Fatty Acid Metabolism

Fatty Acid Metabolism can be divided into two main processes: synthesis and oxidation.

Fatty acid synthesis involves the creation of fatty acids which can be utilized for different metabolic purposes.
Acetyl CoA is a key metabolic intermediate that can either:
- Enter the Krebs cycle to produce energy.
- Be utilized in the synthesis of cholesterol.
- Be converted into ketone bodies, which are important energy substrates.

Ketone bodies are produced mainly in the liver and serve as alternative fuels for the body during certain metabolic states, such as fasting or uncontrolled diabetes mellitus.
- The three primary ketone bodies are:
1. Acetoacetate (AcAc)
2. Beta-hydroxybutyrate (β-HB)
3. Acetone (detected on breath)
Ketone bodies require the action of four enzymes for their synthesis.
Acetoacetate and beta-hydroxybutyrate are water-soluble and can diffuse into the bloodstream, making them accessible fuels during metabolic processes.
The organs, like the heart and renal cortex, prefer utilizing ketone bodies as fuels under normal metabolism.
High levels of ketone bodies are typically associated with conditions such as starvation and uncontrolled diabetes mellitus.
Although the brain usually relies on glucose, it can adapt to utilize ketone bodies when necessary.

Acetoacetate: Converted to acetoacetyl CoA via CoA transferase.
Beta-Hydroxybutyrate: Produced from acetoacetate, this conversion is catalyzed by β-hydroxybutyrate dehydrogenase.
Acetone: A byproduct that is released and can be detected in breath; it does not serve as a fuel.

Initially, during starvation, muscle tissue is broken down to provide amino acids needed for gluconeogenesis.
After approximately three days, the liver begins synthesizing significant quantities of ketone bodies, which the brain adapts to use effectively.
After several weeks, ketone bodies become the primary energy source for the brain when glycogen stores are depleted.
Once lipid reserves are consumed, muscle tissue is again broken down to sustain metabolic functions.

Fatty acid synthesis is distinctly different from fatty acid oxidation and occurs in the cytoplasm of cells.
The process involves several steps catalyzed by a multi-functional enzyme known as fatty acid synthase, which comprises seven catalytic sites on a single polypeptide chain.
Steps in fatty acid synthesis include the introduction of two-carbon units at a time through a process that uses malonyl CoA as a building block and requires the reduction of intermediates by NADPH.
Formation of Malonyl CoA:
- Acetyl CoA is carboxylated to form malonyl CoA in a reaction that utilizes ATP and bicarbonate.
- This reaction is catalyzed by acetyl CoA carboxylase:
  $ext{Acetyl CoA + HCO}_3^- + ext{ATP} <br>ightarrow ext{Malonyl CoA + ADP + Pi + H}^+$

Transfer of Malonyl CoA:
- Enzymes involved:
- Malonyl transacylase: Transfers malonyl CoA to Acyl Carrier Protein (ACP) to form malonyl-ACP.
Condensation Reaction:
- Two-carbon units are added to a growing fatty acid chain, involving:
- Condensation of acetyl-ACP and malonyl-ACP to create a four-carbon unit.
Reduction:
- Subsequent reduction reactions incorporate NADPH, leading to a saturated fatty acid with a chain of 16 carbons, primarily palmitate.
Dehydration and Further Reduction:
- Upon further processing, dehydration occurs followed by a reduction to achieve double bond formation when applicable.

Longer-chain fatty acids and fatty acids with double bonds are synthesized in the endoplasmic reticulum.
The synthesis process is limited to double bonds at certain locations, particularly not beyond the 9th carbon position.

Certain fatty acids, referred to as essential fatty acids, must be obtained from the diet because the body cannot produce them.
- Examples:
- Linoleate (18:2): Fatty acid with double bonds at positions 9 and 12.
- Linolenate (18:3): Fatty acid with double bonds at positions 9, 12, and 15.
Arachidonate (20:4): Derived from linoleate; important for the creation of eicosanoids (local hormones) which are synthesized in various tissues.