Fatty Acids and Their Structures

Arachi Donic Acid

• Arachi donic acid is a polyunsaturated fatty acid with 20 carbons and 4 double bonds
• It's a ω-6 fatty acid that can be described as 20:4, Δ5,8,11,14
• Naturally occurring fatty acids have double bonds in the cis configuration
• In cis configuration, hydrogens are on the same side of the double bond and the acyl chains are on the other side
• In trans fatty acids, the acyl chains are on opposite side of the double bond
• Margarine and the fat used in preparing French fries are major sources of trans fatty acids found in humans
• Trans fatty acids are produced by the chemical hydrogenation of polyunsaturated fatty acids in vegetable oils, and are not a natural food product

Base Chain Length of Hydrocarbon Chain

• Short chain fatty acid - 2-6 carbon atoms
• Medium chain fatty acid - Octanoic acid (8 carbons) and Decanoic acid (10 carbons)
• Long chain fatty acid - 16-22 carbons
• Very long chain fatty acid - >24 carbons

Based on Source/Nutritional Classification

• Essential fatty acids (EFAs) - Fatty acids that the body cannot produce on their own
• Non-essential fatty acids (NEFAs) - Fatty acids that the body can produce from other fatty acids
• Endogenous polyunsaturated fatty acids (PUFAs) - These are a type of non-essential fatty acid
• PUFAs are a type of NEFAs, and the body can produce them from other fatty acids

Arachidonic Acid Derivatives - Eicosanoids

• Arachidonate and other C20 polyunsaturated fatty acids give rise to a diverse set of eicosanoids
• Eicosanoids are physiologically and pharmacologically active compounds
• Eicosanoids are best known as prostaglandins (PG), thromboxanes (TX), leukotrienes (LT), and lipoxins (LX)
• Physiologically, eicosanoids are considered to act as local hormones (autocoids) via G-protein linked receptors
• Eicosanoids elicit biochemical effects by acting on receptors
• There are three groups of eicosanoids that are synthesized from C20 eicosanoic acids
• These eicosanoic acids are derived from essential fatty acids linoleate and α-linolenate, or directly from dietary arachidonate and eicosapentaenoate
• Arachidonate is usually derived from the 2 position of phospholipids in the plasma membrane by the action of phospholipase A2
• Arachidonate can also come from diet
• Arachidonate is the substrate of the synthesis of PG2, TX2 series (prostanoids) by the cyclooxygenase pathway
• Arachidonate is the substrate of the LT4 and LX4 series by the lipoxygenase pathway
• The two pathways compete for the arachidonate substrate
• Eicosanoids, including prostaglandins, thromboxanes, and leukotrienes, control cellular function in response to injury
• These compounds are all derived from arachidonic acid, a 20 carbon polyunsaturated fatty acid usually present in cells as part of the membrane lipid phosphatidylcholine
• Although almost every cell in the body produces eicosanoids in response to tissue injury, different cells produce different eicosanoids
• Eicosanoids are the principal mediators of paracrine and autocrine functions, affecting the cells that produce them or neighboring cells
• For example, vascular endothelial cells (cells lining the vessel wall) secrete the prostaglandin PGI2 (prostacyclin)
• Prostacyclin acts on nearby smooth muscle cells causing vasodilation (expansion of the blood vessel)
• Eicosanoids are derived from arachidonic acid and retain its original 20 carbons (hence the name eicosanoids)
• All prostaglandins, such as prostacyclin, have an internal ring### Functions of Lipids
• Nutritional Value: Lipids provide taste, aroma, and palatability to food.
• Enhanced Absorption of Fat-Soluble Vitamins: Lipids enhance the absorption of vitamins A, D, E, and K.
• Vitamin A: Acts as a substrate for rhodopsin pigment in the retina, improving vision, especially in darkness.
• Vitamin A: Stimulates epithelial cell growth and differentiation, enhancing healing processes.
• Vitamin A: Serives as an exogenous antioxidant.
• Vitamin D: Promotes calcium absorption in the gastrointestinal tract, facilitating bone mineralization.
• Vitamin E: Serves as a potent antioxidant, particularly for -tocopherol.
• Vitamin K: Involved in the γ-carboxylation of vitamin K-dependent clotting factors.
• Protective Function: Lipids provide protection and cushioning for internal organs like the heart, kidneys, and mesentery.
• Thermal Insulator: Fat deposition in adipose tissue and subcutaneous tissues regulates body temperature.
• Electrical Insulator: Myelin sheath, rich in lipids, protects axons and enhances the propagation of nerve impulses.
• Energy Generation: Triacylglycerol provides a significant source of energy storage, mobilized for energy production.
• Neurotransmitter Generation and Transmission: Cholesterol, a precursor for steroid hormones, plays a role in neurotransmitter synthesis and transmission via hydrolysis of phosphatidylcholine to provide choline for acetylcholine generation.
• Metabolic Fuel During Prolonged Fasting and Starvation: Lipolysis provides metabolic fuel during prolonged fasting and starvation.
• Cholesterol as a Precursor for Steroid Hormones: Cholesterol serves as a precursor for steroid hormones like corticosteroids.
• Cholesterol in Bile Acid Synthesis: Cholesterol is a precursor for bile acids, crucial for fat emulsification and absorption, along with the participation of bile acids in the metabolism of lipids.
• Cholesterol in Adrenal Cortex Function: Cholesterol is a precursor for corticosteroid synthesis in the adrenal cortex, contributing to fluid and electrolyte balance, cardiovascular health, and energy substrate homeostasis.
• Arachidonic Acid in Cell Injury: Arachidonic acid is a precursor for inflammatory mediators involved in cell injury and damage.
• Eicosanoids in Inflammation: Eicosanoids, derived from arachidonic acid, are potent autocoids (local hormones) that participate in tissue injury and homeostasis.
• Phosphoglycerides in Surfactant: Phosphoglycerides are components of surfactant, preventing alveolar collapse.
• Glycolipids in Intercellular Communication: Glycolipids play a role in intercellular communication and cell recognition, with their oligosaccharides acting as cell recognition factors, as seen in the ABO blood group system.
• Lipid Transport and Solubility: Plasma lipoproteins transport plasma lipids in circulation, transferring them to peripheral tissues for utilization. They also ensure solubility and prevent polymerization of lipids.
• Lipid Role in Vesicular Transport: Lipids contribute to the formation of vesicles involved in vesicular transport processes.
• Maintenance of Plasma Membrane Fluidity and Integrity: Lipids contribute to plasma membrane fluidity and integrity.
• Globoside Function in Erythrocytes: Globosides, ceramide-derived oligosaccharides, are components of erythrocyte membranes.
• Ganglioside Function in Neuron Stability: Gangliosides, complex glycosphingolipids, contribute to the stability of paranodal junctions and channel clustering in myelinated nerve fibers.
• Cardiolipin Function in Mitochondrial Membranes: Cardiolipin, a diphosphatidylglycerol, is a major component of mitochondrial membranes. Its deficiency or absence leads to mitochondrial dysfunction, causing heart failure, hypothyroidism, and certain myopathies.

This quiz covers the characteristics and classifications of various fatty acids, including arachi donic acid and others based on chain length. Learn about polyunsaturated, cis, and trans fatty acids, and their significance in human nutrition. Test your knowledge on fatty acid structures and their properties.