Biochem 3.26 Metabolism and Biochemical Pathways

Topics covered: hormones, metabolism, metabolic pathways.
Focus on insulin, glucagon, epinephrine, and serotonin.
Reviewing metabolic pathways: citric acid cycle, oxidative phosphorylation, glycolysis, gluconeogenesis, etc.

Cytoplasm:
- Glycolysis: Breakdown of glucose into pyruvate.
- Pentose Phosphate Pathway (PPP): Generates NADPH and ribose-5-phosphate for nucleic acid synthesis.
Mitochondrial Matrix:
- CAC occurs (except fumarate oxidation).
- Fatty acid oxidation and ketone body formation.
Inner Mitochondrial Membrane:
- Electron Transport Chain (ETC).

Central metabolic organ, processing fats, carbohydrates, and proteins.
Hepatocytes: Transform dietary nutrients into fuels for other tissues.
Enzyme flexibility based on nutritional state.
Roles in:
- Carbohydrate metabolism (e.g., glycogen synthesis, gluconeogenesis).
- Amino acid catabolism; produces urea via the urea cycle.
- Fatty acid metabolism; primary fuel source; capable of beta oxidation.
- Cholesterol synthesis regulated by HMG-CoA reductase.

Stores fatty acids as triacylglycerol (TAG).
Functions in TAG synthesis and breakdown in response to hormones (insulin, glucagon).
Glyceroneogenesis: Converts glucose to glycerol for TAG formation.

Energy sources vary with activity level:
- Rest: Fatty acids, ketone bodies.
- Moderate Work: Adds blood glucose.
- Heavy Work: Uses glycogen and phosphocreatine.
Phosphocreatine provides quick energy through phosphate transfer.

ATP Usage: Seconds.
Creatine Phosphate: Brief, seconds, high energy release.
Anaerobic Pathways: Provide energy via fermentation (lactate production).
Aerobic Metabolism: Dominates after initial oxygenation, involves oxidative phosphorylation in the ETC.

Interaction between muscle and liver during intense exercise:
- Muscle breaks down glycogen to glucose then pyruvate.
- Pyruvate -> lactate (anaerobic conditions).
- Lactate transported to the liver for gluconeogenesis, producing glucose for muscle replenishment.

Topics covered include a wide range of metabolic processes such as hormones, metabolism, and specific metabolic pathways pivotal in bioenergetics.
Focuses particularly on critical hormones like insulin, glucagon, epinephrine, and serotonin, each playing a distinct role in the regulation of metabolism.
In-depth review of major metabolic pathways, notably the citric acid cycle (CAC), oxidative phosphorylation, glycolysis, and gluconeogenesis, along with their interconnections and regulatory mechanisms influencing metabolic homeostasis.

The CAC serves as a central hub for metabolism, functioning in both catabolic and anabolic processes, thus described as amphibolic in nature.
Key functions include:
- Extracting high-energy electrons from acetyl-CoA, crucial for ATP production in subsequent metabolic processes.
- Producing three NADH, one FADH2, and one GTP (convertible to ATP), which are essential for energy transfers.
Important intermediates within the cycle include:
- Oxaloacetate: A critical four-carbon molecule that initiates the cycle by condensing with acetyl-CoA.
- Alpha-ketoglutarate: Plays a key role in amino acid metabolism and is involved in the regulation of energy production.
- Citrate: Central for connecting carbohydrate and fatty acid metabolism.

Cytoplasm:
- Glycolysis: The anaerobic breakdown of glucose into pyruvate, leading to ATP production.
- Pentose Phosphate Pathway (PPP): Important for generating NADPH, which is essential in reductive biosynthesis and ribose-5-phosphate crucial for nucleic acid synthesis.
Mitochondrial Matrix:
- CAC occurs here, with the exception of fumarate oxidation.
- Also involves fatty acid oxidation and the formation of ketone bodies during periods of prolonged fasting or carbohydrate restriction.
Inner Mitochondrial Membrane:
- Home to the Electron Transport Chain (ETC), where the high-energy electrons carried by NADH and FADH2 are finally used to produce ATP through oxidative phosphorylation.

Acts as a central metabolic organ capable of processing various macronutrients including fats, carbohydrates, and proteins.
Hepatocytes: Special cells that transform dietary nutrients into various fuels that other tissues can utilize.
Exhibits enzyme flexibility depending on the body's nutritional state, altering metabolic pathways accordingly.
Key roles include:
- Carbohydrate metabolism:
- Glycogen synthesis and gluconeogenesis to maintain blood glucose levels.
- Amino acid catabolism: Leads to the production of urea through the urea cycle, which detoxifies ammonia.
- Fatty acid metabolism: The liver serves as a primary source of ketone bodies via beta-oxidation, especially during fasting.
- Cholesterol synthesis: Regulated by the enzyme HMG-CoA reductase, important for steroid hormone production and maintaining cellular membrane integrity.

Serves as a major energy reservoir, storing fatty acids in the form of triacylglycerol (TAG).
Engages in TAG synthesis and mobilization in response to hormonal signals from insulin and glucagon.
Glyceroneogenesis: An essential process that converts glucose to glycerol for TAG formation, allowing for energy storage and management.

Energy sources vary significantly based on physical activity levels:
- At Rest: Primarily utilizes fatty acids and ketone bodies.
- Moderate Work: Blood glucose becomes an additional energy source, useful for sustaining activity.
- Heavy Work: Reliance on stored glycogen and phosphocreatine for rapid energy release.
Phosphocreatine plays a crucial role in providing immediate energy through the transfer of a phosphate group to ADP, rapidly regenerating ATP.

ATP Usage: Utilized immediately within seconds of activity.
Creatine Phosphate: Offers short bursts of energy lasting just a few seconds but releases energy rapidly to aid high-intensity activities.
Anaerobic Pathways: Involve processes that provide energy through fermentation, resulting in lactate production when oxygen levels are low.
Aerobic Metabolism: Becomes predominant after several minutes of sustained exercise, involving oxidative phosphorylation to generate ATP through the ETC.