Electron Transport System
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Electron Transport System

In this section we'll focus on how the hydrogen and electrons combine with oxygen to form water. This involves a series of oxidation-reduction reactions, in which elctrons (and hydrogen) are tranferred from one chemical to another to another in what is called the electron transport system. It comprises the lower left-hand portion of this diagram.

66rxn04.JPG (4780 bytes)


Oxidation - Reduction

Let's return to one of the reactions in which hydrogen atoms along with their electrons are removed from a molecule (step 9 in the citric acid cycle). You should know from your earlier work with oxidation-reduction reactions (in CH-105) that in a reaction like this hydrogen and the electrons don't just simply leave, they have to be removed. The reactant in this process is being oxidized. That doesn't happen by itself, there has to be an oxidizing agent.

Equation for oxidation of malic acid to oxaloacetic acid (step 9 of citric acid cycle). [66rxn12.JPG]

Two oxidizing agents that operate in the citric acid cycle are simply referred to as FAD and NAD. Each one is a fairly complicated molecule and each is a nucleic acid derivative. (We will study nucleic acids in Lesson 9.) The structures of these compounds can be found in your textbook or other references.

FAD stands for flavin adenine dinucleotide and it is synthesized in our bodies from the vitamin riboflavin. Its oxidized form is FAD and its reduced form is FADH2. Note that FAD is reduced by taking on two hydrogens and two electrons.

NAD stands for nicotinamide adenine dinucleotide and it is synthesized in our bodies from the vitamin niacin. Its oxidized form is NAD+ and its reduced form is NADH. Note that NAD+ is reduced by taking on two electrons but only one hydrogen.

NAD is the oxidizing agent that works with this reaction. Two hydrogens and two electrons are removed from the reactant (malic acid) to oxidize it into the the product (oxaloacetic acid). NAD takes both of the electrons, but it takes only one of the hydrogens. The other hydrogen, without its electron, floats around in solution as an H+ ion.

Equation for oxidation of malic acid to oxaloacetic acid (step 9 of citric acid cycle). [66rxn12a.JPG]

Redox Review

When we first dealt with oxidation-reduction reactions (in CH-105), we didn't deal with electrons coming off along with hydrogen atoms. Instead, we dealt with electrons coming off from simple metal atoms.

At that time, we were able to develop an oxidation potential list that rated these chemicals with regard to how easily the electrons could be pulled off. We were even able to assign voltages to reflect how readily those electrons could be removed.

Short oxidation potential list including K, Na and Mg. [66tab05.JPG.

We developed the idea that in an oxidation-reduction reaction, not only did oxidation take place, but so did reduction.
For example, a chemical such as a potassium atom would lose electrons only if there were something available to take electrons, such as a sodium ion or a magnesium ion. So in order for potassium reaction to proceed to the right, a reaction further down on the table, such as sodium, has to proceed to the left. In essence, the potassium atom (K in top box) transfers an electron to a sodium ion (Na+ in top box) and converts it to the sodium atom (Na in lower box). The electron has been passed from potassium to sodium.
K    K+
Na    Na+
  Mg    Mg2+
Na    Na+
  Mg    Mg2+
Once that has happened, the sodium in its reduced form with the electron (Na in new top box) can now react with something else that is lower than it on this oxidation potential list, such as Mg2+ (in the same box). (Yes, that would actually have to happen twice in order for the magnesium to become reduced because it requires two electrons.)  The electron(s) has been passed from sodium to magnesium.
Na    Na+
  Mg    Mg2+
Na    Na+
  Mg    Mg2+
I hope you can visualize that if the process could be controlled in just the right way, electrons could be passed stepwise from potassium to sodium to magnesium (and right on down the line, if we had more redox chemicals lined up). Each exchange is an electron transfer reaction, and the sequence of reactions would be an electron transfer (or transport) system. A voltage can be calculated for each step and that voltage is a measure of the amount of energy released in each step.


Biochemical Redox

These same ideas apply to biochemical oxidation-reduction reactions. Oxidation potential lists for biochemical compounds have been figured (even with voltages, but we won't need those).

Biochemical oxidation potential list (without potential values). [66tab06.JPG]


In the reaction that we were just looking at, the starting compound lost two hydrogens along with its electrons to form a double bond and released two hydrogens and electrons. Essentially, the hydrogens and their electrons have been transferred to the NAD.

