L9 - Notes on Lipid Metabolism, Digestion, and Energy Extraction, and Ketogenesis

Follow-up on Glucose Homeostasis and Glycogen Usage

• Carbohydrate Loading (Carb Loading):

  • The practice of consuming large amounts of carbohydrates (e.g., a bowl of pasta) before an athletic event is intended to maximize glycogen stores.

  • The objective is to maximize these storage molecules so they can be utilized as an energy source during exercise.

  • Glycogen stores in the muscles can fuel approximately one hour or more of medium to high-intensity exercise before depletion occurs.

• Tissue-Specific Roles of Glycogen:

  • Muscle Glycogen: This storage is reserved exclusively for the muscle's own energy requirements and ATP production. Muscle cells lack the specific enzyme required to convert $\text{glucose-6-phosphate}$ back into glucose, meaning they cannot release glucose into the bloodstream to assist other organs.

  • Liver Glycogen: The primary role of liver glycogen is to maintain blood glucose homeostasis to ensure a constant supply of fuel for the brain.

• Signal Transduction Differences:

  • Liver Cells: Possess glucagon receptors. During a fasting state, glucagon is perceived, triggering the breakdown of glycogen into glucose for systemic release.

  • Muscle Cells: These cells are "blind" to glucagon because they lack the glucagon receptor. Instead, they possess adrenaline receptors. Adrenaline (epinephrine) triggers a similar signal transduction pathway using the secondary messenger cyclic AMP ($\text{cAMP}$) and activating Protein Kinase A ($\text{PKA}$). This activates enzymes that break down glycogen for immediate local energy use (e.g., the "chased by a gorilla" scenario).

Introduction to Lipids and Fatty Acids

• Lipids as Macronutrients: This section marks the transition from studying carbohydrates to fats (lipids), to be followed by proteins (amino acids).

• Chemical Characteristics: Fats are generally waxy, greasy, or oily. They are highly reduced, energy-rich molecules.

• Basic Building Blocks:

  • Fatty Acids: Composed of a carbonyl group and a long, highly reduced acyl (hydrocarbon) chain.

  • Triacylglycerol (TAG): The chemical name for fat, consisting of three acyl units attached to a glycerol backbone via ester bonds.

  • Cholesterol: A sterol-based fat crucial for membrane structure and signaling.

• Biological Functions of Lipids:

  • Energy storage and extraction.

  • Structural components of membranes (compartmentalization).

  • Signaling molecules.

Structural Classification and Naming of Fats

• Saturated Fats:

  • Contain only single bonds between carbons in the hydrocarbon tail.

  • Example: Stearic acid ($18:0$), which has an $18$-carbon chain and zero double bonds.

  • Characteristics: Rigid structure, high melting point, typically solid at room temperature (e.g., animal fats and tropical oils like coconut oil).

• Unsaturated Fats:

  • Contain at least one double bond in the fatty acid tail, which imparts flexibility.

  • Monounsaturated: Contains one double bond (e.g., Oleic acid, with a double bond between carbons $9$ and $10$).

  • Polyunsaturated: Contains multiple double bonds (e.g., linoic acid).

• Essential Fatty Acids: Includes $\text{Omega-3}$ fatty acids, which humans cannot synthesize and must obtain from dietary sources like plants and fish.

• Trans Fats:

  • Rare in nature; predominantly produced during food manufacturing.

  • Health Risks: A 2 \text{%} increase in energy intake from trans fats is associated with a 23 \text{%} increase in cardiovascular risk.

  • Impact: They are associated with increased LDL ("bad cholesterol") and reduced HDL ("good cholesterol") levels.

Efficiency of Fat as an Energy Reservoir

• Energy Density:

  • $1 \text{ mole}$ of fat yields approximately $120 \text{ moles}$ of $\text{ATP}$.

  • $1 \text{ mole}$ of glucose yields approximately $32 \text{ moles}$ of $\text{ATP}$.

• Hydrophobicity and Storage:

  • Fats are non-polar and hydrophobic (anhydrous), meaning they do not associate with water.

  • In contrast, $1 \text{ gram}$ of glycogen associates with $2 \text{ grams}$ of water.

  • This lack of water association allows fat to have a much higher energy-to-weight ratio, making it an extremely efficient storage medium compared to glycogen, which takes up too much space due to its water weight.

Digestion and Absorption of Dietary Fats

• The Solubilization Problem: Enzymes function in aqueous (water-based) environments, while fats are hydrophobic. The body must partially solubilize fats to digest them.

• Bile Salts:

  • Act as the body’s "detergent."

  • Synthesized in the liver from cholesterol and stored in the gallbladder.

  • They have both hydrophobic and minor hydrophilic properties, allowing them to emulsify large lipid droplets into smaller ones.

