Metabolism and Metabolic Pathways.

METABOLISM

Definition of Metabolism: Refers to the entire set of life-sustaining chemical reactions in organisms. It can be categorized into two broad types: catabolism (breaking down molecules to extract energy) and anabolism (building up molecules for cellular functions).

Definition: Metabolic pathways are sequences of chemical reactions occurring within a cell, where different enzymes facilitate each step.
- Each step may require specific coenzymes to assist in transferring products from one reaction to another.
Branching Metabolic Pathways:
- The direction of pathway flow is influenced by the presence and activity of specific enzymes.
- Enzymes (Enz₁, Enz₂, Enz₃, etc.) catalyze reactions to convert substrates through intermediates to final products.

Regulatory Mechanism: Pathways may be regulated based on the availability and accumulation of products.
- Example: If a final product accumulates, it may inhibit an earlier enzyme in the pathway (end-product inhibition).
- Control of enzyme activation is essential for metabolic efficiency and balance.

Inborn errors of metabolism arise from damage or defects to specific enzymes in metabolic pathways. Examples include:
- Defective Enz₁: Phenylketonuria (PKU)
- Defective Enz₅: Alcaptonuria (black urine disease)
- Defective Enz₆: Albinism

Definition: Cellular respiration utilizes fuel molecules (like glucose, fatty acids, and amino acids) to generate ATP for cellular energy.
Storage Molecules: The body stores energy in forms such as glycogen (carbohydrates) and triglycerides (fats).
- Goal: To collect hydrogen (H) from fuel molecules, facilitating ATP synthesis from stored fuels.

General Process: The cellular respiration process can proceed under different oxygen conditions, namely:
- Anaerobic Respiration: Occurs when oxygen supply is limited.
- Aerobic Respiration: Occurs in the presence of sufficient oxygen.

Step 1: Glycolysis is the initial step of glucose metabolism.
- Location: Cytosol (not in mitochondria).
- Characteristics: It occurs regardless of aerobic or anaerobic conditions.
Steps of Glycolysis:
1. Add 2 ATP to glucose.
2. Split glucose into two three-carbon molecules.
3. Collect 2 hydrogen atoms on NAD+
4. Produce 4 ATP (net gain of 2 ATP after consumption).
5. The end-product of glycolysis is 2 pyruvate molecules.
Overall Products of Glycolysis:
- Net production of 2 ATP.
- Generating 2 NADH for later use (hydrogen carriers).
- Producing 2 pyruvate, which can follow two pathways:
1. Lactic acid pathway (anaerobic respiration), or
2. Citric acid cycle (aerobic respiration).

After glycolysis, if oxygen is low:
- Anaerobic Respiration: Lactic acid is produced, netting only 2 ATP.
- Lactic acid is sent to the liver for conversion back to glucose.
If oxygen is available:
- Aerobic Respiration proceeds through various stages:
1. Conversion of Pyruvate to Acetyl CoA.
2. Enter the Citric Acid Cycle (Krebs Cycle).
3. Electron Transport Chain (ETC) for ATP production.

This transformation connects glycolysis to further energy-yielding processes.
Process: Each pyruvate from glycolysis is converted into Acetyl CoA, releasing CO2 and generating another NADH.

Purpose: Further breakdown the Acetyl CoA to harvest energy.
Functionality: The Citric Acid Cycle completes the oxidation of carbohydrates and fats, producing:
- 1 ATP per cycle, as well as CO2 and electron carriers NADH and FADH2.
- It consists of a series of transformations collecting each hydrogen for further oxidative phosphorylation.

Final Stage: Utilizes the harvested hydrogen ions collected by earlier metabolic processes:
- Electron Transport Chain (ETC): Electrons from NADH and FADH2 are transferred through mitochondrial proteins.
- As electrons move through the chain, energy released is used to pump hydrogen ions into the intermembrane space, creating an electrochemical gradient.
- Production of ATP: Hydrogen ions flow back into the mitochondrial matrix through ATP synthase, generating ATP from ADP and inorganic phosphate through a process called oxidative phosphorylation.
- Oxygen Role: Oxygen serves as the final electron acceptor, forming water as a byproduct.

Step 1: Beta-Oxidation in mitochondria: Fatty acids are metabolized to produce Acetyl CoA (no anaerobic respiration occurs).
- Segments of fatty acids are cleaved into 2-carbon units that enter the citric acid cycle.
Step 2: Continue through the Citric Acid Cycle and ETC for further ATP generation.

This process only occurs if necessary:
- Step 1: Oxidative Deamination, preparing amino acids for entry into the citric acid cycle.
- Step 2: Amino acids enter the Citric Acid Cycle at various points, producing waste nitrogen in the form of ammonium (NH3), which is converted to urea in kidneys.

Glycogen Metabolism: Crucial metabolic processes occur in the liver:
- Glycogenolysis: Conversion of glycogen into glucose for energy.
- Glycogenesis: Formation of glycogen from glucose.
- Gluconeogenesis: Production of glucose from non-carbohydrate precursors.
- Lipogenesis: Synthesis of lipids from excess carbohydrates or proteins.

Metabolic pathways can be categorized as either anabolic (building up) or catabolic (breaking down).
- These processes are interlinked and can switch based on the body's needs, particularly in the liver.

The body tends to prioritize fatty acids as fuel during energy production. This is crucial during prolonged periods when glucose availability is low, ensuring that glucose is preserved for essential functions, particularly for the brain, which predominantly relies on glucose under normal conditions.