29.2 Catabolism of Triacylglycerols: The Fate of Glycerol

Catabolism of Triacylglycerols

Triacylglycerols (TAGs) are the body's main long-term energy storage molecules. Breaking them down releases both fatty acids and glycerol, each of which follows a distinct metabolic path. This section focuses on how TAGs are hydrolyzed and, in particular, what happens to the glycerol that's released.

Mechanism of Triacylglycerol Hydrolysis

TAGs are broken down by lipases through a process called lipolysis. These enzymes catalyze the hydrolysis of the ester bonds linking fatty acids to the glycerol backbone, producing free fatty acids and glycerol.

Lipases belong to the serine hydrolase class. Their active sites contain a catalytic triad of serine, histidine, and aspartate (or glutamate) residues. The hydrolysis of each ester bond proceeds through a four-step mechanism involving an acyl-enzyme intermediate:

1. The active-site serine attacks the carbonyl carbon of the ester bond, forming a tetrahedral intermediate stabilized by the oxyanion hole.
2. This intermediate collapses, releasing the alcohol component (glycerol or a partial glyceride) and forming an acyl-enzyme intermediate.
3. A water molecule attacks the acyl-enzyme intermediate, generating a second tetrahedral intermediate.
4. That intermediate collapses, releasing the free fatty acid and regenerating the active-site serine for another catalytic cycle.

Lipases also show substrate specificity and regioselectivity:

• They can preferentially hydrolyze ester bonds at specific positions on the glycerol backbone (sn-1, sn-2, or sn-3).
• Some lipases are selective for short-, medium-, or long-chain fatty acids, depending on the shape of their binding pockets.

In adipose tissue, hormone-sensitive lipase (HSL) is the key regulatory enzyme. Its activity is controlled by hormonal signals (e.g., epinephrine activates it via phosphorylation, while insulin suppresses it), making it central to the regulation of fat mobilization.

Fate of Glycerol After Breakdown

Once released, glycerol enters the cytosol and is converted into dihydroxyacetone phosphate (DHAP), an intermediate of glycolysis. This conversion takes two enzymatic steps:

1. Phosphorylation: Glycerol kinase transfers a phosphate group from ATP to glycerol, producing glycerol-3-phosphate.
2. Oxidation: Glycerol-3-phosphate dehydrogenase (GPD), an $\text{NAD}^+$-dependent enzyme, oxidizes glycerol-3-phosphate to DHAP. This reaction involves transfer of a hydride ion ($H^-$) from C2 of glycerol-3-phosphate to $\text{NAD}^+$, generating NADH.

From DHAP, glycerol's carbons can take two major routes:

• Glycolysis: Triose phosphate isomerase (TPI) converts DHAP to glyceraldehyde-3-phosphate (GAP), which continues through glycolysis to pyruvate, generating ATP and NADH.
• Gluconeogenesis: DHAP can be used to build glucose. Aldolase combines DHAP with GAP to form fructose-1,6-bisphosphate, which is then processed through the gluconeogenic pathway to glucose-6-phosphate. In the liver and kidney, glucose-6-phosphatase hydrolyzes this to free glucose, which can be exported to the blood.

Prochiral Nature of Glycerol

Glycerol is a prochiral molecule. It has two hydroxyl groups at C1 and C3 that are enantiotopic: they look identical in a free molecule, but an enzyme can distinguish between them because of the molecule's three-dimensional arrangement around C2.

This matters because glycerol kinase doesn't phosphorylate randomly. It specifically phosphorylates the pro-S hydroxyl (at C1 by Fischer convention), producing sn-glycerol-3-phosphate (also called L-glycerol-3-phosphate). The enzyme's active site has a defined 3D shape that binds one enantiotopic group preferentially over the other.

This enzymatic prochiral selectivity is a recurring theme in biochemistry:

• Alcohol dehydrogenases stereospecifically reduce prochiral ketones, producing only one enantiomer of the alcohol product.
• Lipases and esterases selectively hydrolyze one enantiotopic ester group over another.

The selective modification of prochiral substrates ensures that downstream metabolites have the correct stereochemistry for further enzymatic processing.

Fate of Fatty Acids

The fatty acids released during lipolysis follow a separate catabolic path. They undergo beta-oxidation in the mitochondrial matrix, where they're sequentially shortened by two carbons per cycle, generating acetyl-CoA, FADH$_2$, and NADH.

During prolonged fasting or in uncontrolled diabetes, the rate of fatty acid oxidation can exceed the capacity of the citric acid cycle. When this happens, excess acetyl-CoA is diverted to ketone body production (ketogenesis) in the liver.

Adipose tissue serves as the primary storage depot for TAGs and plays a central role in energy homeostasis by regulating the balance between lipogenesis (fat storage) and lipolysis (fat mobilization) in response to hormonal and nutritional signals.