Biochem Oct. 22nd

Midterm Information

Date: Saturday, November 1
Time: 2:00 PM - 5:00 PM
Coverage: 13 topics, including the first set of content. A total of 90 questions are planned, raising concerns about the number of questions relative to the extensive content coverage.

Review Session

Organizer: Back End Mentorship Program
Format: In-person and Zoom options available
Date: Friday (prior to the midterm)
Facilitators: Two graduate students will lead the review session, ensuring comprehensive coverage of pertinent material for effective studying.
Access: Zoom link available through Topic 13 slides.

Lecture Overview

Current Topic: Continuation of Topic 12
Comic Reference: A reference to a comic with Calvin returning from a birthday party—illustrative of social dynamics.
Key Concepts Introduction:
- Triacylglycerol Breakdown: Triacylglycerols, stored in adipose tissue, are broken down into fatty acids and glycerol by hormone-sensitive lipase. These products are then released into the bloodstream.
- Functionality of Glycerol:
- Glycerol, once in the liver, can be phosphorylated to glycerol-3-phosphate and then oxidized to dihydroxyacetone phosphate (DHAP), which enters the gluconeogenesis pathway to produce glucose.
- The brain predominantly uses glucose as its primary fuel source. While fatty acids can be used by other tissues, they are not directly taken up or metabolized by the brain due to their inability to cross the blood-brain barrier effectively.

Energy Supply from Fatty Acids

Energy Supply Mechanism:
- Tissues, particularly the liver and muscle, can efficiently uptake fatty acids from the bloodstream.
- Inside the mitochondrial matrix of these cells, fatty acids undergo beta-oxidation, a catabolic process that converts them into two-carbon units of acetyl CoA.
- This acetyl CoA then enters the citric acid cycle, leading to the production of reduced cofactors (NADH and $QH_2$ ) that subsequently fuel ATP generation through oxidative phosphorylation.

Hormonal Regulation of Lipase

Lipase Functionality:
- Hormone-sensitive lipase is the key enzyme responsible for hydrolyzing triacylglycerols into fatty acids and glycerol.
- Insulin: When blood glucose levels are high (e.g., after a meal), insulin is released. Insulin signals energy abundance and primarily inhibits hormone-sensitive lipase activity, thus promoting fat storage and reducing fatty acid mobilization.
- Glucagon: Conversely, when blood glucose levels are low (e.g., during fasting), glucagon is released. Glucagon activates hormone-sensitive lipase through a cAMP-dependent phosphorylation cascade, promoting the breakdown of triacylglycerols and the release of fatty acids for energy extraction.

Fatty Acid Activation and Beta-Oxidation

Activation Process:
- Enzymatic Process: Before fatty acids can be catabolized, they must be activated. This occurs in the outer mitochondrial membrane and cytosol where fatty acids are converted to acyl-CoA by acyl-CoA synthetase (also known as fatty acyl-CoA ligase).
- The reaction requires ATP, which is hydrolyzed to AMP and pyrophosphate ( $ATP \rightarrow AMP + PPi$ ), equivalent to two ATP molecules. A high-energy thioester linkage is formed between the fatty acid and coenzyme A. $Fatty Acid + CoA + ATP \xrightarrow{Acyl-CoA\ Synthetase} Acyl-CoA + AMP + PPi$
Beta-Oxidation Process:
- Acyl-CoA is transported into the mitochondrial matrix via the carnitine shuttle system (for long-chain fatty acids).
- Beta-oxidation consists of a repeating sequence of four enzymatic reactions:
1. Dehydrogenation (forms a double bond and produces $FADH_2$ )
2. Hydration (adds water across the double bond)
3. Dehydrogenation (forms a keto group and produces $NADH$ )
4. Thiolysis (cleaves off acetyl CoA)
- Each cycle results in the cleaving of two carbon molecules from the acyl-CoA chain, yielding one molecule of acetyl CoA and a shortened acyl-CoA molecule that re-enters the cycle.
- For example, a 16-carbon saturated fatty acid (palmitoyl-CoA) undergoes 7 cycles of beta-oxidation, producing 8 molecules of acetyl CoA, 7 molecules of $NADH$ , and 7 molecules of $QH2$ (or $FADH2$ ), all contributing to increased ATP production.
Two Main Outputs in Beta-Oxidation:
- Acetyl CoA, which proceeds to the citric acid cycle.
- Reduced cofactors (NADH and $QH_2$ ), which donate electrons to the electron transport chain for ATP synthesis.

