This summary focuses on the biochemistry of fatty acids, triacylglycerols, and ketone body metabolism, as covered in Chapter 16 of the Lippincott's Biochemistry textbook review.

Fatty Acid Structure and Properties
📌 Free fatty acids (FFAs) serve as a high-energy substrate, with blood levels increasing during fasting to mobilize energy stores for tissues like the liver and muscle.
💧 Fatty acids are amphipathic due to a terminal carboxyl group ($\text{COO}^-$), making them predominantly hydrophobic and requiring protein association for transport in circulation.
🔗 Structure naming conventions denote total carbons followed by the number of double bonds (e.g., $\text{18:2}$ means 18 carbons with 2 double bonds), with the location of double bonds specified in brackets.
🥑 Essential fatty acids like linoleic and alpha-linolenic acid must be obtained through diet as the body cannot produce them.

Fatty Acid Synthesis
🏭 Synthesis occurs when there is excess carbohydrate/protein diet, requiring the conversion of excess acetyl-coenzyme A ($\text{Acetyl-CoA}$) from the mitochondria to the cytosol via the citrate shuttle mechanism.
🔄 Cytosolic $\text{Acetyl-CoA}$ is converted to malonyl-CoA by Acetyl-CoA carboxylase ($\text{ACC}$), which requires biotin ($\text{Vitamin B7}$).
🛑 $\text{ACC}$ activity is inhibited by high palmitoyl-CoA (the final product) and by phosphorylation catalyzed by $\text{AMP-activated protein kinase}$, epinephrine, and glucagon.
⛓️ The main synthesis involves the multi-functional enzyme Fatty Acid Synthase ($\text{FAS}$), which repeatedly adds two-carbon units from malonyl-CoA to build a chain, culminating in the 16-carbon saturated fatty acid, palmitate.

Fat Storage: Triacylglycerol (TAG) Synthesis
💧 $\text{TAG}$ is formed by attaching three fatty acids (as fatty acyl-CoA) to a glycerol 3-phosphate backbone, resulting in a neutral fat suitable for major energy reserve storage.
💧 Glycerol 3-phosphate is produced from glucose in the liver and adipose tissue, but only the liver can convert free glycerol into glycerol 3-phosphate.
🔗 The attachment process involves four reactions: adding two fatty acyl-CoAs, removing a phosphate group, and adding the third fatty acid, resulting in $\text{TAG}$.

Fat Mobilization and $\beta$-Oxidation
⚡ The complete oxidation of fatty acids yields a very high energy output, producing 9 $\text{kcal/gram}$ compared to 4 $\text{kcal/gram}$ for protein or carbohydrates.
🏃 Lipolysis (fat breakdown) releases FFAs from $\text{TAG}$ stores, activated by lipases spurred by catecholamines (e.g., during the fight-or-flight response).
↔️ Insulin promotes fatty acid synthesis and inactivates lipases, while epinephrine activates lipases and inhibits synthesis, ensuring energy use when required.
🚗 Fatty acids travel bound to serum albumin to tissues, where long chains (>$12$ carbons) require the carnitine shuttle to cross the inner mitochondrial membrane for $\beta$-oxidation.

$\beta$-Oxidation and Energy Production
🔥 $\beta$-oxidation involves sequentially chopping off two-carbon fragments (as $\text{Acetyl-CoA}$) from the fatty acid chain.
⚙️ Each two-carbon removal cycle produces one $\text{Acetyl-CoA}$, one $\text{FADH}_2$, and one $\text{NADH}$.
📈 Oxidation of one 16-carbon palmitate molecule yields a total of 129 $\text{ATP}$ molecules through the $\text{TCA}$ cycle and electron transport chain.
🔗 If an unsaturated fatty acid is oxidized, the double bond must first be removed/oxidized to allow the process to continue.
🔗 Odd-numbered chains result in a 3-carbon molecule, propanoyl-CoA, which is converted to succinyl-CoA to enter the $\text{TCA}$ cycle.
🦚 Very long-chain fatty acids (>$22$ carbons) undergo initial breakdown via $\beta$-oxidation in peroxisomes before mitochondrial processing.

Ketone Body Metabolism
💡 Ketone bodies (acetoacetate, 3-hydroxybutyrate, and acetone) are alternative fuels produced by the liver when there is excess $\text{Acetyl-CoA}$ due to relative oxaloacetate depletion.
❌ The liver cannot utilize ketone bodies for energy because it lacks the enzyme thiophorase.
🩸 Ketones are water-soluble and travel freely in the blood to peripheral tissues (like muscle) to be converted back to $\text{Acetyl-CoA}$ for energy, thus sparing glucose.
⚠️ High levels of ketones, coupled with dehydration from excessive urination (due to high blood glucose), lead to severe acidosis, resulting in Diabetic Ketoacidosis ($\text{DKA}$) in uncontrolled diabetes.

Key Points & Insights
➡️ Fatty acids are the most energy-dense fuel source, yielding 129 $\text{ATP}$ per 16-carbon palmitate molecule upon complete oxidation.
➡️ Fatty acid synthesis and oxidation are tightly regulated: Insulin promotes synthesis (inhibiting lipases), while epinephrine/glucagon promote fat mobilization (activating lipases).
➡️ The carnitine shuttle is essential for transporting fatty acids ($>12$ carbons) into the mitochondria; its inhibition by malonyl-CoA prevents oxidizing fatty acids being synthesized for storage.
➡️ Ketone body production becomes critical during fasting/starvation when the cell needs an alternative fuel source to spare glucose, but pathological overproduction leads to dangerous acidosis ($\text{DKA}$).

📸 Video summarized with SummaryTube.com on Oct 11, 2025, 08:48 UTC