beta-ketoacyl-acyl-carrier-protein synthase III activity

Definition

Target type: molecularfunction

Catalysis of the reaction: acetyl-CoA + malonyl-[acyl-carrier protein] = acetoacyl-[acyl-carrier protein] + CoA + CO2. [EC:2.3.1.180]

Beta-ketoacyl-acyl-carrier-protein synthase III (KAS III) activity is a crucial step in the biosynthesis of very-long-chain fatty acids (VLCFAs). KAS III is an enzyme that catalyzes the condensation of a malonyl-acyl carrier protein (ACP) with a growing acyl-ACP chain, specifically in the elongation of very long chain fatty acids. This elongation process occurs in the endoplasmic reticulum (ER) and is essential for the synthesis of VLCFAs, which are important components of various cellular structures and functions.

KAS III activity is characterized by the following key steps:

1. **Substrate Binding:** KAS III binds to both the malonyl-ACP and the acyl-ACP substrate. The acyl-ACP substrate is typically a long-chain fatty acid with a chain length of 16 or 18 carbons, while the malonyl-ACP provides the two-carbon unit for chain elongation.

2. **Decarboxylation:** Upon binding, the malonyl-ACP undergoes decarboxylation, releasing carbon dioxide (CO2). This decarboxylation is crucial for providing the energy required for the subsequent condensation reaction.

3. **Condensation:** The decarboxylated malonyl unit then reacts with the acyl-ACP substrate. This condensation reaction forms a new beta-ketoacyl-ACP molecule, which is two carbons longer than the initial acyl-ACP.

4. **Product Release:** The newly synthesized beta-ketoacyl-ACP is released from the enzyme, ready to undergo further elongation cycles.

KAS III activity plays a vital role in the synthesis of various VLCFAs, including:

* **Sphingolipids:** These are essential components of cell membranes and play crucial roles in signal transduction, cell growth, and differentiation.
* **Waxes:** Waxes are hydrophobic substances that provide a protective barrier for plants and animals.
* **Cutin:** Cutin is a waxy substance that forms a protective layer on the surface of plants, preventing water loss and protecting against pathogens.

Mutations or dysregulation of KAS III activity can lead to various human diseases, including:

* **X-linked adrenoleukodystrophy (X-ALD):** This is a genetic disorder that affects the brain, adrenal glands, and other tissues.
* **Refsum disease:** This is a rare genetic disorder that affects the nervous system, causing nerve damage and impaired vision.

Overall, KAS III activity is essential for the biosynthesis of VLCFAs, which are crucial for various cellular functions. Understanding the molecular function of KAS III is vital for developing new treatments for diseases associated with VLCFA biosynthesis defects.'
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Proteins (1)

Compounds (2)