BCMB 301 L2( Dr. Sarpong)

NAD+ and NADPH Overview

  • NADPH is a crucial cofactor in fatty acid synthesis, providing a reducing environment necessary for the process.

  • In animal cells, particularly hepatocytes (liver cells), the concentration of NADPH is very high, around 75 in the cytosol.

  • The reducing environment assists in the synthesis of fatty acids through various reduction reactions.

Fatty Acid Synthesis

Key Reactions Involved

  1. Condensation reaction

  2. Reduction reaction

    • First Reduction: Converts a ketone to an alcohol.

    • Dehydration (Elimination of Water): Occurs after the first reduction.

    • Second Reduction: Reduces a double bond to form a saturated carbon acid.

Role of NADPH

  • NADPH is essential for the two reduction reactions during fatty acid synthesis.

  • A strong reducing environment is required for these key reactions.

Sources of NADPH in Hepatocytes

  • Pentose Phosphate Pathway: Converts glucose to glucose-6-phosphate, which is then transformed into ribulose-5-phosphate.

  • Malate to Pyruvate Conversion: Catalyzed by malic enzyme, this also produces NADPH.

Acetyl CoA: Precursor for Fatty Acid Synthesis

Origin of Acetyl CoA

  1. Methyl Oxidation: Fatty acids can be oxidized to yield acetyl CoA.

  2. Decarboxylation of Pyruvate: Converting pyruvate (3 carbons) to acetyl CoA (2 carbons).

Transport of Acetyl CoA

  • Acetyl CoA is generated in the mitochondria and must be transported to the cytosol.

  • Transport Mechanism:

    • Acetyl CoA reacts with oxaloacetate to form citrate, catalyzed by citrate synthase.

    • Citrate is transported out of the mitochondria via the citrate transporter.

    • In the cytosol, citrate is cleaved back into acetyl CoA and oxaloacetate by ATP-dependent cleavage.

Role of Oxaloacetate

  • Oxaloacetate is also critical for the regeneration of acetyl CoA and must return to the mitochondrial matrix after cytosolic conversion.

Elongation and Desaturation of Fatty Acids

Fatty Acid Structure and Functionalities

  • Palmitate (C16 fatty acid) is the principal product of fatty acid synthesis.

  • There are various pathways for elongation and desaturation, which can occur in the endoplasmic reticulum (ER) and mitochondria:

    1. Adding Carbons (Elongation): Increasing the chain length of fatty acids.

    2. Introducing Double Bonds (Desaturation): Converting saturated fatty acids to unsaturated forms.

Elongation Process

  • Elongation involves the addition of two carbon units, with similar steps (reduction, dehydration, reduction) as seen in fatty acid synthesis.

  • Two distinct elongation pathways:

    • Endoplasmic Reticulum (ER) Pathway: Key enzymes located on the cytosolic side.

    • Mitochondrial Pathway: Primarily processes fatty acids with 14 or fewer carbon atoms, but also palmitate.

Desaturation Process

  • The mechanism exists for desaturation of fatty acids at specific carbon positions (4, 5, 6, 9) but not beyond carbon 9 in mammals.

  • Lack of Desaturases: Essential fatty acids with more than one double bond must be obtained from the diet (e.g., linoleic acid).

Enzymatic Machinery

  • Desaturation requires specific enzymes, such as:

    1. Desaturases (e.g., cytochrome b5, cytochrome c reductase).

    2. Oxygen Incorporation: Molecular oxygen serves as the final electron acceptor in the conversion of saturated fatty acids to unsaturated counterparts.

Arachidonic Acid: A Key Metabolite

  • Arachidonic Acid is a 20-carbon fatty acid with four double bonds.

  • Arachidonic acid serves as a precursor for signaling molecules known as eicosanoids.

  • Importance of Eicosanoids:

    • Derived from arachidonic acid during cell signaling, particularly in inflammatory responses and other physiological processes.

    • When stimulated, phospholipases (e.g., phospholipase A2) cleave arachidonic acid from membrane phospholipids, initiating signaling pathways.

Eicosanoid Synthesis Pathway

  1. Release of Arachidonic Acid: Phospholipase A2 releases arachidonic acid from membrane phospholipids.

  2. Conversion to Prostaglandins: Enzymes in the ER convert arachidonic acid to prostaglandins, beginning with the formation of PGG2.

  3. Role of COX Enzymes: Cyclooxygenase (COX) facilitates the introduction of oxygen into arachidonic acid, resulting in lipid mediators that fulfill various biological roles.

Summary of Key Enzymes and Their Functions

  • Cyclooxygenases (COX-1, COX-2):

    • COX-1: Involved in synthesizing prostaglandins that maintain gastric mucosal integrity.

    • COX-2: Induced during inflammation and stress to produce prostaglandins.

Conclusion

  • The study of fatty acid metabolism is central to understanding cellular energy production and signaling pathways. From the synthesis of palmitate to the subsequent elongation and desaturation processes leading to important signaling molecules like arachidonic acid, these biochemical pathways reflect the intricate regulation of fat metabolism in biology.