2

PUFAs and Their Essential Role in Diet

  • Polyunsaturated Fatty Acids (PUFAs):

    • Defined as fatty acids with ≥ 2 double bonds, with at least one beyond carbon 9 (C9).
    • Essential fatty acids that must be consumed in the diet because humans and animals cannot synthesize them after C9.
    • Key PUFAs:
    • Linolenic acid (C18:3): Omega-3 fatty acid (ω-3)
    • Linoleic acid (C18:2): Omega-6 fatty acid (ω-6)

  • Conversion to Other PUFAs:

    • Linoleic acid can be converted to Arachidonic acid (C20:4) (ARA).
    • Linolenic acid can be converted to Eicosapentaenoic acid (C20:5) (EPA).
    • ARA and EPA act as precursors for eicosanoids, which play roles in various biological functions.

Eicosanoids and Their Synthesis

  • Derived from C20 PUFAs:

    • Examples include:
    • Eicosatrienoic acid (C20:3)
    • Arachidonic acid (C20:4)
    • Eicosapentaenoic acid (C20:5)

  • Synthesis Pathways:

    • Cyclooxygenase (COX) pathway: Produces prostaglandins and thromboxanes.
    • Lipoxygenase (LOX) pathway: Produces leukotrienes.

  • Biological Actions:

    • Eicosanoids exert actions through binding to specific plasma membrane receptors.
    • Function as local signaling molecules in:
    • Inflammation (example: COX-2 inhibitors like aspirin)
    • Regulation of vascular tone
    • Numerous other biological processes

Structure and Function of Triacylglycerols (TAGs)

  • Composition:

    • Composed of a glycerol backbone esterified to 3 fatty acids (FAs) through the -COOH group.

  • Functions:

    • Major energy storage form in animals, primarily stored in adipose tissue.
    • Highly reduced and energy-dense, yielding approximately 38extkJ38 ext{ kJ} of energy per gram (twice as much as carbohydrates or proteins).
    • Advantages include:
    • Less solvation and more compact than highly hydrated carbohydrates
    • Provides insulation

Lipases and Fatty Acid Release

  • Role of Lipases:
    • Enzymes that cleave ester bonds in triacylglycerols, releasing fatty acids.
    • Fatty acids may vary in type and can be saturated or unsaturated.

Glycerophospholipids (GPLs) Structure

  • Base Structure:

    • The simplest GPL, phosphatidate, consists of glycerol-3-phosphate esterified with 2 fatty acids, plus a phosphate group on the 3rd -OH.

  • Specifics:

    • If a GPL contains an unsaturated fatty acid, it is typically located at the second carbon (C2) position.
    • A variety of polar alcohols can be esterified to the phosphate group of phosphatidate, including:
    • Choline (Resulting in phosphatidylcholine aka lecithin)
    • Serine (Phosphatidylserine)
    • Ethanolamine (Phosphatidylethanolamine)
    • Inositol (Phosphatidylinositol)
    • Glycerol (Phosphatidylglycerol)

Breakdown of GPLs by Phospholipases

  • Phospholipases:
    • Enzymes that hydrolyze GPLs into various components:
    • Phospholipase D
    • Phospholipase C
    • Phospholipase A1
    • Phospholipase A2

Biological Implications of Phospholipase Activity

  • Toxicity:
    • Venoms of poisonous snakes, such as the Indian cobra, contain high levels of phospholipase A2, which breaks down phospholipids in cell membranes.
    • Breakdown product Lysolecithin acts as a biological detergent, dissolving membranes of red blood cells, leading to cell rupture and contributing to the toxicity of the venom.