Glycerol to Acetyl CoA
Home Up Hydrolysis of Fats Glycerol to Acetyl CoA Fatty Acid Spiral Citric Acid Cycle Electron Transport System

 

Glycerol to Acetyl CoA

The next part, converting glycerol to pyruvic acid, is quite a bit more involved. Because of that, I would like to introduce you to the kind of reasoning that I would like you to apply as you continue with this process of oxidation of fats as we look at more and more steps along the way.

66rxn04.JPG (4780 bytes)

Glycerol to Pyruvic Acid

If we look at the structural formula for glycerol and compare it to pyruvic acid (also shown in Example 10 in your workbook), you can see that a number of changes are going to have to be made.

Equation for the conversion of glycerol to pyruvic acid. [66rxn06.JPG]

The basic carbon structure is okay. However, starting at the top we need to somehow remove an oxygen. On the second carbon atom we need to remove two hydrogen atoms and take their electrons along with them. On the third carbon atom we need to add an oxygen atom and also remove two hydrogen atoms along with their electrons.

Annotated equation for the conversion of glycerol to pyruvic acid. [66rxn06a.JPG]

 

Now, think about how many steps are involved. The process should require at least four steps, because we need to

remove an oxygen,
remove two hydrogens,
replace an oxygen, and then
again remove two hydrogens.

As far as the types of reactions, removing and replacing an oxygen are kind of a wash, but removing hydrogen atoms twice is definitely going to make this an oxidation reaction overall.

This equation is in your workbook (Exercise 10) and, while this is still fresh in your mind, I would like you to try your hand at examining the structural formulas of the compounds and figuring out what steps need to be taken in order to convert glycerol into pyruvic acid. So please take a moment to do that before continuing.

Pyruvic Acid to Acetyl CoA

Now let's take a look at the next step - changing pyruvic acid into acetyl CoA (also shown in Example 11 in your workbook). An enzyme is necessary to carry out this reaction and a couple things happen.

Equation for the conversion of pyruvic acid into acetyl CoA. [66rxn07.JPG]

The hydrogens from both the HSCoA and the acid group are removed. Also, the carbon and the two oxygens from the acid group are removed so that we get carbon dioxide being released. At the same time these bonds are broken, the sulfur attaches to the middle carbon atom, replacing the carboxylic group.

Equation for the conversion of pyruvic acid into acetyl CoA highlighting the affect hydrogens. [66rxn07a.JPG]

In all, carbon dioxide is released, hydrogen and electrons are released, and the coenzyme A is attached to what remains of the pyruvic acid molecule. Those remains are kind of like an acetic acid group except that it has a bioester bond instead of an -OH. Consequently, this group is called an acetyl group and the new molecule is called acetyl CoA.

In summary, the CoA attaches to the acetyl group, carbon dioxide is evolved and two hydrogens along with their electrons are released.

Now you know, of course, that electrons and hydrogens don't just disappear, they go somewhere. If two hydrogens are removed, then this chemical is being oxidized. If it is being oxidized, then there must be an oxidizing agent.
The oxidizing agent for this particular reaction is a chemical that for now we will just refer to as NAD. In its oxidized form it has a positive charge and it picks up these hydrogen atoms and electrons and is reduced to the reduced form of NAD which is NADH, it gets one of the hydrogens and both of the electrons and the other hydrogen ion just floats around in solution.

Equation for the conversion of pyruvic acid into acetyl CoA also showing the involvement of NAD. [66rxn07b.JPG]

 

Quite often these two reactions are combined  in the kind of touch-and-go representation shown here. This is intended to show that the NAD+ comes in and the hydrogens and electrons from the other molecules are transferred to the NAD+ converting it into NADH. You may very well see this kind of reaction represented in this way.

Alternate representation of equation for the conversion of pyruvic acid into acetyl CoA, including NAD. [66rxn07c.JPG]

For now we will just say that the hydrogens and the electrons with them went to the NAD. Later, we will look in more detail at what happens to them in another part of the process.

Ions vs. Molecules

In quite a number of the books that I have consulted, they talk about pyruvate ion instead of pyruvic acid, so that the acidic hydrogen is already missing. That alters the equation slightly. The NAD+ still gets one hydrogen as well as the two electrons, but the H+ ion is already out of the picture.

 

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E-mail instructor: Sue Eggling

Clackamas Community College
2001, 2003 Clackamas Community College, Hal Bender