Alpha-Glycerol Phosphate Shuttle and Malate Shuttle
We saw that one of the fates of the NADH made in glycolysis was to be recycled for use, by donating its proton and electrons to make either alcohol or lactic acid. If the electrons were not subsequently donated from the NADH, cells would eventually become depleted of working NAD, and energy metabolism would come to a halt at this enzymatic step. So there has been evolutionary pressure to select cells that can recycle this electron carrier to keep energy metabolism rolling along.
In those tissues which do not carry out fermentations, this pair of electrons represents a certain amount of potential energy in the form of additional ATPs that could be made if the electron pair could be transferred to the electron transport chain inside the mitochondrial membranes. Unfortunately, the inner mitochondrial membrane is impermeable to NAD & protons. Thus there are two pools of NAD in most cells. One is the inner mitochondrial pool and the other is the cytoplasmic pool of NAD.
With the electron transport chain being sequestered in the inner membrane of the mitochondria, some cells have evolved a mechanism to move the electron pair, associated with the cytoplasmic pool of NAD, into the electron transport chain?
SKELETAL MUSCLE & BRAIN tissues operate the glycerol-3-phosphate shuttle.
The glycolytic pathway's cytosolic NAD accepts an electron pair and becomes NADH (+ H+). The electron pair is then transferred to dihydroxyacetone phosphate (DHAP). This step regenerates the NAD, thereby allowing the glycolytic pathway to continue its important operations using the oxidized form of this coenzyme (NAD). [see figure]
Reducing the DHAP by addition of electrons converts it from DHAP to glycerol-3- phosphate. This newly formed compound easily migrates across the outer mitochondrial membrane. Again, the membrane is a barrier to proton and NAD flux, but not to glycerol-3-P. Once inside the mitochondria, the electron pair is donated from glycerol-3-P to FAD which becomes FADH 2, and which converts the glycerol-3-phosphate back into DHAP. The mitochondrial DHAP is free to wander back out into the cytosol.
The FADH produced can cycle into the mitochondrial electron transfer chain and produce 2 ATP for each pair of electrons transferred from the cytosolic NADH.
Two important points are demonstrated by the glycerol-3-phosphate shuttle:
1. the NAD is regenerated for use by glycolysis and
2. the potential energy of the cytoplasmic captured e's are realized to make ATPs.
LIVER, KIDNEY & HEART MUSCLE tissue operate on the same principle, but use a different shuttle.
This other pathway, which moves reducing equivalents from cytosol into mitochondria, is called the MALATE SHUTTLE, because malate is formed by oxidation of oxaloacetic acid -
[OAA + NADH ---> malate + NAD]. The malate moves into the mitochondria, where it is converted back into OAA and NADH.
[ malate + NAD ---> oxaloacetate + NADH ]
The NADH can the be used by the electron transfer chain of the miotchondria to produce ATP. For each pair of electrons transferred from the cytosol to NADH, 3 molecules of ATP can be made. [see figure]
Note: a new value for ATP per NADH is 2.5 and ATP per
FADH2 is 1.5.
You may see the numbers ATP per NADH = 3 and ATP per FADH 2 = 2 in some texts. For more information on the re-evaluation of the number of ATP's/nucleotide coenzyme see Hinkle, et al. "Mechanistic stoichiometry of mitochondrial oxidative phosphorylation". Biochemistry 30:3576-82, 1991.
The different values of 30 or 32 ATP/glucose depend on the method used to transport cytoplasmic NADH, formed by glycolysis, into the mitochondria, i.e. the shuttles.