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CASE FILES: BIOCHEMISTRY
glycerol 3-phosphate is released from the cytosolic form of the enzyme and
crosses to and is bound to the mitochondrial FAD-linked glycerol-3-phosphate
dehydrogenase, which is bound to the cytosolic side of the mitochondrial inner
membrane. There the mitochondrial glycerol-3-phosphate dehydrogenase
reoxidizes glycerol 3-phosphate to dihydroxyacetone phosphate (preserving
mass balance) reducing its FAD cofactor to FADH2. Electrons are then passed
to coenzyme Q of the electron transport chain and on to oxygen generating
two additional ATP molecules per electron pair and therefore per glycerol
3-phosphate.
The malate-aspartate shuttle, however, functions primarily in the heart,
liver, and kidney (Figure 27-3). This shuttle requires cytosolic and mitochon-
drial forms of malate dehydrogenase and glutamate-oxaloacetate transaminase
and two antiporters, the malate-a-ketoglutarate antiporter and the glutamate-
aspartate antiporter, which are both localized in the mitochondrial inner mem-
brane. In this shuttle cytosolic NADH is oxidized to regenerate cytosolic
NAD+ by reducing oxaloacetate to malate by cytosolic malate dehydrogenase.
CYTOPLASM
INNER
MITOCHONDRIAL
MEMBRANE
MITOCHONDRIAL
MATRIX
NAD+
NADH
1.
malate dehydrogenase (cytosolic)
2.
malate - KG antiporter (mitochondrial inner membrane)
3.
malate dehydrogenase (mitochondrial matrix)
4.
glutamate - OAA transaminase (mitochondrial matrix)
5.
glutamate - aspartate antiporter (mitochondrial inner membrane)
6.
glutamate - OAA transaminase (cytosolic)
Figure 27-3. Malate-aspartate shuttle.
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