Module 9 - Fatty Acid Synthesis Flashcards
How many stages are there to fatty acid synthesis?
Three
The first step is to shuttle the acetyl CoA from the mitochondria to the cytosol where fatty acid synthesis occurs
the second stage, acetyl CoA is activated to form malonyl CoA
the third and final stage, palmitate is synthesized in a five-step elongation cycle
How is acetyl CoA shuttled out of the mitochondria?
First, acetyl CoA reacts with oxaloacetate to form citrate by the enzyme citrate synthase
the citrate can be shuttled out of the mitochondria through a transport protein that resides in the mitochondrial membrane
The net result is that acetyl CoA has now been brought into the cytosol at the expense of an ATP
What happens once citrate is in the cytosol?
citrate is acted upon by the enzyme ATP-citrate lyase, which cleaves citrate into acetyl CoA and oxaloacetate as shown:
Citrate + ATP + CoA + H2O → Acetyl CoA + ADP + Pi + Oxaloacetate
The oxaloacetate is reduced to malate using NADH, followed by the conversion of malate to pyruvate by malic enzyme which produces a molecule of NADPH
Pyruvate is able to be transported back into the mitochondria and converted by a carboxylation reaction to oxaloacetate catalyzed by pyruvate carboxylase
How is acetyl CoA activated?
by combining it with HCO3-, the form of CO2 in water
This carboxylation reaction, catalyzed by acetyl CoA carboxylase, is driven by the hydrolysis of ATP and is the committed step in fatty acid synthesis
How does the carboxylation reaction occur?
In the first step, biotin is carboxylated and coupled with ATP hydrolysis which drives the reaction forward
Step 1: biotin-enzyme + ATP + HCO3- → CO2-biotin + ADP + Pi + H+
In step 2, the carboxyl group is transferred from biotin to acetyl CoA to form malonyl CoA, a 3-carbon molecule.
Step 2: CO2-biotin + acetyl CoA → malonyl CoA + biotin-enzyme
What is palmitate?
a 16-carbon saturated fatty acid
How is palmitate formed?
through a series of four reactions
Step 1: Condensation reaction catalyzed by β-ketoacyl synthase
Step 2: Reduction step catalyzed by β-ketoacyl reductase
Step 3: Dehydration step catalyzed by 3-hydroxyacyl dehydratase
Step 4: Reduction step catalyzed by enoyl reductase.
A large enzyme system called fatty acid synthase performs the task of synthesizing fatty acids
In bacteria and plants, the fatty acid synthase complex is made up of individual proteins that each contain a separate catalytic activity;
in yeast, all of the catalytic activities are present in two polypeptides;
while in vertebrates, all of the enzyme activities are contained within one large polypeptide
An important component of the fatty acid synthase complex is the acyl carrier protein or ACP
ACP has a phosphopantetheine prosthetic group linked to it.
The phosphopantetheine group has a free sulfhydryl group, and it is to this group that the intermediates of fatty acid synthesis are linked.
ACP thus acts as an arm, which moves the fatty acid intermediate from one enzyme to the other as it synthesizes palmitate.
Step 1: Condensation reaction catalyzed by β-ketoacyl synthase
In this reaction, the acetyl group from acetyl ACP reacts with malonyl ACP to form acetoacetyl ACP (4-carbons), with the release of CO2 and the ACP from acetyl ACP
there is a substantial release of energy which drives this reaction forward and makes it energetically favorable
Step 2: Reduction step catalyzed by β-ketoacyl reductase
In this reaction, acetoacetyl ACP is reduced to 3-hydroxybutyryl ACP using NADPH as the reducing agent.
Note that the keto group is now a hydroxyl group as a result of the reduction.
Step 3: Dehydration step catalyzed by 3-hydroxyacyl dehydratase
a water molecule is removed (thus a dehydration), forming crotonyl ACP.
This removes the oxygen group and leaves a double bond.
Step 4: Reduction step catalyzed by enoyl reductase.
The double bond in crotonyl ACP is reduced through the consumption of another molecule of NADPH, to form butyryl ACP.
At this point, a 4-carbon molecule has been formed.
The final product of fatty acid synthesis is palmitate, a 16-carbon molecule.
Thus, six more rounds of fatty acid synthesis (the four reactions above) have to occur.
Look at Figure 9-5 again and note the product of the first round, butyryl ACP.
If you take butyryl ACP and replace acetyl ACP with it at the top of the pathway, you see how this works.
How is the synthesis of the fatty acid completed?
In order to complete the synthesis of a free fatty acid, palmitate has to be released from ACP.
This is accomplished by the action of an enzyme called thioesterase, which hydrolytically (using H2O) cleaves the link between the carboxyl group of the fatty acid and the sulfhydryl group of ACP
What is the Overall Stoichiometry of Palmitate Synthesis?
8 Acetyl CoA + 7 ATP + 14 NADPH → palmitate + 14 NADP+ + 8 CoA + 6 H2O + 7 ADP + 7 Pi
Not surprising for a biosynthetic reaction, both ATP and NADPH are consumed to provide the energy and reducing equivalents, respectively.