4.8: Overview of Fatty Acid Metabolism

Overview of Fatty Acid Metabolism
JoVE Core
Cell Biology
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JoVE Core Cell Biology
Overview of Fatty Acid Metabolism

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01:28 min
April 30, 2023

Overview

Lipids also are sources of energy that power cellular processes. Like carbohydrates, lipids are composed of carbon, hydrogen, and oxygen, but these atoms are arranged differently. Most lipids are nonpolar and hydrophobic. Major types include fats and oils, waxes, phospholipids, and steroids.

Fatty acids are catabolized in a process called beta-oxidation, which takes place in the matrix of the mitochondria and converts their fatty acid chains into two-carbon units of acetyl groups. The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle. Excess fats are stored in adipocytes as triacylglycerides and can be used as energy reserves.

Trans Fats

The food industry artificially hydrogenates oils to make them semi-solid and of a consistency desirable for many processed food products. Hydrogen gas is bubbled through oils to solidify them. During this hydrogenation process, double bonds of the cis– conformation in the hydrocarbon chain may convert to double bonds in the trans– conformation.

Recent studies have shown that an increase in trans fats in the human diet may lead to higher levels of low-density lipoproteins (LDL), or "bad" cholesterol, which in turn may lead to plaque deposition in the arteries, resulting in heart disease. Many fast food restaurants have recently banned using trans fats, and food labels are required to display the trans fat content.

Omega Fatty Acids

Essential fatty acids are those that the human body requires but does not synthesize. Consequently, they must be supplemented through ingestion via the diet. Omega-3 fatty acids  and omega-6 fatty acids are the only two that humans require. These Polyunsaturated fatty acids are termed omega-3 because a double bond connects the third carbon from the hydrocarbon chain's end to its neighboring carbon, while in omega-6 the double bond is present on the sixth carbon.

Phospholipids

Phospholipids are major plasma membrane constituents that comprise cells' outermost layer. Like fats, they are fatty acid chains attached to a glycerol or sphingosine backbone. However, instead of three fatty acids attached as in triglycerides, there are two fatty acids forming diacylglycerol, and a modified phosphate group occupies the glycerol backbone's third carbon. A phosphate group alone attached to a diacylglycerol does not qualify as a phospholipid. It is phosphatidate (diacylglycerol 3-phosphate), the precursor of phospholipids. An alcohol modifies the phosphate group. Phosphatidylcholine and phosphatidylserine are two important phospholipids that are in plasma membranes.

Steroids

Cholesterol is the most common steroid. Cholesterol is mainly synthesized in the liver and is the precursor to many steroid hormones such as testosterone and estradiol, which are secreted by the gonads and endocrine glands. It is also the precursor to Vitamin D. Cholesterol is also the precursor of bile salts, which help in the emulsification of fats and their subsequent absorption by cells.

This text is adapted from Openstax, Biology for AP Courses, Section 3.3: Lipids

Transcript

Fatty acid metabolism includes the synthesis of fatty acids in the cytoplasm and their breakdown in the mitochondria and peroxisomes. Both of these processes are tightly regulated depending on nutrient availability.

For example, consider the breakdown of an eight-carbon fatty acid molecule. It is first esterified to produce fatty acyl-CoA in the cytoplasm and subsequently transported into mitochondria for beta-oxidation.

Here, the fatty acyl-CoA molecule loses two carbon atoms to produce a six carbon fatty acid chain along with a molecule of acetyl coenzyme A, NADH, and FADH2. This process is repeated until the fatty acid is completely converted into acetyl CoA, which then enters the citric acid cycle producing more NADH and FADH2.

These reduced coenzymes donate their electrons to the electron transport chain to produce ATP.

During fatty acid synthesis, malonyl-CoA, a three-carbon molecule, donates two carbons to an acetyl group forming a four-carbon fatty acid molecule, while the third carbon is released as carbon dioxide. This process repeats until the desired eight-carbon fatty acid is formed.

Key Terms and definitions​

  • Lipids – Energy sources powering cellular processes composed of carbon, hydrogen, and oxygen atoms.
  • Fatty acids – Catabolized through beta-oxidation, converting their chains into two-carbon units of acetyl groups.
  • Trans Fats – Artificially hydrogenated oils that may increase LDL (bad cholesterol) levels.
  • Omega Fatty Acids – Essential fatty acids that the human body requires but does not produce such as Omega-3 and Omega-6.
  • Phospholipids – Major plasma membrane constituents comprising fatty acid chains attached to a glycerol or sphingosine backbone.

Learning Objectives

  • Define Lipids – Explain its role as energy sources for cellular processes (e.g., Lipids).
  • Contrast Saturated vs Unsaturated Fatty Acids – Explain the differences in their formation and health effects (e.g., Trans Fats vs Omega Fatty Acids).
  • Explore Beta-Oxidation – Describe the process of catabolizing fatty acids into acetyl groups (e.g., Beta Oxidation).
  • Explain Fatty Acid Synthesis – Detail how the body forms and stores fatty acids for energy reserves.
  • Apply in Phospholipids Understanding – Explore how phospholipids contribute to the structure and function of cell membranes.

Questions that this video will help you answer

  • [Question 1] What are lipids and how are they catabolized into energy for cellular processes?
  • [Question 2] What's the difference between Trans Fats and Omega Fatty Acids and their effects on human health?
  • [Question 3] How does the process of beta-oxidation convert fatty acids into acetyl groups?

This video is also useful for

  • Students – Understanding Lipid Metabolism enhances comprehension of body's energy production process.
  • Educators – Provides a clear framework for concepts around lipid types and their roles in human health.
  • Researchers – Offers insights into the biochemistry of lipid metabolism for further study.
  • Science Enthusiasts – In-depth exploration into the fascinating world of fats; Lipids, Fatty Acids, Omega Fatty Acids, Phospholipids.