Fatty Acid Metabolism Study Notes

β-oxidation of Fatty Acids
- Definition: The process of breaking down fatty acids to produce acetyl-CoA, which enters the citric acid cycle for energy production.
Nomenclature of Biosynthetic Enzymes
- Kinases: Enzymes that add phosphate groups to substrates.
- Synthases: Enzymes that synthesize molecules without the use of ATP.
- Hydrolases: Enzymes that catalyze the hydrolysis of chemical bonds.
- Hydratases: Enzymes that catalyze the addition of water to double bonds.
Key Terms Distinguishing Synthase and Synthetase Enzymes
- Synthetases: Enzymes that require energy from ATP hydrolysis to form chemical bonds. They cleave an ATP molecule to function.
- Synthases: Enzymes that do not require ATP, instead synthesizing molecules directly through existing substrates.

The biosynthesis of fatty acids is an anabolic process that occurs primarily in the cytosol under well-fed conditions where there is high glucose availability.
As glucose is metabolized, an increase in flux through the TCA cycle generates excess citrate, which is exported via the citrate shunt into the cytosol for fatty acid synthesis.

The stages of biosynthesis include:
1. Transfer
2. Elongation
3. Reduction
4. Dehydration
5. Reduction
This cyclical process continues until palmitoyl-ACP (an acyl carrier protein bound to palmitic acid) is formed.

Activation of Acetyl-CoA:
- Acetyl-CoA is activated by the addition of carbon dioxide producing Malonyl-CoA:
  $ext{Acetyl-CoA} + ext{CO}_2 <br>ightarrow ext{Malonyl-CoA}$
- Catalyzed by Acetyl-CoA carboxylase.

Consists of two essential carrier proteins:
1. ACP1: Serves as a holding station for acetyl or fatty acyl groups, linked to a cysteinyl sulfhydryl group.
2. ACP2: Binds the growing fatty acyl chain during the condensation and reduction reactions of the biosynthetic cycle, with the acyl group being carried on a long phosphopantetheine.

Transfer of Acetyl Group
- Catalyzed by Acetyl-CoA:ACP transacylase, where an acetyl group is transferred from Coenzyme A to the cysteinyl-S group on ACP1.
Transfer of Malonyl Group
- Catalyzed by Malonyl-CoA:ACP transacylase, transferring a malonyl group from Coenzyme A to pantetheinyl-S of ACP2.
Formation of β-ketoacyl Group
- Upon decarboxylation, the carbon dioxide leaves the malonyl group, triggering a nucleophilic attack by the acyl group on ACP1, facilitated by Ketoacyl-ACP synthase. This reaction prepares the β-ketoacyl group for the reverse reactions of β-oxidation.

Reduction to Alcohol:
- Catalyzed by β-ketoacyl-ACP reductase, converting the keto-group into an alcohol.
- $ext{β-ketoacyl-ACP} <br>ightarrow ext{(Alcohol form)}$
Dehydration:
- Catalyzed by Enoyl-ACP hydrase, resulting in the formation of the cis-2,3-enoyl group.
Second Reduction:
- Performed by Enoyl-ACP reductase, using NADPH instead of FADH2 to convert the enoyl into a saturated acyl group.
Acyl Transfer:
- Final step facilitated by ACP-acyltransferase, where the acyl group is transferred from ACP2 to ACP1, allowing ACP2 to bind to another malonyl moiety and continue the cycle.

Longer fatty acids are synthesized via a fatty acid elongation system found in the endoplasmic reticulum (ER).
The same reactions as in synthesis occur, but utilizing individual enzymes.
Example: Palmitate is activated to palmitoyl-CoA.

Sequential Action of Enzymes:
- Each round of elongation sees the β-keto group reduced, forming a saturated chain with the help of a ketoreductase (KR), dehydratase (DH), and enol reductase (ER).
- The growing fatty acid chain remains attached to a phosphopantetheine prosthetic group of ACP until the chain reaches a length of 16 (palmitidic acid), then released by a thioesterase (TE).

If fatty acid synthase primarily produces palmitate, where do the additional fatty acids originate?
1. Shortening by β-oxidation.
2. Fatty acid elongation in the ER: Enzymes prefer a substrate length of C-16 or less, producing mostly stearoyl-CoA.
3. Longer Unsaturated Fatty Acids:
  - Unsaturated fatty acids bind effectively due to their kinks (cis double bonds), resulting in the synthesis of longer fatty acids (C-20, C-22, C-24).

A second fatty acid elongation system exists in the mitochondria for long-chain fatty acid provision.
Utilizes most functions of β-oxidation but employs an NADPH-dependent enoyl-CoA reductase instead of the FAD-dependent dehydrogenase.

Fatty acid synthesis occurs in the cytosol, while acetyl-CoA production occurs in mitochondria and cannot freely cross the inner membrane.
The Pyruvate-Malate Cycle (or Citrate-Pyruvate Cycle) facilitates the transfer of acetyl groups to the cytosol, coupling fatty acid synthesis with glycolysis.
- Acetyl-CoA is initially converted to citrate with oxaloacetate, which is transported out of the mitochondria using a co-transporter.
Upon arrival in the cytosol, citrate is cleaved back into acetyl-CoA and oxaloacetate, a reaction that necessitates ATP to proceed favorably.

The process of generating oxaloacetate post fatty acid synthesis includes:
1. Dehydrogenation of cytosolic oxaloacetate to produce malate and NAD+.
2. Malic enzyme oxidizes malate to produce pyruvate, which regenerates oxaloacetate and provides NADPH crucial for biosynthetic reactions.

Glucose undergoes glycolysis to produce pyruvate, which is converted into acetyl-CoA and oxaloacetate, then further cycled through malate and back again, facilitating fatty acid synthesis while recycling an essential metabolite for mitochondrial operations.

β-oxidation: Breakdown of fatty acids to acetyl-CoA.
Anabolic pathway: Building larger molecules from smaller units.
Citrate shunt: Process connecting mitochondrial and cytosolic metabolic routes.
Malonyl-CoA: Key intermediate in fatty acid biosynthesis.
ACP (Acyl Carrier Protein): Essential protein for fatty acid elongation and transfer.
NADPH: Reducing agent involved in anabolic reactions.