Lipid Metabolism: Lipolysis and Beta-Oxidation of Fatty Acids

Comprehensive vocabulary flashcards covering triacylglycerol mobilization (lipolysis), hormonal regulation, fatty acid activation, the carnitine transport system, beta-oxidation enzymatic reactions, and ATP stoichiometry.

Lipolysis

The breakdown or hydrolysis of triacylglycerol in adipose tissue into glycerol and $3$ fatty acids by intracellular lipases.

Triacylglycerols (Triglycerides)

The principal storage form of energy in human adipose tissue, constituting approximately $84\%$ of total stored body energy.

Hormone-sensitive lipase (HSL)

The key regulatory lipase in adipose tissue that catalyzes the sequential removal of fatty acids from triacylglycerol to form diacylglycerol and monoacylglycerol; activated via phosphorylation by cyclic AMP-dependent protein kinase.

Monoacylglycerol lipase (MAG lipase)

The enzyme that hydrolyzes monoacylglycerol into free glycerol and a fatty acid during the final step of lipolysis.

Epinephrine (in lipolysis)

An anti-insulin hormone that stimulates lipolysis during fasting, stress, and hypoglycemia by binding to membrane receptors and activating adenylate cyclase to elevate cyclic AMP levels.

Insulin (in lipolysis regulation)

A hormone that inhibits lipolysis by inhibiting adenylate cyclase, activating phosphodiesterase ($\text{cAMP} \rightarrow \text{AMP}$), and activating phosphatase to dephosphorylate and inactivate hormone-sensitive lipase.

Glycerol kinase

An enzyme present in the liver but absent in adipose tissue, which prevents adipose cells from directly reusing free glycerol for triacylglycerol synthesis.

Non-esterified fatty acids (NEFA)

Free fatty acids released by lipolysis that bind reversibly to plasma albumin to enable soluble transport through the bloodstream to peripheral tissues.

Beta-oxidation ($\beta$-oxidation)

The major catabolic pathway occurring in the mitochondrial matrix that degrades fatty acids at the $\beta$-carbon to release $2$-carbon units in the form of acetyl-CoA per cycle along with $\text{NADH}$ and $\text{FADH}_2$.

Alpha-oxidation ($\alpha$-oxidation)

A fatty acid degradation pathway occurring predominantly in the brain and liver that removes a single carbon unit per cycle in the form of $\text{CO}_2$.

Omega-oxidation ($\omega$-oxidation)

A minor pathway of fatty acid catabolism that becomes physiologically significant under conditions of impaired beta-oxidation.

Fatty acid activation

The conversion of a free fatty acid to fatty acyl-CoA in the cytosol by esterification with $\text{CoASH}$, requiring the cleavage of $2$ high-energy phosphate bonds ($\text{ATP} \rightarrow \text{AMP} + 2\,\text{P}_i$).

Carnitine

A carrier compound required to transport activated long-chain fatty acyl groups across the inner mitochondrial membrane into the mitochondrial matrix.

Acylcarnitine translocase

An inner mitochondrial membrane transport protein that moves fatty acyl-carnitine into the matrix while transporting free carnitine back out to the cytosolic side.

Acyl-CoA dehydrogenase

The first enzyme in the $\beta$-oxidation spiral, catalyzing the dehydrogenation of fatty acyl-CoA to form trans-$\Delta^2$-enoyl-CoA and reducing $\text{FAD}$ to $\text{FADH}_2$.

Enoyl-CoA hydratase

The second enzyme in the $\beta$-oxidation spiral, catalyzing the hydration of the trans-$\Delta^2$ double bond to produce $\text{L-}\beta\text{-hydroxyacyl-CoA}$.

$\beta$-hydroxyacyl-CoA dehydrogenase

The third enzyme in the $\beta$-oxidation spiral, catalyzing the oxidation of the $\beta$-hydroxyl group to form $\beta\text{-ketoacyl-CoA}$ while reducing $\text{NAD}^+$ to $\text{NADH} + \text{H}^+$.

Thiolase (Acyl-CoA acetyltransferase)

The fourth enzyme in the $\beta$-oxidation spiral, catalyzing thiolytic cleavage of $\beta\text{-ketoacyl-CoA}$ with $\text{CoA-SH}$ to release acetyl-CoA and a fatty acyl-CoA shortened by two carbons.

Net ATP yield of Palmitate ($\text{C}_{16:0}$)

The total energy harvested from the complete oxidation of palmitate: $129\,\text{ATP}$ ($35\,\text{ATP}$ from $7$ cleavage cycles yielding $\text{NADH}$ and $\text{FADH}_2$, plus $96\,\text{ATP}$ from $8$ acetyl-CoA units in the citric acid cycle, minus $2\,\text{ATP}$ utilized during initial activation).

Net ATP yield of Lauric Acid ($\text{C}_{12}$)

The total net energy produced from lauric acid degradation: $95\,\text{ATP}$ ($15\,\text{ATP}$ from $5\,\text{NADH}$, $10\,\text{ATP}$ from $5\,\text{FADH}_2$, and $72\,\text{ATP}$ from $6$ acetyl-CoA, minus $2\,\text{ATP}$ consumed in activation).