section 14.3
Mitochondrial Energy States
265
C O O H
:c=c—
c h
3
'H
Ql_la
A tractyloside
FIGURE 14-20
Inhibitors of adenine nucleotide (ATP/ADP) translocation. Bongkrekic acid binds to the ATP binding site, whereas
atractyloside binds to the ADP binding site of the translocator.
phosphate to glycerol 3-phosphate by NAD+-dependent
glycerol- 3 -phosphate dehydrogenase :
C H 2O H
C H 2OH
I
!
C = 0
+ NADH + H + --------â–º
C H O H
+
NAD +
I
I
C H
2
0 P 0 32'
C H
2
0 P 0 32 “
D ih y d ro x y a c e to n e
G ly cero l 3 - p h o s p h a te
p h o s p h a te
Glycerol 3-phosphate in the intramembrane space is ox-
idized to dihydroxyacetone phosphate by the FAD-linked
dehydrogenase. The FADH
2
thus generated is then oxi-
dized by CoQ of the respiratory chain. This shuttle dif-
fers from the malate-aspartate shuttle in three important
respects:
1. The substrate glycerol 3-phosphate, which carries the
reducing equivalents, does not penetrate the inner
mitochondrial membrane, since it is oxidized on the C
side of the membrane.
2. The oxidation of one molecule of cytoplasmic NADH
yields only two molecules of ATP, since the reducing
equivalents enter the respiratory chain at the CoQ
level.
3. The shuttle is bidirectional.
The malate-aspartate shuttle is quantitatively more
significant in all vertebrate tissues. It is unidirectional,
requiring cytoplasmic and mitochondrial malate dehy-
drogenases and aspartate aminotransferases as well as
two membrane-bound carrier systems (Figure 14-22).
In this process, the reducing equivalents of NADH are
NADH,
NAI
Cytoplasm
of cell
D ih yd roxyaceton e
p h o sp h a te
Cytoplasmic
glycerol-3-phosphate
dehydrogenase
G lycerol 3 -p h o sp h a te
I
Outer
!
'
mitochondrial
'
I
membrane
j
I
.
'
l
Intermembrane
space
D ih yd roxyaceton e
p h o sp h a te
Mitochondrial
glyceroi-3-phosphate
dehydrogenase
G lycerol 3 -p h o sp h a te
1
Inner
;
Matrix of
, mitochondrial,
mitochondrion
;
membrane i
1
' E lectron
I
transport
I
I
I
1
FADHj
;
FAD
FIGURE 14-21
Glycerol 3-phosphate shuttle for the transport of cytoplasmic reducing equivalents to the inner mitochondrial
membrane. Glycerol 3-phosphate, which carries the reducing equivalents, is oxidized to dihydroxyacetone phosphate
by an FAD-linked dehydrogenase located on the C side of the inner membrane.
