Metabolism of fatty acids and proteins: MCAT

Medistudents Team
May 3, 2023

The metabolism of fatty acids and proteins is included in the Biological and Biochemical Foundations of Living Systems section of the MCAT. The syllabus breaks down the topic further into the following:

  • Description of fatty acids
  • Digestion, mobilization, and transport of fats
  • Oxidation of fatty acids
  • Ketone bodies
  • Anabolism of fats
  • Non-template synthesis: biosynthesis of lipids and polysaccharides
  • Metabolism of proteins

This guide will provide you with a clear overview of everything you need to know about the metabolism of fatty acids and proteins for the MCAT.

Description of fatty acids

Fatty acids are a group of lipids that have a long hydrocarbon chain with one end having a carboxylic functional group (COOH) (highlighted in green in figure 1 below). Naturally occurring fatty acids usually have between 4-28 carbons – making some very long chains.

Fatty acids can then be further grouped into saturated and unsaturated, with unsaturated fatty acids being any FAs that contain one or more carbon-carbon double bonds. These types of fatty acids are liquid at room temperature as the C-C double bond causes a kink in the FA chain – interrupting the uniform structure usually seen in a fatty acid hydrocarbon chain. Saturated fatty acids have no C-C double bonds and therefore have a uniform structure that makes them solid at room temperature.

Saturated and unsaturated fatty acid diagram
Figure 1: Saturated and unsaturated fatty acid diagram. Red highlights the carbon-carbon double bond that gives unsaturated fatty acids its name.

Digestion, mobilization and transport of fats

For the MCAT, you need to know about the following:


First, lingual lipase in the saliva may break down the initial fat molecule (a triglyceride) into a diglyceride or the triglyceride will move to the stomach where gastric lipase will break it down. However, most digestion of the fat occurs when it enters the small intestine; hormones are released that stimulate the production of pancreatic lipase from the pancreas and bile from the liver. Both of these compounds help the fat to break down completely into 3 fatty acid molecules and one monoglyceride molecule.

Mobilization and transport

The components of a triglyceride (fatty acids and monoglyceride) are packaged in micelles and absorbed into the small intestine by the microvilli. After they have crossed the epithelial cell layer, the components assemble back into triglycerides which accumulate to form a lipoprotein known as a chylomicron. This chylomicron then exits the cell and enters the bloodstream via the lymphatic system to be transported to wherever it is needed in the body.

In the liver, fats can also be transported by other lipoproteins like very low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins as well as a blood protein known as albumin.

Oxidation of fatty acids

Beta-oxidation is the process used to degrade fatty acids into acetyl CoA (used in the TCA cycle). It also makes NADH and FADH₂- which deposit electrons into the electron transport chain.

Beta-oxidation is an aerobic process that occurs in the mitochondrial matrix and it follows these general steps:

  1. Activation of fatty acid by adding CoA to form to make fatty acyl-CoA
  2. Fatty acyl-CoA is oxidized into enoyl-CoA and FADH₂ is made
  3. Enoyl-CoA is hydrated to form an alcohol
  4. This alcohol is oxidized to form a carbonyl, with its lost electron added to NAD to make NADH
  5. Acetyl CoA is cleaved off from the carbonyl leaving an acyl-CoA that is two carbons shorter than when it first entered the beta-oxidation process

This cycle will repeat until all that remains is an acetyl CoA which will be fed into the TCA cycle.

Unsaturated fatty acids need to be metabolized before they enter the beta oxidation cycle as the presence of carbon-carbon double bonds would interfere. A double bond is removed either through hydration (adding water (H₂O)) or hydroxylation (adding a hydroxyl group (-OH)).

Ketone bodies

Ketone bodies are produced by the liver and used as an energy source when there are insufficient amounts of glucose in the body. They are made from excess acetyl-CoA (made during beta oxidation).

Excess acetyl-CoA gets diverted to the ketogenesis pathway (instead of the TCA cycle) where it will be converted into HMG CoA and then eventually into β-hydroxybutyrate (the most common ketone body found in the blood).

Anabolism of fats

Anabolism of fats is also known as fatty acid synthesis. There are several steps to this process:

  1. Acetyl CoA is converted into citrate in the TCA cycle so that it can be transported out of the mitochondria into the cytosol
  2. Citrate is then cleaved to make acetyl-CoA and oxaloacetate by the enzyme ATP citrate lyase
  3. Acetyl-CoA is carboxylated to make malonyl-CoA
  4. Fatty acid synthase (a complex of multiple enzymes) then begins fatty acid chain elongation by adding acetyl-CoA onto the malonyl-CoA
  5. The addition of acetyl-CoA repeats until the original malonyl-CoA becomes a 16-carbon fatty acid chain (known as palmitic acid)
  6. The palmitic acid can then be further processed to make more useful molecules – like triglycerides used for storage

Non-template synthesis

Unlike protein synthesis that uses a template (mRNA), fatty acid synthesis does not have a template. Instead, the enzyme acetyl-CoA carboxylase recognises the sugars it is linking together. Similarly, in polysaccharide synthesis (also known as gluconeogenesis) a template is not needed to direct this process.

Metabolism of proteins

Protein metabolism begins in the stomach where protease enzymes like pepsin, break down complex protein molecules into polypeptides. Then enzymes like chymotrypsin are released into the small intestine to further break down the polypeptides into their constituent amino acids. These amino acids are then transported to the bloodstream where they will be moved to other cells of the body to create new proteins or transported to the liver to break down amino acids further.

When there is an excess amount of amino acids, amino acids are taken to the liver where they will undergo amino acid catabolism. In a state of starvation the amino acids will be broken down to create energy:

  1. The amino group (-NH₂) is removed from the amino acid (deamination)
  2. The amino group is then converted into urea and excreted
  3. The remaining portion of the amino acid is oxidized to create an α-keto acid
  4. The keto acid will enter the TCA cycle to produce energy

If the body is not in starvation mode then excess amino acids can be stored as glucose, fats or ketone bodies.

Hopefully this has helped your MCAT revision of the metabolism of fatty acids and proteins. You’ll find more revision materials to help your MCAT preparation on our website, including essential MCAT topics, such as the principles of metabolic regulation. There’s also more information about the MCAT syllabus and format of the exam, key dates and what’s a good score.