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.

66rxn08.JPG (16217 bytes)

Okay, keep your work in front of you while we go through this.

Step 1

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.

Equation for step 1 of the fatty acid spiral. [66rxn08a.JPG]

Step 2

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.)

Equation for step 2 of the fatty acid spiral. [66rxn08b.JPG]

Step 3

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 reaction.

Equation for step 3 of the fatty acid spiral. [66rxn08c.JPG]

Step 4

What next? The -OH becomes a carbonyl group so we have to remove two hydrogens along with their electrons. Therefore, this is another oxidation reaction.

Equation for step 4 of the fatty acid spiral. [66rxn08d.JPG]

Step 5

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.

Equation for step 5 of the fatty acid spiral. [66rxn08e.JPG]

Spiral

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.

Recap

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.

66rxn04.JPG (4780 bytes)

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E-mail instructor: Sue Eggling

Clackamas Community College
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