BIOCHEM CH10

Biochemistry II - Lecture 10 Notes

Mobilization of Triacylglycerols

• Stored in Adipose Tissue

  • Glucagon: Hormone that stimulates the mobilization of triacylglycerols from adipose tissues.

  • Key Components:

  • Adenylyl cyclase: Enzyme that converts ATP to cAMP when activated.

  • Receptor: Interacts with glucagon and activates the Gs protein.

  • Gs Protein: Stimulatory G protein that activates adenylyl cyclase.

  • ATP, cAMP: Energy currency and secondary messenger, respectively.

  • CGI-58: Comparative Gene Identification 58, activates ATGL.

  • ATGL: Triacylglycerol lipase that hydrolyzes triacylglycerol to diacylglycerol.

  • Lipid droplet: Store of fats within adipocytes (fat cells).

  • Monoacylglycerol: Product of diacylglycerol hydrolysis.

  • Perilipin: Structural protein associated with lipid droplets, serves as a barrier to lipolysis.

  • HSL (Hormone-sensitive lipase): Further hydrolyzes diacylglycerol into monoacylglycerol and releases free fatty acids and glycerol.

  • Glycerol: Released into the bloodstream; can enter the glycolytic pathway.

  • MGL (Monoacylglycerol lipase): Hydrolyzes monoacylglycerol to glycerol and free fatty acids.

  • Fatty acid transporter: Transports fatty acids into cells, especially into myocytes (muscle cells).

Entry of Glycerol into the Glycolytic Pathway

• Energy Content: Most biological energy in triacylglycerols resides in their three long-chain fatty acids.

• Glycerol kinase: Converts glycerol into glycerol 3-phosphate via phosphorylation.

• Glyceraldehyde 3-phosphate: Can enter the glycolysis pathway following conversion from glycerol.

Fatty Acids Activation and Transport into Mitochondria

• Small Fatty Acids: Fatty acids with fewer than 12 carbons diffuse freely across mitochondrial membranes.

• Carnitine Shuttle: Mechanism transporting long-chain fatty acids (≥14 carbons) across mitochondrial membranes, requires conversion to fatty acyl-CoA and attachment to carnitine.

Fatty Acyl-CoA Synthetase

• Function: Activates fatty acids by converting them to fatty acyl-CoA thioesters.

  • Reaction:
    ext{fatty acid} + ext{CoA} + ext{ATP}
    ightleftharpoons ext{fatty acyl-CoA} + ext{AMP} + ext{PPi}

Formation of a Fatty Acyl-CoA

• Structure: Contains a high-energy thioester linkage between the fatty acid carboxyl group and the thiol group of coenzyme A.

Overall Reaction for Fatty Acyl-CoA Formation

• Steps:

  • Two-step formation of the fatty acyl-CoA derivative.

  • Hydrolysis of created pyrophosphate.

• Overall reaction:
  ext{fatty acid} + ext{CoA} + ext{ATP}
  ightleftharpoons ext{fatty acyl-CoA} + ext{AMP} + 2 ext{Pi}
  ext{ΔG'}^ ext{°} = -34 ext{ kJ/mol}

Carnitine

• Role: Transports fatty acyl-CoAs into mitochondria for oxidation.

• Chemical Structure:
  ext{CH}3- ext{[N(CH}2 ext{)}_3]- ext{COO}^-

Carnitine Acyltransferase 1 (CAT1)

• Function: Catalyzes the attachment of carnitine to fatty acyl-CoA, forming fatty acyl-carnitine.

  • Reaction: Transesterification reaction.

Acyl-Carnitine/Carnitine Cotransporter

• Function: Passively transports fatty acyl-carnitine into the mitochondrial matrix while moving carnitine into the intermembrane space.

Carnitine Acyltransferase 2 (CAT2)

• Function: Transfers fatty acyl group back to CoA from carnitine to regenerate fatty acyl-CoA and free carnitine.

• Location: Inner mitochondrial membrane.

