Fatty Acid Spiral
In this section, we will look at how fatty acids are converted into acetyl
CoA. (This sequence of reactions is also shown in your workbook in Exercise 12.)
|To help you develop your powers of observation and reasoning, I'd like you
to take a look at this diagram (and the rest of the sequence in Exercise 12) first and see
what you can figure out on your own about what is happening in each step of this process.
Take a few minutes to look at each step of this reaction process to see what happens (look
for changes) and determine what kind of reaction it is. Then continue when you're finished
evaluating these reactions.
Okay, keep your work in front of you while we go through this.
In this first reaction, notice that we start with a fatty acid and coenzyme A. They
react to form a compound which is no longer an acid but contains an acid-like group bonded
to coenzyme A. It is referred to as an acyl CoA (or sometimes this is called a fatty acyl
CoA). Note how similar this terminology is to acetyl CoA.
|What has changed here is that the acid group has lost its -OH
and the coenzyme A has lost a hydrogen. Therefore, this is an intermolecular
dehydration reaction. Removing water can be represented by writing "dehydration"
next to the arrow, writing "- H2O" next to the
arrow, or "+ H2O" by the product. But you wouldn't
normally do all three as is shown here.
|Okay, next step. What happens here? Well, I think you can see that the
double bond has been formed by the removal of two hydrogens along with their electrons.
Therefore, this is an oxidation reaction. (Ignore the H2O from
the previous reaction.)
|What next? The double bond disappears and an -OH and an -H
appear. So we've added a water molecule and this, of course, is an addition
|What next? The -OH becomes a carbonyl group
so we have to remove two hydrogens along with their electrons. Therefore, this is another oxidation
|After that, what happens? In this reaction the acetyl CoA at the end of
the molecule splits off and is replaced by another CoA. The new hydrogen on the acetyl CoA
comes from the coenzyme A that attaches to the acyl group. What has happened is something
like hydrolysis, but using coenzyme A instead of water.
Notice that the product of the last step is a fatty acyl CoA, very
much like the product of the first step only shorter by two carbon atoms. This shorter
acyl CoA can now go through steps 2-5, make another acetyl CoA and be shortened by two
carbon atoms. The resulting compound can then undergo the process again and again and
again as long as there is carbon remaining in the chain. Because of the repetitious aspect
of this process, it is sometimes referred to as the fatty acid spiral.
Notice that each time through the process an additional acetyl CoA is made. When the
process is finished for a particular fatty acid, there will be a lot of acetyl CoA
produced. Also, even more hydrogen atoms with their electrons will have been removed.
|So far, that brings the fats through the steps of glycerol and fatty
acids, through pyruvic acid, and ultimately to acetyl CoA (as shown in the top part of
this diagram). At this point the acetyl CoA is combined with another compound to make
citric acid. In the next section we will look at the details of the citric acid cycle
which removes additional carbon dioxide and hydrogen from the molecules.
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