So the amount of energy is well sufficient for electron transfer to produce ATP. But recall from the course introduction that if energy is released in large chunks is is difficult to capture and convert the energy efficiently. Electron transfer occurs in smaller chunks where the change in reduction potential is not so overwhelmingly large.
We also have another problem to overcome... that is in electron count! If you look back on the calculations you see that we used half of an O2 molecule to reduce to water. This is of course ridiculous! But for a whole O2 to get reduced requires 4 electrons to get all the way to water. NADH is and OBLIGATE 2 electron donor! We must get oxygen reduced all the way in a single 4 electron step
The consequences of failing to reduce oxygen completely to water can indeed be dire.
The intermediate reduction molecular oxygen products are:
Every one of these is dangerous to have around and could easily lead to modification of DNA which would lead to mistakes in DNA replication which could ultimately lead to cancer. We need to ensure transfer of 4 electrons at once.... This means COFACTORS and lots of them of several different varieties. Listed below are a few of them and the number of electrons that each can handle at one time
| Cofactors | NAD+/NADH | O2 | Flavin ubiquinone |
iron sulfur clusters
copper ions hemes |
| # electrons transferred at a time by cofactor | ONLY 2 | ONLY 4 | 1 OR 2 | ONLY 1 |
All of these cofactors are required to be associated with a protein complex, with the exception of ubiquinone. It is the only one that CAN be associated with a protein complex or can be allowed free diffusion.
While individual cofactors handle one or two electrons at a time, several are combined into large protein complexes. more about this in the next few pages.