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.
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.
|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.
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
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.
|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.
|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
|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.
|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.
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
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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