Biosynthesis of Fatty Acids: Detailed Study Notes and Palmitic Acid

Biosynthesis Distinction: Contrary to what might be assumed, fatty acid synthesis is not a direct reversal of fatty acid oxidation (beta-oxidation). It involves a completely different set of reactions and enzyme activities.
Primary Enzymes: The two central enzymes involved in this process are:
- Acetyl CoA Carboxylase (ACC)
- Fatty Acid Synthase (FAS)
Example Model: The transcript uses palmitic acid (also known as palmitate), a $16$ -carbon fatty acid, as the primary example for synthesis.
Scope of Synthesis: The focus is on the biosynthesis of the fatty acid component itself, not the subsequent assembly of these fatty acids into triacylglycerols.
Biological Significance:
- All organisms possess the ability to synthesize fatty acids.
- Fatty acids are essential for the formation of lipid bilayers, cell membranes, and other cellular/tissue components.
- They serve as a vital storage form for carbon. Excessive carbohydrate (sugar) intake is converted into fats for long-term energy storage.

Compartmentalization:
- Fatty Acid Oxidation: Occurs in the mitochondria.
- Fatty Acid Biosynthesis: Occurs in the cytosol.
Covalent Linkages (Carriers):
- Oxidation: Intermediates are covalently linked to the sulfhydryl ( $-SH$ ) groups of Coenzyme A ( $CoA$ ).
- Biosynthesis: Intermediates are covalently linked to the sulfhydryl groups of an Acyl Carrier Protein (ACP).
Enzymatic Organization:
- Oxidation: Requires a set of four separate enzymes working in a sequence.
- Biosynthesis: Catalyzed by a single, large multi-enzyme complex called Fatty Acid Synthase. All required activities are contained within this one enzyme structure.
Electron Carriers (Reductant vs. Oxidant):
- Oxidation: Generates $NADH$ (and $FADH_2$ ).
- Biosynthesis: Uses $NADPH$ as the source of electrons and reducing power.
Metabolic Separation: Having distinct pathways and locations minimizes the occurrence of a futile cycle, where fatty acids are simultaneously synthesized and broken down without net gain.

Two-Carbon Units: Both processes involve the manipulation of the carbon chain in units of two. Oxidation removes two carbons at a time, while biosynthesis adds two carbons at a time.
Acetyl CoA Precursor: In biosynthesis, the two-carbon units are initially derived from Acetyl CoA, which is also the product released during oxidation.

Conversion Process: Before synthesis can proceed, Acetyl CoA must be converted into a three-carbon compound called Malonyl CoA.
Enzyme: Acetyl CoA Carboxylase (ACC).
Reaction Equation: $Acetyl-CoA + CO_2 + ATP \rightarrow Malonyl-CoA + ADP + P_i + H^+$
- Note: The $CO_2$ is often provided in the form of dissolved bicarbonate ( $HCO_3^-$ ).
Characteristics:
- This reaction is essentially irreversible.
- It serves as a key point of regulation in fatty acid biosynthesis.
Cofactor/Prosthetic Group: Biocitin:
- ACC uses a prosthetic group called biocitin (also known as the biotin carrier protein).
- Dietary Origin: The vitamin biotin is the dietary precursor for this prosthetic group.
Mechanism of ACC:
- The biotin carrier protein has a flexible "arm" that moves between two active sites.
- Step 1: The arm picks up a carboxylate group ( $CO_2$ ) at the first site using energy from $ATP$ .
- Step 2: The arm flips over to the transcarboxylase active site.
- Step 3: The carboxyl group is transferred to Acetyl CoA, forming the three-carbon Malonyl CoA.

Mammalian Form: In mammals, the enzyme is known as Fatty Acid Synthase I.
Composition: It is a homodimeric protein, meaning it consists of two identical polypeptide subunits.
Multi-functional Nature: Each individual polypeptide chain contains seven different active sites.
- This allows one single enzyme to catalyze seven distinct enzymatic steps.
Acyl Carrier Protein (ACP) Domain:
- ACP is a specific part of the FAS enzyme.
- It contains pantothenic acid (the same vitamin found in Coenzyme A) attached to a serine residue of the protein.
- It terminates in a sulfhydryl group ( $-SH$ ), which forms a high-energy thioester bond with the malonyl groups. This bond provides the necessary energy for the synthesis reactions.

Iterative Process: The synthesis is a cyclic process where each round adds two carbons to the growing chain.
Priming the Enzyme:
- An acetyl group from Acetyl CoA is first attached to the KS (ketoacyl-ACP synthase) active site.
- A malonyl group from Malonyl CoA is attached to the ACP site.

Condensation: The acetyl group (on KS) and the malonyl group (on ACP) react. This involves the release of a carbon dioxide molecule ( $CO_2$ ). The loss of $CO_2$ makes this step highly exergonic (energy-releasing). The result is a four-carbon chain attached to the ACP.
Reduction: The keto group is reduced using $NADPH$ as the electron donor.
Dehydration: A molecule of water ( $H_2O$ ) is removed, creating a double bond.
Reduction: The double bond is reduced using a second molecule of $NADPH$ , resulting in a saturated carbon chain.

The Source of NADPH: The majority of the $NADPH$ used in these biosynthetic reactions is generated by the Pentose Phosphate Pathway, which branches off from glycolysis.

Translocation: After the first cycle, the resulting saturated four-carbon chain is moved (translocated) from the ACP site to the KS site.
Repetition: This leaves the ACP free to bind a new Malonyl CoA molecule. The four-carbon chain then undergoes another round of condensation with the new malonyl group, becoming a six-carbon chain.
Stoichiometry for Palmitic Acid:
- Palmitic acid has $16$ carbons.
- To reach this length, the cycle must be repeated seven times (adding $2$ carbons in each round to the original $2$ -carbon acetyl primer).
Final Product: Palmitic acid ( $16:0$ ) is the standard end product of the Fatty Acid Synthase enzyme in the cytoplasm of animal and yeast cells.

While palmitic acid is the primary product, organisms can create more complex fatty acids through further processing:

Location: These modifications occur in the Endoplasmic Reticulum (ER) (the system surrounding the nucleus) in animal and yeast cells. In plants, this occurs in the chloroplasts.
Elongation: Specialized enzymes can extend the fatty acid chain beyond $16$ carbons.
Desaturation: Enzymes can introduce double bonds into the saturated carbon chains to create unsaturated fatty acids.