Lipids: Classification and Functions

Lipids

• Lipids are a broad group of organic compounds insoluble in water and soluble in organic solvents.
• They are found in plants, animals, and microorganisms.
• Lipids are used in cell membranes, energy storage, insulation, and hormones.
• Lipids contain hydrogen, carbon, and oxygen atoms.

Classification of Lipids

• Simple lipids: Esters of fatty acids with various alcohols, including fats and waxes.
• Compound lipids: Lipids with additional groups such as phosphate, nitrogen, or a carbohydrate.
• Derived lipids: Substances derived from simple or compound lipids.

Simple Lipids

Fats

• Fats are tri-esters of glycerol and fatty acids.
• Fats are solid at room temperature due to the presence of saturated fatty acids.
• Oils are liquid at room temperature and contain unsaturated fatty acids.

Waxes

• Waxes are esters of fatty acids with high molecular weight alcohols.
• Waxes contain one molecule of fatty acid and one molecule of alcohol.
• Beeswax is an example of a wax composed of palmitic acid and myristic palmitate.

Fatty Acids

• Fatty acids are obtained from the hydrolysis of fats.
• Fatty acids are long-chained carboxylic acids with 4-36 carbon atoms.
• They are not found free but remain associated with alcohol to form triglycerides.
• Fatty acids are stored as an energy reserve through ester linkage to glycerol, forming triglycerides.

Saturated Fatty Acids

• Saturated fatty acids have the maximum possible number of hydrogen atoms bonded to each carbon.
• Examples include butaric acid (C4) and caproic acid (C6) found in milk.
• Palm kernel oil is rich in fatty acids with 8 and 10 carbons (C8 and C10).

Unsaturated Fatty Acids

• Unsaturated fatty acids have one or more carbon-carbon double bonds.
• Monounsaturated fatty acids have one double bond, while polyunsaturated fatty acids have two or more double bonds.
• Oleic acid is an example of a monounsaturated fatty acid, while linoleic acid and eleostearic acid are examples of polyunsaturated fatty acids.
• The cis configuration of double bonds is common in biological unsaturated fatty acids.

Cellular Membranes

• Cell membranes are formed by two phospholipid monolayers associating with their tails in an aqueous solution to create a phospholipid bilayer.

Protein Modification

• Fatty acids play critical roles in protein interactions.
• Polyunsaturated fatty acids are important for protein acylation, which is crucial for anchoring, folding, and function of proteins.
• Fatty acids can interact with nuclear receptors and promote gene expression, acting as transcription factors.

Degradation of Fatty Acids

Beta (β) Oxidation of Fatty Acid

• Beta-oxidation is the catabolic process breaking down fatty acid molecules in the cytosol (prokaryotes) or mitochondria (eukaryotes) to produce acetyl-CoA.
• This process involves multiple steps to break down fatty acid molecules for energy.

Location of Beta-Oxidation

• Beta-oxidation occurs in the mitochondria of eukaryotes and the cytosol of prokaryotes.

Products of Beta-Oxidation

• For each removal of a two-carbon group from the fatty acid chain, one acetyl CoA, one NADH, and one FADH2 are produced.

Stages of Beta-Oxidation

• Activation of fatty acids in the cytosol: Fatty acid protein transporters facilitate the entry of fatty acids into the cytosol. Fatty Acyl-CoA Synthase (FACS) adds a CoA group to the fatty acid chain, forming acyl-CoA.
• Transport of activated fatty acids into mitochondria (via carnitine shuttle): The enzyme Carnitine Palmitoyl Transferase I (CPT1) converts acyl-CoA to acylcarnitine, which is transported across the mitochondrial membrane by carnitine translocase (CAT). Inside the mitochondria, Carnitine Palmitoyl Transferase II (CPT2) converts the acylcarnitine back to acyl-CoA.
• Beta oxidation proper in the mitochondrial matrix: This process involved four steps:
  • Dehydrogenation: A double bond is formed between the second and third carbon (C2 and C3) of the acyl-CoA chain, forming trans-Δ -enoyl-CoA. This step involves FAD and produces FADH2.
  • Hydration: The double bond between C2 and C3 of trans-Δ -enoyl-CoA is hydrated to form L-β-hydroxyacyl CoA. This reaction requires water and is catalyzed by enoyl CoA hydratase.
  • Oxidation: The hydroxyl group in C2 of L-β-hydroxyacyl CoA is oxidized by NAD+ to form β-ketoacyl CoA and NADH + H. This step involves 3-hydroxyacyl-CoA dehydrogenase.
  • Thiolysis: The β-ketoacyl CoA is cleaved by a thiol group (SH) of another CoA molecule, forming an acetyl-CoA molecule and an acyl-CoA chain two carbons shorter than the original. This reaction is catalyzed by β-ketothiolase.

End of Beta Oxidation

• For even-numbered acyl-CoA chains, beta oxidation ends when a four-carbon acyl-CoA chain is broken down into two acetyl-CoA units. Each acetyl-CoA enters the citric acid cycle to generate ATP.
• For odd-numbered acyl-CoA chains, beta oxidation ends with a five-carbon acyl-CoA chain broken down into a three-carbon propionyl-CoA and a two-carbon acetyl-CoA. Propionyl-CoA is then converted to succinyl-CoA, which enters the citric acid cycle to produce ATP.

Ketone Bodies

• Ketone bodies are water-soluble compounds containing ketone groups produced by the liver during periods of fasting and starvation.
• They are used to provide energy when glucose is low or absent.
• The three main ketone bodies are acetone, acetoacetate, and β-hydroxybutyrate.
• Ketone bodies are produced during gluconeogenesis, a process that creates glucose from noncarbohydrate sources.

Why Ketone Bodies are Produced?

• During prolonged fasting or starvation, the body's glucose levels decrease.
• This triggers the liver to produce ketone bodies from fatty acids.
• Ketone bodies can be used as an alternative fuel source by the brain and other tissues when glucose is limited.
• Ketone bodies are also produced in conditions like diabetes, where glucose metabolism is impaired.