Oxidative Phosphorylation

By the end of glycolysis and the Krebs Cycle, the glucose molecules are fully consumed. Carbon dioxide, 4 ATPs, 2 FADH, and 8 NADH have been produced. The reduced coenzymes (FADH and NADH) carry some electrons from glucose, and they are full of potential energy that need to extracted in order to recycle these molecules, and also to produce way more ATP. In this post we will see an general idea of what happens to these coenzymes and how different NADH is from FADH.

Coenzymes do not directly react with oxygen. The electrons carried by NADH are passed though different electron carriers until it reach the terminal electron receptor, oxygen. In fact, oxygen also picks up some water from the surrounding and forms water. Most of the electron carriers are embedded inside of the membrane (bacterias - cell membrane, and eukaryotes - mitochondrial membrane.)

The free energy resulted from this redox reactions is used in the phosphorylation of ADP. The overall process is known as "Oxidative Phosphorylation" or simply OxPhos.

Storing Energy in the Electrochemical Gradient

The potential energy acquired during the redox reactions of OxPhos must be stored temporarily before it is used to make ATP. This is accomplished by pumping hydrogen ions across the membrane (prokaryotes - plasma membrane, eukaryotes - mitochondrial membrane.) Enzymes that are responsible to catalyse the transfer of electron from one electron carrier to another (redox reactions) also helps to pump hydrogen ions across the membrane. Pumping of hydrogen requires active energy, because it is moving against its gradient in order to store potential energy in the electrochemical gradient.

Energy for pumping hydrogen ions

As we have previously seen, the process of transferring hydrogen atoms across membrane is a active process, but where does the energy comes from? It comes from secondary active transport. Thus, it acquires energy from other redox reactions that occur between electron carriers.

NADH transfer its electron to CoQ, which is a hydrophobic coenzyme that is located within the membrane. The enzyme the catalyses this redox reaction is known as Complex I. Now, CoQ in reduced form will transfer its electron to a hydrophilic enzyme called Cytochrome C, or only CytoC. The enzyme the catalyses this transfer is known as Complex III. Finally, CytoC transfers its electron to oxygen, through the help of Complex IV.

What about FADH?

The process of transfer of electrons from FADH to oxygen is very similar to the one regarding NADH. The difference however is associated with the use on enzyme Complex II. The use of this enzyme will NOT result in the pumping of hydrogen ions across the membrane, meaning that the oxidation of FADH will not pump as much hydrogen ion as of the one of NADH.