requires the removal of four H atoms and is catalyzed
by two dehydrogenases (Chapter 18). One dehydrogenase
yields FADH
; the other yields NADH.
The term “one reducing equivalent” means that 1 mol
of electrons is present in the form or one gram-equivalent
of a reduced electron carrier. One mole of NAD+, when
reduced to 1 mol of NADH, utilizes 2 mol of electrons
from 1 mol of substrate (Chapter 13):
(reduced substrate)
NADH + H+ + S (oxidized substrate)
In this reaction, the 2 mol of electrons is transferred in the
form of 1 mol of hydride ion (H:)_.
Mitochondrial Structure and Properties
Mitochondria are present in the cytoplasm of aerobic eu-
karyotic cells. They are frequently found in close prox-
imity to the fuel sources and to the structures that require
ATP for maintenance and functional activity (e.g., the con-
tractile mechanisms, energy-dependent transport systems,
and secretory processes). The number of mitochondria in
a single cell varies from one type of cell to another; a rat
liver cell contains about
1 0 0 0
, while one giant amoeba has
about 10,000. In a given cell, the number of mitochondria
may also depend on the cell’s stage of development or
functional activity.
The size and shape of mitochondria vary considerably
from one cell type to another. Even within the same cell,
mitochondria can undergo changes in volume and shape
depending on the metabolic state of the cell. In general,
they are 0.5-1.0 gm wide and 2-3 /xm long and are
known to aggregate end to end, forming long filamentous
Mitochondria consist of two membranes, one encir-
cling the other, creating two spatial regions: the inter-
membrane space and the central space, called the matrix.
The outer membrane is 6-7 nm thick, smooth, unfolded
(Figure 14-1), and freely permeable to molecules with
molecular weights below 10,000. It contains a heteroge-
neous group of enzymes that catalyze certain reactions
of lipid metabolism as well as hydroxylation reactions
(Table 14-1). The intermembrane space (5-10 nm) con-
tains the enzymes that catalyze interconversion of adenine
The inner membrane (
6 - 8
nm thick) has many folds di-
rected toward the matrix. These invaginations, known as
cristae, increase the surface area of the inner membrane.
The lipid component, almost all of which is phospholipid,
constitutes 30-35% by weight of the inner membrane.
chapter 14
Electron Transport and Oxidative Phosphorylation
Morphology of a mitochondrion (transverse section).
Phospholipids are asymmetrically distributed in the lipid
bilayer, with phosphatidylethanolamine predominating on
the matrix side and phosphatidylcholine on the cytoplas-
mic side. Seventy-five percent of the cardiolipin is present
on the matrix side of the membrane. The fatty acid compo-
sition of the phospholipids depends on the species, tissue,
and diet. In all cases, sufficient unsaturated fatty acids are
contained in the phospholipids to provide a highly fluid
membrane at physiological temperatures.
The inner membrane is studded with spheres, each
8-10 nm in diameter, that are attached via stalks 4-5 nm in
length. These inner membrane spheres are present on the
matrix side (M-side) but absent from the cytoplasmic side
(C-side). The components of the inner membrane include
respiratory chain proteins, a variety of transport molecules,
and a part of the ATP-synthesizing apparatus (the base
piece of ATP synthase). The ATP-ADP translocase and
previous page 278 Bhagavan Medical Biochemistry 2001 read online next page 280 Bhagavan Medical Biochemistry 2001 read online Home Toggle text on/off