Fatty acid synthesis

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Fatty acid synthesis



Fatty acid synthesis


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USMLE® Step 1 questions

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Fatty acid synthesis

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USMLE® Step 1 style questions USMLE

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Biochemistry researchers are studying the pathway of fatty acid synthesis with students in the laboratory. Which of the following is the rate-limiting step of fatty acid synthesis?  

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In addition to carbohydrates and proteins, lipids are the third main macromolecule we consume in our diet.

Fatty foods include red meat, dairy products, and even peanut butter.

And lipids come in many forms, including cholesterol, glycerol, phospholipids, and fatty acids.

Of these, fatty acids are the simplest form of lipids - they’re basically just long chains of carbon and hydrogen, that are grouped by length into short, medium, long and very long chain fatty acids.

Fatty acids can also combine with glycerol to make triacylglycerides, which is made of 3 fatty acids attached to a glycerol molecule, and is the main storage form of fat in our body.

Now, short and medium-chain fatty acids are primarily obtained from the diet, but the liver and fat cells can synthesize long chain fatty acids.

This occurs by combining lots of 2-carbon molecules, called acetyl-coenzyme A or acetyl-CoA, into a single 16-carbon, long chain fatty acid called palmitoyl-coenzyme A, or palmitoyl-CoA.

Palmitoyl-CoA can then serve as a precursor to even longer chain fatty acids.

To make palmitoyl-CoA, acetyl-CoA provides the carbon atoms, and nicotinamide adenine dinucleotide phosphate, or NADPH provides the hydrogen atoms.

As it turns out, most of the acetyl-CoA used to make fatty acids comes from carbohydrate metabolism - specifically glucose, which is a 6-carbon sugar molecule.

After eating a glucose-rich dinner, like cake and cookies, glucose levels in the blood rise quickly.

In response, the pancreas secretes insulin, a hormone which makes our cells take in and process a lot more glucose.

Inside the cells, glucose can enter glycolysis where it’s broken down into two 3-carbon pyruvate molecules, and that yields a bit of energy in the form of adenosine triphosphate - or ATP.


Fatty acids are one of the essential forms of energy storage. The biosynthesis of fatty acids is a multi-step process in the cytoplasm, mainly of the liver and fat cells. The process takes place in three major steps: the citrate shuttle, acetyl-CoA carboxylase (the rate-limiting step), and fatty acid synthase complex.


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