Download Energy Yields from Aerobic Respiration: Some Alternatives

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Appendix G
Energy Yields from
Aerobic Respiration:
Some Alternatives
In Chapter 21 we described catabolic processes as occurring in three stages. In
stage I, dietary protein, carbohydrate, and lipid are hydrolyzed into small subunits
that can cross the membranes of the cells of the intestine and are transported to the
cells of the body. In stage II, these monomers enter cells of the body and are converted into a form that can be completely oxidized. For carbohydrates, glucose is
used as a substrate for the glycolysis pathway, the first stage of carbohydrate metabolism. In this pathway, glucose is converted into two pyruvate molecules. In the
process, two ATP, net, are produced by substrate level phosphorylation and two
NADH are formed by oxidation of glyceraldehyde. Under aerobic conditions, that
is, when oxygen is present, the pyruvate is transported into organelles called
mitochondria. Here the pyruvate dehydrogenase complex is involved in the reaction
pyruvate coenzyme A NAD
acetyl CoA CO2 NADH
In stage III, the two-carbon acetyl group is completely oxidized in the reactions of
the citric acid cycle.
When glycolysis occurs under anaerobic conditions, it is followed by fermentation reactions, such as the lactate and alcohol fermentations. These reactions reduce pyruvate—or a molecule produced from pyruvate—and simultaneously
oxidize the NADH produced in glycolysis. As a result, the net energy yield from
glycolysis under anaerobic conditions is only two ATP. No further ATP energy is
harvested from the oxidation of the NADH. It is simply reoxidized in the fermentation reactions.
Under aerobic conditions, the energy yield of glycolysis is much greater because the high-energy electrons carried by NADH are shuttled into mitochondria
and used in oxidative phosphorylation to produce more ATP.
There are two shuttle systems: the glycerol-3-phosphate shuttle found in skeletal
muscle and nerve cells and the oxaloacetate-malate shuttle found in heart and liver
cells. Because skeletal muscle produces the majority of the ATP for the body, it is
the glycerol-3-phosphate shuttle that is used most commonly when discussing
metabolic energy yields. In Example 22.1, calculation of the ATP harvest of glycolysis is based on this shuttle.
Let’s consider the reactions involved in the glycerol-3-phosphate shuttle. In
this shuttle, the NADH produced in the cytoplasm is oxidized in a reaction that reduces dihydroxyacetone phosphate to glycerol-3-phosphate.
CH2OH
A
NADH
CPO
A
CH2OPO32
CH2OH
A
NAD
CHOH
A
CH2OPO32
Dihydroxyacetone phosphate
Glycerol-3-phosphate
The glycerol-3-phosphate then passes through the outer mitochondrial membrane and is oxidized to dihydroxyacetone phosphate by the enzyme glycerol-3phosphate dehydrogenase. This enzyme, which is located in the inner mitochondrial
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Appendix G Energy Yields from Aerobic Respiration: Some Alternatives
membrane, simultaneously reduces FAD to FADH2. The electrons from each FADH2
are then used during oxidative phosphorylation to produce two ATP. The dihydroxyacetone phosphate returns to the cytoplasm to continue the shuttle process.
The main drawback of the glycerol-3-phosphate shuttle is that only two ATP
are produced for each cytoplasmic NADH. The reason is that the electrons are
shuttled to FADH2, which yields only two ATP by oxidative phosphorylation. (The
energy yield of the oxidation of mitochondrial NADH is three ATP.) Thus the total
energy yield from glycolysis under aerobic conditions in muscle and nerve cells is
two ATP, produced by substrate level phosphorylation, plus four ATP (two ATP
per NADH), produced by oxidative phosphorylation. This provides an energy
yield of six ATP per glucose.
The oxaloacetate-malate shuttle system is more efficient. In this system, cytoplasmic NADH reduces oxaloacetate to malate.
COO
A
CPO NADH
A
CH2
A
COO
COO
A
CHOH NAD
A
CH2
A
COO
Oxaloacetate
Malate
Malate is then transported into the mitochondrion where it is reoxidized to oxaloacetate. Mitochondrial NAD is reduced in the process. These electrons are
then used in oxidative phosphorylation to produce three ATP per NADH. Thus
the energy yield of glycolysis in heart and liver cells is two ATP, produced by substrate level phosphorylation, plus six ATP (three ATP per NADH), produced by
oxidative phosphorylation. This gives an energy yield of eight ATP per glucose.
The oxaloacetate cannot cross the mitochondrial membrane to return to the
cytoplasm to continue the cycle. It is able to return only after a series of reactions
involving the amino acids glutamate and aspartate.
In summary, the energy yield of glycolysis depends on the conditions present
(aerobic versus anaerobic) and the type of cell. The following table summarizes the
energy gains from glycolysis under various conditions.
Condition
Anaerobic
Aerobic, muscle
(glycerol-3-phosphate shuttle)
Aerobic, heart
(oxaloacetate-malate shuttle)
ATP by Substrate
Level Phosphorylation
ATP by Oxidative
Phosphorylation
2
0
2
4
2
6