Two Pools of Coenzyme A

• Cytosolic Pool: Used for the biosynthesis of fatty acids.

• Mitochondrial Pool: Used in oxidative degradation of pyruvate, fatty acids, and some amino acids.

Carnitine Shuttle as Control Point

• Significance: Rate-limiting step for fatty acid oxidation in mitochondria.

• Inhibition: Carnitine acyltransferase 1 is inhibited by malonyl-CoA, preventing simultaneous fatty acid synthesis and degradation.

Stages of Fatty Acid Oxidation

• Stage 1: β Oxidation of fatty acids to release acetyl-CoA.

• Stage 2: Oxidation of acetyl-CoA into CO2 in the citric acid cycle, yielding NADH, FADH2, and GTP.

• Stage 3: Electron transport chain and oxidative phosphorylation, generating ATP from NADH and FADH2.

β Oxidation of Saturated Fatty Acids - Four Basic Steps

1. Dehydrogenation: Catalyzed by acyl-CoA dehydrogenase, yielding trans-∆2-enoyl-CoA.

2. Hydration: Addition of water to the double bond of trans-∆2-enoyl-CoA, forming L-β-hydroxyacyl-CoA.

3. Oxidation: Dehydrogenation of L-β-hydroxyacyl-CoA, resulting in β-ketoacyl-CoA.

4. Cleavage: Catalysis by acyl-CoA acetyl-transferase (thiolase) produces acetyl CoA and a shortened fatty acyl-CoA.

The Chemical Logic of β-Oxidation Sequence

• Reactions: Convert stable single bonds between methylene groups into less stable bonds, allowing nucleophilic attack and cleavage.

• Functionality: Terminal –CH2–CO–S-CoA serves as a leaving group facilitating breakage of the α–β bond.

Overall Reaction for One Pass Through Stage 1 of β Oxidation

• Starting with Palmitoyl-CoA:
  ext{palmitoyl-CoA} + ext{CoA} + ext{FAD} + ext{NAD}^+ + ext{H}2 ext{O} ightarrow ext{myristoyl-CoA} + ext{acetyl-CoA} + ext{FADH}2 + ext{NADH} + ext{H}^+

ATP Yield from FADH2 and NADH

• FADH2: Each generates 1.5 ATP.

• NADH: Each generates 2.5 ATP.

• Total ATP: 4 ATP generated per pass through β oxidation.

Complete Oxidation of Palmitoyl-CoA

• Overall Reaction Including Electron Transfers:
  ext{palmitoyl-CoA} + 7 ext{CoA} + 7 ext{O}2 + 28 ext{Pi} + 28 ext{ADP} ightarrow 8 ext{acetyl-CoA} + 28 ext{ATP} + 7 ext{H}2 ext{O}

Conclusions

• Final Yield for Complete Oxidation:
  ext{palmitoyl-CoA} + 23 ext{O}2 + 108 ext{Pi} + 108 ext{ADP} ightarrow ext{CoA} + 108 ext{ATP} + 16 ext{CO}2 + 23 ext{H}_2 ext{O}

Enzymatic Processes in Oxidation Steps

Oxidation of Monounsaturated Fatty Acids

• Isomerization Requirement: Requires enoyl-CoA isomerase to convert cis-∆3-enoyl-CoA to trans-∆2-enoyl-CoA.

Oxidation of Linoleoyl-CoA and Odd-Number Fatty Acids

• Oxidation Requirements: Complete oxidation of odd-number fatty acids requires extra steps involving propionyl-CoA.

Reactions of Propionyl-CoA Oxidation

1. Step 1 - Catalyzed by propionyl-CoA carboxylase; requires biotin, forms D-methylmalonyl-CoA.

2. Step 2 - Epimerization by methylmalonyl-CoA epimerase to L-methylmalonyl-CoA.

3. Step 3 - Rearrangement by methylmalonyl-CoA mutase to succinyl-CoA (requires coenzyme B12).