The body metabolizes macronutrients through electrolysis and the TCA to make ATP. Little ATP is produced in these two steps. However, the production of NADH and a FADH2 in the process leads to much ATP production later in the electron transport chain.
Summary of ATP Production in Food Metabolism
| ATP | Reduced Coenzyme | |
| Glycolysis | 2 | 2 NADHb |
| Oxidation of 2 pyruvates | 0 | 2 NADH |
| Two Turns of TCA Cycle | 2 |
6 NADH + 2 FADH2b |
| ATP Produced | 4 | 34 |
aEach NADH produces 3 ATPs; bEach FADH2 produces 2 ATPs 2
Some ATP is made in the cytsol of the cell will most is made in the mitochondrion. That is why this little machine is called the powerhouse of the cell. The mitochondria is an interesting little machine having two membranes. One is smooth (the outer) and the other crimply (the inner). The crimply one is where most of the excitement take place. That’s where you have the electron transport chain; the main location for ATP production. The diagram below illustrates the big picture of what we are talking about here, and where important stuff are located.
So let’s now explore how ATP is made using the next diagram (below). This diagram is referred to as the Chemiosmotic model. It is a snap shot of the mitochondrion inner membrane. Along this membrane is where the electron transport chain (ETC) happens. We say ETC because electrons are transported along the membrane in a chain format. Key components of the electron transport chain include five enzymes complexes (Complex I, II, III, IV, V) and two electron carriers
Enzymes Complexes
- NADH dehydrogenase (Complex I)
- Succinate dehydrogenase (Complex II) -this enzyme is also used in the citric acid cycle
- Cytochrome oxidoreductase (Complex III)
- Cytochrome oxidase (Complex IV)
- ATP synthase (Complex V)
Electron Carriers
- Coenzyme Q (ubiquinone)
- Cytochrome c
NADH from the citric acid cycle interacts with Complex I and gives up two electrons. These electrons are snatched up by CoQ and carried over to Complex III. Another carrier, Cyt c snatches the two electrons and shuttles them across to Complex IV. The electrons travels down Complex IV and delivers the electrons to oxygen. At the same time, oxygen reacts with hydrogen protons to form water.
This movement of electrons from one enzyme to the next creates a proton (H+) pump i.e. protons are moved into the inter membrane space via Complex I, III, and IV. For every two electrons transferred to the chain, there are 4, and 2 protons pumped into the intermembrane space by Complex I, II and IV respectively. This creates a backup of protons in the intermembrane space. These protons can be transferred back into the mitochondrial matrix via ATP synthase. For every four protons that ATP synthase accepts, 1 ADP is converted to ATP.
I have not mentioned Complex II in this whole transaction. Complex II is very much involved. Remember that FADH2 is also produced in the citric acid cycle. The FADH2 interacts with complex II in the ETC to give up two electrons. These two electrons is snatched up by CoQ and then shuttled off along the chain to also contribute to ATP production in just about the same way as NADH. However the only difference is that FADH2 contributes to 1.5 ATPs for every two electrons compared to NADH which contributes to 2.5 ATPs for every two electrons it transfers. Process of making ATP along the electron transfer chain in the presence of oxygen is called oxidative phosphorylation.
Reference: Timberlake K. C. (2013). General, organic, and biological chemistry structures for life, 4th edition. Boston, MA: Pearson Education Inc.
