Metabolism

  • Definition: The sum of chemical reactions occurring in an organism.

Lipids

  • Definition: Organic molecules that are insoluble in water but soluble in organic solvents (hydrophobic solvents).

  • Classification:

    • Phospholipids:

    • Lipids that contain phosphate groups.

    • Simple Lipids:

    • Characteristics: Contain only carbon (C), hydrogen (H), and oxygen (O).

    • Complex Lipids:

    • Characteristics: Contain C, H, O, and other elements (N, P, S).

  • Formation: Lipids are esters formed through the esterification of alcohol and acid.

Types of Lipids Based on Saponification

  • Saponifiable Lipids: Can react with KOH.

  • Non-saponifiable Lipids: Cannot react with KOH.

Types of Lipids Based on Their Alcohol Composition

  • Glycerol-based Lipids: Alcohol in their structure is glycerol.

  • Sphingolipids: Alcohol in their structure is sphingosine.

  • Sterols: Another category of lipids that includes cholesterol.

Glycerides

  • Divided into:

    • Glycerolipids: Lipids derived from glycerol.

    • Phosphoglycerides: Glycerol lipids with phosphate groups.

Fatty Acids

  • Definition: Long-chain molecules with a carboxylic (–COOH) group at one end and an aliphatic tail.

  • Classification:

    • Saturated Fatty Acid: No double bonds between carbon atoms.

    • Monounsaturated Fatty Acid (C18:1): One double bond.

    • Polyunsaturated Fatty Acid (C18:2): Multiple double bonds.

  • Omega Fatty Acids:

    • Essential fatty acids that cannot be synthesized by the body (e.g., Linoleic and Linolenic acids).

  • Position of Unsaturated Fatty Acids in Phospholipids:

    • Position 1 typically has unsaturated fatty acids in phospholipids.

Phosphatidate Molecule

  • General structure of phosphatidate as the basis for phospholipids.

    • Phosphatidic Acid (PA) is formed when a head group is added.

Biosynthesis of Triglycerides

  • Starts from glycolytic pathway, particularly Dihydroxyacetone Phosphate (DHAP).

  • In the liver, triglycerides are formed from Glycerol-3-phosphate dehydrogenase (DHAP):

    • Reaction catalyzed by Glycerol Kinase:
      extGlycerol+ATP<br>ightarrowextGlycerol−3−phosphate+ADPext{Glycerol + ATP} <br>ightarrow ext{Glycerol-3-phosphate + ADP}

  • Acyltransferase transfers acyl groups from acyl-CoA to form triglycerides.

Classification of Enzymes

  • Oxidoreductases (EC 1): Catalyze oxidation and reduction reactions.

  • Transferases (EC 2): Catalyze transfer of functional groups (e.g., kinases).

  • Hydrolases (EC 3): Catalyze hydrolysis reactions (e.g., lipases, proteases).

  • Lyases (EC 4): Break down without hydrolysis.

  • Isomerases (EC 5): Catalyze isomerization (racemization, epimerization).

  • Ligases (EC 6): Join molecules using energy from ATP.

Biosynthesis of Phospholipids

  • Two Strategies to Attach Head Group:

    1. Attachment of Head Group to CDP-Glycerol:

    • Resulting in glycerophospholipid.

    1. Salvation Pathway: Synthesis from existing phospholipids.

Biosynthesis of Phosphatidylethanolamine

  • De novo Pathway: Synthesizing from small precursors.

  • Salvation Pathway: Synthesizing from similar molecules.

Biosynthesis of Cardiolipin

  • Pathway differs between eukaryotes and prokaryotes.

  • Biosynthesis in E. coli involves:

    • Starting from Phosphatidate, leading to the formation of Diphospatidylglycerol.

Degradation of Phospholipids

  • Phospholipids are hydrolyzed by phospholipases.

    • Phospholipase A1: Hydrolyzes fatty acid from position 1.

    • Phospholipase A2: Hydrolyzes fatty acid from position 2.

    • Phospholipase B (Lysophospholipase): Hydrolyzes bonds of remaining phospholipid.

    • Phospholipase D: Cleaves the bond between the phosphate group and the head group.

Biosynthesis of Eicosanoids

  • Essential fatty acids like Arachidonic acid, Linoleic acid, and Linolenic acid are precursors.

  • Arachidonic acid serves as the basis for eicosanoids, acting as second messengers in the body.

    • Enzyme Phospholipase A2 releases Arachidonic acid from membrane phospholipids.

Prostaglandins and Thromboxanes

  • Prostaglandins: Involved in inflammation, vasodilation, and other functions.

    • Synthesized by Cyclooxygenase (COX) enzymes.

    • Aspirin and ibuprofen inhibit COX to prevent thromboxane synthesis, reducing blood clot risk.

Lipoxygenase and Leukotrienes

  • Lipoxygenase enzymes convert arachidonic acid into leukotrienes, critical in immune responses.

Kreb’s Cycle

  • Facilitates entry of Acetyl CoA into energy production pathways.

  • In absence of Oxygen, Lactic acid is produced, which is reconverted to Acetyl CoA when oxygen is present, allowing for the Krebs Cycle to proceed.

  • Acetyl CoA can either participate in carbohydrate metabolism or lipid metabolism in mitochondria.