66rxn12a.JPG (3327 bytes)

In turn, the NAD will transfer those hydrogens and electrons to another chemical (FAD) and as soon as it has given those electrons to another chemical, it's free to react with another molecule and take its hydrogens and electrons. The FADH2, in turn, will pass those hydrogens and electrons on to other biochemical redox compounds.

Equation showing the transfer of electrons from NAD to FAD. [66rxn13.JPG]

The sequence of chemicals (in part "disguised" here as X and Y) which transfer electrons and hydrogen atoms from one to the next to the next to the next is referred to as the electron transport system. Ultimately, those electrons and the hydrogen atoms are transferred to oxygen to make water.

Extended biochemical oxidation potential list (still without potentialvalues). [66tab07.JPG]

Just as with the inorganic oxidation-reduction reactions, every time electrons are transferred some voltage (or energy) is provided. With an SOP list you are able to calculate the voltage. Similarly, in the electron transport system, as the electrons and hydrogen atoms are transferred from one chemical to another, some energy is provided at each step along the way. That is the energy that living things use to carry on their activities.


"Storing" Energy in ATP

One of the things for which that energy is used is to convert a chemical known as ADP (adenosine diphosphate) into a molecule of ATP (adenosine triphosphate) by adding a phosphate group to it.

Equation showing the transfer of electrons from NAD to FAD and the transfer of released energy into converting ADP to ATP. [66rxn13a.JPG]

The reaction can be reversed and the process of converting ATP back to ADP can release energy. ATP is quite often represented as A-P-P~P where the "~" is characterized as a "high energy bond" which can be broken and release energy that is needed for biological processes.  There is a problem with this kind of representation.

energy + A-P-P + P A-P-P~P

A-P-P~P A-P-P + P + energy

The problem here is that this implies that the formation of the last phosphate bond requires energy, and that breaking that bond releases energy. Now if you think about that from a chemical point of view, it shouldn't make any sense at all. That is because the formation of bonds releases energy and breaking bonds requires energy. Just the opposite of what is implied here. The resolution of that dilemma involves taking a closer look at the molecules that are involved in this reaction.

Shown here are the structural formulas for ADP and ATP and phosphoric acid. In biochemistry, phosphoric acid is quite often represented by Pi to mean inorganic phosphate. I have chosen to use molecular formulas in this equation even though, at biological pH, these chemicals will be partially ionized.

Equation with structural formulas showing the conversion of ADP to ATP. [66rxn14.JPG]

Notice that when the hydrogens and oxygens are taken into account, the nature of this reaction looks a bit different. In order to change from ADP to ATP, a dehydration reaction occurs in which an -H and an -OH are taken from the ADP and the Pi to form a water molecule.
In summary, bonds need to be broken and formed in order for this reaction to occur. This reaction does require energy and it is endothermic. However, the endothermic portion of the reaction is in breaking bonds (indicated by / in the diagram), not in forming the new "high energy" bond.

Annotated equation with structural formulas showing the conversion of ADP to ATP. [66rxn14a.JPG]


In a biochemical situation where energy is required, an ATP molecule can be hydrolyzed to form ADP and the inorganic phosphate. This reaction will require breaking the "high energy" bond and also a bond in the water molecule. But two new bonds will be formed (in the ADP and in the Pi), which apparently are stronger than the bonds that were broken. For that reason, this reaction is exothermic and this process will release energy. That released energy can be used by enzymes to force reactions to take place that are necessary for a living system to carry out the functions that it needs to carry out.


Electron Transport System in Perspective

Let's again place the electron transport system in the overall context of the the oxidation of fats. It is the portion of the process in which the hydrogen and electrons that have been stripped from the fat molecules are shuffled along to an eventual combination with oxygen to make water molecules. The energy released in that process is temporarily stored (in ATP and other "high energy" compounds) for use by enzymes to catalyze endothermic biochemical reactions.

This diagram shows that process in the lower left hand portion. That, however, is a gross oversimplification because hydrogen and electrons are removed from the breakdown products of fat molecules at many steps throughout this overall process. Electrons and hydrogen from all of those steps are run through the electron transport system to ultimately combine with oxygen and become water.

66rxn04.JPG (4780 bytes)


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