BIOC*2580 - 8

Beta-Oxidation Pathway Overview

  • Previous Discussion Recap

    • The beta-oxidation pathway was examined, consisting of four distinct steps.

    • Each step involves the oxidation of the beta carbon, which is labeled as follows:

    • Carbon 1: Alpha carbon

    • Carbon 2: Beta carbon

  • Pathway Process

    • Initialization from alkane (CH2, reduced form) to ketone involves the following oxidation steps:

    1. Alkane → Alkene (dehydrogenation)

    2. Alkene → Alcohol

    3. Alcohol → Ketone

    • Preparation for hydrolysis of the bond between alpha and beta carbons occurs after these transformations.

Detailed Steps of Beta-Oxidation

Step 1: Dehydrogenation

  • Process Details

    • Carbon removal leads to the formation of a double bond (alkene).

    • Oxidative dehydrogenation removal of two hydrogen atoms (oxidation) is performed by the cofactor FAD, forming FADH2.

    • Enzyme: Acyl-CoA dehydrogenase; the product is trans-delta2enoyl-CoA.

    • Trans-delta2enoyl-CoA:

      • "delta" denotes double bond position (between carbon 2 & 3).

      • "trans" specifies the configuration of the double bond.

Step 2: Hydration

  • Function

    • Water is added, leading to the molecule becoming hydrated across the double bond.

  • Process Details

    • Hydroxyl group attaches to the beta carbon, while a hydrogen attaches to the alpha carbon.

    • Product forms: Beta-hydroxy acyl-CoA.

    • Enzyme: Enoyl-CoA hydratase.

Step 3: Another Oxidation

  • Process

    • The alcohol at the beta carbon is oxidized to a ketone.

    • This oxidation is mediated by NAD, producing NADH and a free proton.

  • Result

    • Product forms: beta-ketoacyl-CoA.

    • Enzyme: Beta-hydroxy acyl-CoA dehydrogenase.

Step 4: Thiolysis

  • Action

    • Cleavage of the bond between α and β carbons occurs via thiolysis, using coenzyme A as the nucleophile rather than hydrolysis (which uses water).

  • Outcomes

    • Resulting products: Acetyl-CoA (two carbons released) and a new acyl-CoA chain (14-carbons remain).

Repetition of the Beta-Oxidation Cycle

  • Each cycle of beta-oxidation releases one acetyl-CoA and reduces the fatty acid chain by two carbons.

    • Example: Starting from a 16-carbon fatty acid, undergo 7 cycles to yield 8 acetyl-CoA.

  • Acetyl-CoA enters the citric acid cycle and is further oxidized into CO2 making more GTP, NADH and FADH2.

Stoichiometry and Energy Production

  • Summary of energy yields and cofactors formed:

    • For a 16-carbon fatty acid:

    • 8 Acetyl-CoA

    • 7 FADH2

    • 7 NADH

  • These reduced cofactors will later yield ATP during electron transport.

Oxidation of Glucose

  • Complete oxidation of glucose to CO2 and H2O is also accomplished in three stages

  • Stage 1: Glycolysis + Pyruvate dehydrogenase

  • Stage2: TCA Cycle

  • Stage 3: ETC

Glycolysis Overview

  • Preparation for Carbohydrate Catabolism

    • Glycolysis serves as the initial pathway for breaking down glucose into pyruvate (stopping at pyruvate before forming acetyl-CoA).

    • This pathway is present in the cytoplasm, unlike beta-oxidation which occurs in the mitochondria.

    • It is the only pathway that can provide energy under anaerobic conditions

    • The first five reactions - preparatory phase (ATP is used to phosphorylate and activate glucose)

    • The next 5 reactions – pay off phase (net generation of ATP)

Overview of carbohydrate metabolism

Glucose transport into cells

  • Glucose is a highly polar molecule and cannot enter cells by passive
    diffusion across the membrane.

    • Transporter proteins called GLUTs (GLUcose Transporters), residing in the cell membrane, catalyze glucose import.

    • One of the many actions of the hormone insulin is to stimulate GLUT-mediated glucose uptake in skeletal muscle and adipose tissue

Glycolysis Phases

Preparatory Phase (PIPAI)

  • Steps Involved

    • Step 1: Glucose → Glucose-6-phosphate (phosphorylation)

    • Enzyme: Hexokinase.

    • Step 2: Glucose-6-phosphate → Fructose-6-phosphate (isomerization)

    • Enzyme: Phosphohexose isomerase.

    • Step 3: Fructose-6-phosphate → Fructose-1,6-bisphosphate (phosphorylation)

    • Enzyme: Phosphofructokinase.

    • Step 4: Cleavage of fructose-1,6-bisphosphate into two 3-carbon units: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

    • Both products utilized further in glycolysis, favoring G3P.

    • Step 5: Dihydroxyacetone phosphate (DHAP) → Glyceraldehyde-3-phosphate (G3P) (isomerization)

    • Enzyme: Triose phosphate isomerase.

    • DHAP is converted into G3P so that both three-carbon fragments produced in Step 4 can continue through glycolysis.

Payoff Phase

  • Key Steps

    • Step 6: Glyceraldehyde-3-phosphate → 1,3-Bisphosphoglycerate (oxidation + phosphorylation) (requires NAD, producing NADH).

    • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.

    • G3P is oxidized (aldehyde → carboxylic acid) and phosphorylated, producing a high-energy mixed anhydride bond and reducing NAD⁺ → NADH. High-energy molecule formation.

    • Step 7: 1,3-Bisphosphoglycerate → 3-Phosphoglycerate (substrate-level phosphorylation)

    • Enzyme: Phosphoglycerate kinase.

    • The high-energy phosphate of 1,3-BPG is transferred to ADP, producing ATP. Since two G3P molecules are formed per glucose, this step yields 2 ATP per glucose. 

    • Step 8: 3-Phosphoglycerate → 2-Phosphoglycerate (isomerization / mutase reaction)

      Enzyme: Phosphoglycerate mutase.
      A mutase shifts the phosphate from carbon 3 to carbon 2, becoming 2-phosphoglycerate..

    • Step 9: 2-Phosphoglycerate → Phosphoenolpyruvate (dehydration)

      Enzyme: Enolase.
      Water is removed, creating the high-energy enol phosphate, phosphoenolpyruvate (PEP).

    • Step 10: Phosphoenolpyruvate → Pyruvate (substrate-level phosphorylation)

      Enzyme: Pyruvate kinase.
      PEP transfers its high-energy phosphate to ADP, forming ATP and pyruvate. The favorable reaction is driven by the spontaneous enol → keto tautomerization of pyruvate.

Final Notes

  • Critical Considerations

    • Different metabolic fates exist for pyruvate:

    • Further enter into TCA cycle as acetyl-CoA.

    • Convert to lactate or ethanol, depending on oxygen availability.

    • Significance of Glucose:

    • Essential structural role; red blood cells and brain primarily rely on it.

  • Glycolysis Location:

    • Only oxidative pathway functional even under anaerobic conditions, vital for energy in hypoxic scenarios.