Study Notes on Biochemical Pathways and Energetics

Inhibition is an important concept in biochemistry that affects enzyme activity.
Different types of inhibition include:
- Feedback Inhibition: A process where the end product of a pathway inhibits an earlier step to prevent overproduction.
- Example: The final product can inhibit the first enzyme to shut down the pathway when sufficient product is present.
- Feedforward Mechanism: Less common and often involves activation rather than inhibition.
- Activation tends to awaken dormant pathways in response to initial products.
- Branching Pathways: Feedback inhibition can occur within pathways that branch out, affecting multiple enzymes.

Linear Pathways: Products can inhibit the upstream enzymes to control production rates.
Self-Inhibition: Enzymes may inhibit themselves when product concentrations reach a certain level to balance production rate.
The existence of interactions needs to be supported by figures or data; anything not visually represented may be disregarded.

The rate limiting step is the most common target of feedback inhibition but not the only one.
Understanding that the presence of specific product feedback influences enzyme activity is crucial for metabolic control.

In a feedback mechanism, when sufficient levels of the end product are produced, this signals earlier enzymes to slow down or stop their activity.
This is a crucial regulatory mechanism to maintain homeostasis in biochemical pathways.

Adenosine Triphosphate (ATP): Known as the body's energy currency.
- When energy is abundant, ATP stores energy in its phosphate bonds.
ATP Synthesis Reaction: The reaction involves ADP, inorganic phosphate $(P_i)$ , and energy to yield ATP and water.
ATP can be hydrolyzed to release energy: $ext{ATP} + ext{H}<em>2 ext{O} ightarrow ext{ADP} + P</em>i + ext{Energy}$ .

Condensation Reaction: Generation of water in forming ATP using ADP and phosphate.
Endergonic Reaction: Energy enters the system, resulting in products with higher potential energy.
Phosphorylation Reaction: Adding a phosphate group to ADP.
Hydrolysis Reaction: Breaking down ATP to ADP and inorganic phosphate, with the release of energy.
Exergonic Reaction: Energy exits the system, leading to products with lower potential energy.
Dephosphorylation: Removing a phosphate group from ATP.

Substrate-Level Phosphorylation: One molecule donates a phosphate group to ADP to form ATP directly.
Oxidative Phosphorylation: ADP binds to free $(P_i)$ without a direct phosphate donor, occurring via the electron transport chain in mitochondria, requiring oxygen.

The breakdown of one mole of glucose (C₆H₁₂O₆, approximately 180.16g) can theoretically yield 686 kilocalories of energy:
- The synthesis of one mole of ATP requires 7 kilocalories, leading to a theoretical maximum of 98 moles of ATP from complete glucose breakdown, but practically yields around 32 moles.
The energy loss occurs primarily as heat during metabolism.

Occurs in the cytoplasm; converts glucose to two pyruvate molecules while yielding a net of 2 ATP and 2 reduced coenzymes (NADH).

Takes place in mitochondria; converts pyruvate to acetyl CoA while releasing CO₂ and reducing a coenzyme (NADH) for each pyruvate.

Also occurs in the mitochondrial matrix.
Each acetyl CoA enters and produces:
- 1 ATP
- 2 CO₂
- 4 reduced coenzymes (3 NADH, 1 FADH₂).
Overall, 2 acetyl CoA yield 2 ATP and 8 reduced coenzymes.

Couples with the process of oxidative phosphorylation to generate ATP from the energy stored in reduced coenzymes, primarily NADH and FADH₂.
A hydrogen gradient is produced across the inner mitochondrial membrane, driving ATP synthesis.
Final Electron Acceptor: Oxygen, which combines with electrons and protons to form water.

The inefficiency arises through "hydrogen leak"—where hydrogen ions may leak out, resulting in lower ATP yields than theoretically expected.
Increased metabolic demands require more glucose due to these efficiency losses.

Oxygen is required as the final electron acceptor in aerobic respiration.
In scenarios where oxygen supply is insufficient (e.g., muscle fatigue), coenzymes become saturated and metabolism halts.
Anaerobic pathways can generate ATP by converting pyruvate into lactate, albeit inefficiently (only yielding 2 ATP per glucose).

Understanding the complex pathways of energy production, regulation through feedback mechanisms, and the impact of insufficient oxygen underscores the intricacy of cellular respiration.