• Enzymatic Breakdown:

  • Pancreatic enzymes lipase and cholipase access the fat droplets facilitated by bile salts.

  • Lipase clips the ester bonds to release free fatty acids and monoacylglycerol (one fatty acid chain on glycerol).

  • These smaller molecules can then be transported across the epithelial cells of the small intestine.

• Re-synthesis and Transport:

  • Inside the epithelial cells, triacylglycerol is re-synthesized.

  • Chylomicrons: Because fats are too hydrophobic to travel directly in the blood (they would fall out of solution), they are packaged into chylomicrons. These are lipoproteins with a core of hydrophobic molecules surrounded by phospholipids (hydrophilic heads facing out) and specific proteins that act as "address markers" for trafficking.

  • Path of Transport: Chylomicrons travel through the lymph system into the bloodstream.

  • Lipoprotein Lipase (LPL): Located on cell walls, this enzyme cleaves fatty acids from the chylomicrons so they can be taken up by muscles (for energy) or adipose tissue (for storage).

  • Liver Uptake: Remaining chylomicron fragments are taken up by the liver via endocytosis.

Fatty Acid Metabolism: Beta Oxidation

• Localization: Occurs within the mitochondrial matrix.

• Phase 1: Activation/Investment:

  • The fatty acid is converted into fatty acyl CoA in the cytosol.

  • This is an energy-requiring step driven by the hydrolysis of $\text{ATP}$ to $\text{AMP}$ and the release of pyrophosphate ($\text{PPi}$).

  • Reaction: $\text{Fatty Acid} + \text{CoA} + \text{ATP} \rightarrow \text{Fatty Acyl CoA} + \text{AMP} + \text{PPi}$.

  • This reaction has a very low $\text{delta G}$, making it highly favorable.

• Phase 2: Localisation (The Carnitine Shunt):

  • Fatty acyl CoA can cross the outer mitochondrial membrane but is too bulky for the inner membrane.

  • It is converted into fatty acyl carnitine, which is transported into the matrix.

  • Once inside, it is converted back into fatty acyl CoA, and the carnitine is recycled.

• Phase 3: The Oxidation Cycle:

  • The process involves removing two carbons at a time from the acyl chain starting from the beta carbon.

  • Step 1: Oxidation by $\text{acyl CoA dehydrogenase}$, reducing $\text{FAD}$ to $\text{FADH}_2$.

  • Step 2: Hydration (addition of $\text{H}_2\text{O}$).

  • Step 3: Oxidation, reducing $\text{NAD}^+$ to $\text{NADH}$.

  • Step 4: Thiolysis, releasing one molecule of acetyl CoA and a fatty acyl chain that is now two carbons shorter.

• Theoretical Yield Example (Stearic Acid - $18$ carbons):

  • Total Acetyl CoA produced: $\frac{18}{2} = 9 \text{ molecules}$.

  • Total cycles required: $8$ (the final cycle naturally leaves two 2-carbon units, so a 9th cycle is unnecessary).

  • Total electron carriers: $8 \text{ FADH}_2$ and $8 \text{ NADH}$.

Ketogenesis and Ketone Bodies

• Necessity: The brain cannot directly utilize fatty acids for energy. During starvation or fasting, when glucose levels are low, the body uses an alternative fuel derived from fat.

• Process: Occurs in the liver. Excess acetyl CoA is converted into transportable forms known as ketone bodies.

• Specific Ketone Bodies:

  • Acetoacetate.

  • D-beta-hydroxybutyrate (DB hydroxybutyrate).

  • Acetone (a byproduct that can cause a distinct aroma on the breath).

• Function: Ketone bodies are water-soluble enough to be transported in the blood to the heart, skeletal muscles, and specifically the brain, where they are converted back into acetyl CoA to enter the TCA cycle.

• Ketogenic Diet:

  • A high-fat, low-carbohydrate diet designed to force the body into ketogenesis.

  • Has been used by some individuals (including a former student) to manage conditions like Type 2 diabetes by stabilizing blood glucose levels.

Questions & Discussion

• Question about Carb Loading: A student asked if "carb loading" with pasta before an event is related to glycogen storage. The lecturer confirmed it is specifically about maximizing carbohydrate stores to ensure sustained energy for medium to high-intensity exercise.

• Interactive Polling/Observation: The lecturer asked how many students cook with coconut oil. Students noted it is solid in the pantry. The lecturer explained this is due to its high saturated fat content and rigid single-bond molecular structure.

• Case Study Anecdote: A past student with Type 2 diabetes used a ketogenic diet to stabilize their blood glucose levels successfully. The lecturer noted the "acetone aroma" as a physical symptom of high ketone body production on this diet.