochondrial Structure and Properties
251
Submitochondrial Particles
Submitochondrial particles (SMPs) are produced by dis-
ruption of the mitochondria by mechanical, osmotic, or
sonic shock treatment. The fragmentation results in the
release of water-soluble components and inner membrane
fragments that re-form into vesicles (Figure 14-2). The
components are then separated by differential centrifuga-
tion. The membranes of SMPs have the characteristic inner
membrane spheres on their outside. SMPs are capable of
electron transport and oxidative phosphorylation (i.e., the
synthesis of ATP from ADP and phosphate). Removal of
the inner membrane spheres by further mechanical treat-
ment, urea, or trypsin results in the dissociation of the elec-
tron transport assembly from ATP synthesis. The ability to
synthesize ATP resides in the overall structure, which in-
cludes the spheres (called Fi), the stalks, and a membrane
protein subunit (called F0). The F0-subunit spans the inner
membrane and thus is retained in the vesicles.
The spheres removed from SMPs do not support ATP
synthesis but do hydrolyze ATP to ADP and phosphate.
Thus, ATP synthesis is carried out by F
0
/Fi-ATPase (ATP
synthase). The subscript “o” in F
0
indicates that it contains
the site at which a potent antibiotic inhibitor, oligomycin,
binds and inhibits oxidative phosphorylation. Oligomycin
does not bind Fj-ATPase and does not inhibit ATP hydrol-
ysis to ADP and phosphate.
F
0
Fi-ATPase
(ATP synthase;
inhibited by oligomycin)
ADP + fi ,
=±. ATP + H20
F]-ATPase
(Not inhibited by oligomycin)
The fully active SMPs can be reconstituted by adding Fi-
ATPase to depleted vesicles under appropriate conditions.
NADH — »
Succinate — »
Complex I_____
___________
_________________
F ^ - F e f n .h .^ Cytb
/
\
|
________^ Q —»
Cyt
c( r ~
Cyt c
y -
Cyt a,a,-Cu - j - 0 2
Ç -F e (n .h .K ^ F e ( n . h . \
X
I
Complex II
Complex III
Complex IV
F I G U R E 1 4 -3
Diagram of the functional complexes of the electron transport system
within the respiratory chain. Fnad = NADH dehydrogenase flavoprotein;
Fs = succinate dehydrogenase flavoprotein; Fe(n.h.) = nonheme iron.
ation procedures, whereas the bonds holding unlike com-
plexes are relatively weak and can be dissociated. The
functional organization of the four complexes in the in-
ner mitochondrial membrane is shown in Figure 14-3. In
addition to these complexes, the F
0
/F]-ATPase, which is
required for ATP synthesis, may be considered as com-
plex V. The relative ratios of complexes I, II, III, IV, and V
have been estimated to be 1:2:3:6:6. Complexes I, II, III,
and IV can be combined in the presence of cytochrome c
(which separates during fractionation) to form a single unit
with all of the enzymatic properties of the intact electron
transport system except coupled phosphorylation.
The individual electron carriers of the four complexes of
the respiratory chain, shown in Figure 14-4, are arranged
in accordance with their redox potentials, with the transfer
of electrons from NADH to oxygen associated with a po-
tential drop of 1.12 V, and that of succinate to oxygen of
0.8 V. In the electron transport system, the electrons can
be transferred as hydride ions (H:)_ or as electrons (e.g.,
in the cytochromes).
Electron Transport Complexes
Components o f the Electron Transport Chain
The electron transport system can be reconstituted into
discrete enzyme complexes that catalyze the following
four reactions:
NADH-coQ reductase
1. NADH + H+ + coenzyme Q (Q )------------------
>
NAD+ + QH
2
Succinate-CoQ reductase
2. Succinate + Q --------------------> fumarate + QH
2
coQ cytochrome c reductase
3. QH
2
+ 2ferricytochrome c ----------------------- ►
Q +
2 ferrocytochrome c + 2H+
cytochrome c oxidase
4. 2 Ferrocytochrome c + 2H+ + 1 /2 0
2
----------------- >
2 ferricytochrome c + H20
Each complex can be considered as a functional unit
composed of a fixed number of electron carriers. The
individual components of the four complexes are firmly
bonded together and are not dissociated by mild fraction-
Complex I
Complex I catalyzes an NADH-CoQ reductase activity,
and it contains the NADH dehydrogenase flavoprotein. It
has two types of electron-carrying structures: FMN and
several iron-sulfur centers. FMN is a tightly bound pros-
thetic group of the dehydrogenase enzyme, and it is re-
duced to FMNH
2
by the two reducing equivalents derived
from NADH:
NADH + H+ + E-FMN — NAD+ + E-FMNH
2
The electrons from FMNH
2
are transferred to the next
electron carrier, coenzyme Q, via the iron-sulfur centers of
the NADH-CoQ reductase. The iron-sulfur centers consist
of iron atoms paired with an equal number of acid-labile
sulfur atoms. The respiratory chain iron-sulfur clusters
are of the Fe
2
S
2
or Fe
4
S
4
type. The iron atom, present
as nonheme iron, undergoes oxidation-reduction cycles
(Fe2+ — Fe3+ + e “ ). In the Fe
4
S
4
complexes, the centers