Acetyl CoA Utilization

Fueling Mechanisms:
- Post-beta-oxidation, acetyl CoA enters the citric acid cycle, where it is completely oxidized. This internal energy conversion results in the production of additional reduced cofactors (NADH and $FADH2$ ), GTP (which can be converted to ATP), and $CO2$ (as a waste product).
- The electron carriers ( $NADH$ and $FADH_2$ ) then feed into the electron transport chain, driving the vast majority of cellular ATP production.

Unsaturated Fatty Acids

Processing:
- Beta-oxidation can process both saturated and unsaturated fatty acids, but specific auxiliary enzymes are required for unsaturated fatty acids due to the presence of double bonds.
- Unsaturated fats may produce less $QH2$ (or $FADH2$ ) because the initial dehydrogenation step (which normally generates $QH_2$ ) is sometimes bypassed if a double bond is already present at the correct position (e.g., by an isomerase enzyme).
- Odd-chain fatty acids, primarily found in plant lipids and some marine organisms, are managed through different biochemical pathways. After successive rounds of beta-oxidation, they yield propionyl CoA (a 3-carbon unit) in the final cycle. Propionyl CoA is then converted to methylmalonyl CoA and subsequently to succinyl CoA, which can directly integrate into the citric acid cycle.

Ketone Bodies and Fasting

Production:
- During prolonged fasting or in conditions of low glucose availability (e.g., uncontrolled diabetes), the liver synthesizes ketone bodies from fatty acids. The liver is the primary, if not sole, site of ketone body synthesis.
- These ketone bodies (primarily acetoacetate and beta-hydroxybutyrate, with acetone as a minor, volatile product) are released into the bloodstream and serve as an alternative fuel source for extrahepatic tissues, critically including the brain.
- This mechanism is crucial because the brain cannot directly metabolize fatty acids, making ketone bodies vital for brain energy supply when glucose is scarce.

Fatty Acid Synthesis from Acetyl CoA

Contrasting Process:
- Fatty acid synthesis occurs in the cytosol when the body has an abundance of energy, typically under high glucose conditions (high insulin levels).
- Citrate Transport System: Acetyl-CoA, which is produced in the mitochondrial matrix (e.g., from glucose or amino acids), must be moved to the cytosol for fatty acid biosynthesis. This is achieved by condensing acetyl-CoA with oxaloacetate to form citrate. Citrate is then transported out of the mitochondria into the cytosol, where it is cleaved back into acetyl-CoA and oxaloacetate by ATP-citrate lyase. This process generally involves ATP consumption and the generation of NADPH.
- Reverse of Beta-Oxidation: Fatty acid synthesis is not a direct reversal of beta-oxidation. It employs a different set of enzymes (Fatty Acid Synthase complex) and occurs in a different cellular compartment (cytosol vs. mitochondria). It involves the sequential addition of two-carbon units from malonyl-CoA, requiring significant energy input in the form of ATP and specifically NADPH as the primary reducing agent for the successive reduction steps.

Regulation of Fatty Acid Metabolism

Summary of Energy Sources:
- During insulin-spiked states (post-high sugar intake), the body prioritizes glucose utilization, and excess energy (including fatty acids) is channeled into storage as triacylglycerols in adipose tissue rather than being mobilized for immediate energy production.
- Glucagon, acting in low glucose states, promotes lipid mobilization and fatty acid oxidation for energy retrieval. This re-emphasizes the coordinated partnership between glucagon and insulin in regulating fatty acid metabolism to maintain energy homeostasis.

Conclusion of the Lecture

Story Integration: Narrative of Sorav's heartbroken experience interspersed with metabolic discussions provides a continuous learning metaphor.

Important Notes for Midterm Preparation

Familiarize with metabolic pathways of fatty acid synthesis and degradation, differentiation between the roles of insulin and glucagon, and the regulation and biochemical underpinnings of energy metabolism in varying body states.