Under aerobic conditions regeneration of cytosolic NAD+ from cytosolic NADH is accomplished by transferring electrons across the mitochondrial membrane barrier to the electron transport chain where the electrons are transferred to oxygen.
There are two different shuttle mechanisms whereby this transfer of electrons across the membrane to regenerate cytosolic NAD+ can be accomplished, the glycerol 3-phosphate shuttle and the malate-aspartate shuttle.
1) The glycerol 3-phosphate shuttle (Figure-1) functions primarily in skeletal muscle and brain. The shuttle takes advantage of the fact that the enzyme glycerol-3-phosphate dehydrogenase exists in two forms, a cytosolic form that uses NAD+ as cofactor and a mitochondrial FAD-linked form.
Figure-1- showing glycerol-3-Phosphate shuttle. G3P- glycerol-3-P, DHAP- Dihydroxy acetone-Phosphate
Cytosolic glycerol-3-phosphate dehydrogenase uses electrons from cytosolic NADH to reduce the glycolytic intermediate dihydroxyacetone phosphate to glycerol 3-phosphate, thereby regenerating cytosolic NAD+. The newly formed 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 through complex II to coenzyme Q of the electron transport chain and on to oxygen generating two ATP molecules per electron pair and therefore per glycerol -3-phosphate.
2) The malate-aspartate shuttle, however, functions primarily in the heart, liver, and kidney (Figure 2). This shuttle requires cytosolic and mitochondrial forms of malate dehydrogenase and glutamate-oxaloacetate transaminase and two antiporters, the malate-α-ketoglutarate antiporter and the glutamate aspartate antiporter, which are both localized in the mitochondrial inner membrane. In this shuttle cytosolic NADH is oxidized to regenerate cytosolic NAD+ by reducing oxaloacetate to malate by cytosolic malate dehydrogenase.(1)
Figure-2-Showing Malate Aspartate shuttle
Malate is transported into the mitochondrial matrix while α-ketoglutarate is transported out by the malate-α-ketoglutarate antiporter, a seeming mass unbalance(2).
Next malate is oxidized back to oxaloacetate producing NADH from NAD+ in the mitochondrial matrix by mitochondrial malate dehydrogenase (3).
Oxaloacetate cannot be transported per se across the mitochondrial membrane. It is, instead transaminated to aspartate from the NH3 donor glutamate by mitochondrial glutamate-oxaloacetate transaminase (4). Aspartate is transported out of the matrix whereas glutamate is transported in by the glutamate-aspartate antiporter (5)in the mitochondrial membrane, obviating the apparent mass unbalance noted above.
The last step of the shuttle is catalyzed by cytosolic glutamate-oxaloacetate transaminase regenerating cytosolic oxaloacetate from aspartate and cytosolic glutamate from α-ketoglutarate(6) both of which were earlier transported in opposing directions by the malate- α- ketoglutarate antiporter.
The net effect of this shuttle is to transport electrons from cytosolic NADH to mitochondrial NAD+. Therefore, those electrons can be presented by the newly formed NADH to electron transport system complex I thereby producing three ATPs by oxidative phosphorylation.
Note that depending on which shuttle is used (i.e., which tissue is catalyzing glycolysis) either two or three ATPs are produced by oxidative phosphorylation per triose phosphate going through the latter steps of glycolysis.
ATP Formation in the Catabolism of Glucose
This assumes that NADH formed in glycolysis is transported into mitochondria by the malate shuttle.
If the Glycerol-phosphate shuttle is used, then only 2 ATP will be formed per mol of NADH. At the step of glyceraldehyde-3-P dehydrogenase-4 ATP will be produced. Hence the total will be-6+30= 36 ATP. ( 6 from Glycolysis and 30 from PDH complex and TCA cycle. Since this shuttle operates. in skeletal muscle and brain, hence the total yield per glucose mol will be 2 ATP less as compared to other tissues.
In nonaerobic glycolysis, as in the case when a tissue is subjected to an ischemic episode (i.e., myocardial infarction), neither the ATP produced by the shuttle nor the ATPs produced by normal passage of electrons through the electron transport chain are produced because of oxygen insufficiency.
Therefore glycolysis must increase in rate to meet the energy demand. In damaged tissue this increased rate is compromised. Moreover the shuttle mechanisms to regenerate NAD+ from NADH formed by glycolysis are unavailable.
Glycolysis under ischemic conditions satisfies the requirement for NAD+ by reducing pyruvate, the glycolytic end product under normal conditions, to lactate with the reducing equivalents of NADH.
The new end product lactate accumulates in muscle cells under ischemic conditions and damages cell walls with its low pH causing rupture and loss of cell contents such as myoglobin and troponin I. These compounds as well as other end products combine to cause increased cell rupture and pain.