Lesson 17 & 18: Cellular Redox Mechanisms, Antioxidants, and Organelles

ROS are reactive oxygen species produced during cellular respiration and various metabolic processes.
ROS can damage macromolecules: DNA, RNA, proteins, lipids, and membranes.
Antioxidant defenses: enzymatic (SOD, CAT, GPx, GRx) and non-enzymatic (GSH, vitamins C/E, carotenoids, etc.).
Balance between ROS production and antioxidant defenses maintains redox homeostasis.

Redox = oxidation-reduction reaction; electrons are transferred between donor and acceptor.
Redox pair dynamics: one species oxidized, one reduced; reactions occur in coupled pairs.
Oxidation vs Reduction (course definitions):
- Oxidation: gain in oxygen; loss of electrons; loss of hydrogen.
- Reduction: loss of oxygen; gain of hydrogen; gain of electrons.
Electron carriers in metabolism: NAD⁺/NADH, FAD/FADH₂, and their roles in energy production.

Electrons from glycolysis, pyruvate oxidation, and TCA are transferred to NAD⁺ (→ NADH) and to FAD (→ FADH₂).
NADH and FADH₂ donate electrons to the electron transport chain (ETC) to regenerate NAD⁺ and FAD for continued metabolism.
Electrons flow from Complexes I/II through the chain to Complex IV and ultimately to O₂, forming water.
Electron leakage at ETC complexes can generate ROS such as superoxide (O₂⁻) and H₂O₂.

In mitochondria, imperfect electron transfer can produce superoxide (O₂⁻) which is converted to H₂O₂ by superoxide dismutase.
ROS include: ext{O}2^{ullet-}, ext{H}2 ext{O}_2, ext{OH}^ullet
ROS are highly reactive and act as oxidizing agents causing cellular damage if not neutralized.

Antioxidants donate electrons to neutralize ROS, forming stable electron pairs.
Vitamin C (ascorbate) is a non-enzymatic antioxidant/reducing agent.
Classification:
- Non-enzymatic endogenous antioxidants: GSH, ceruloplasmin, albumin, bilirubin, transferrin, uric acid.
- Exogenous antioxidants: carotenoids, ascorbic acid, selenium, α-tocopherol, flavonoids.
- Enzymatic antioxidants: SOD, CAT, GPx, GRx.
Glutathione (GSH) is a major intracellular reducing agent; it donates electrons to reduce ROS (e.g., H₂O₂ to H₂O) and becomes oxidized to GSSG. Regeneration of GSH from GSSG requires NADPH via Glutathione Reductase.
NADPH is crucial for maintaining the pool of reduced glutathione and for other reductive biosyntheses.

PPP provides NADPH for reductive biosynthesis and antioxidant defense.
Oxidative phase of PPP generates NADPH and ribose-5-phosphate (for nucleotide synthesis).
NADP⁺ is reduced to NADPH during oxidative PPP; regeneration of NADP⁺ by glutathione reductase sustains PPP flow.
Key link: PPP supplies the reducing power that regenerates GSH via GR, maintaining ROS detoxification.

G6PD is the rate-limiting enzyme in PPP, critical for NADPH production.
Fava beans contain vicine and convicine; metabolites divicine and isouramil are potent oxidants.
In G6PD deficiency, NADPH production is impaired, limiting GSH regeneration via glutathione reductase.
Result: inadequate detoxification of oxidative agents, damage to RBCs, and hemolytic anemia (favism).

Peroxisomes perform initial β-oxidation of very-long-chain fatty acids (≥22 carbons) to shorten them before mitochondrial oxidation.
Acyl-CoA oxidase initiates peroxisomal β-oxidation, producing enoyl-CoA and hydrogen peroxide (H₂O₂).
Catalase in peroxisomes decomposes H₂O₂ to water and oxygen, preventing toxicity.
Once shortened, fatty acids are transferred to mitochondria for complete β-oxidation.

Lysosomes: membrane-bound organelles with acid hydrolases; form in the Golgi and fuse with endosomes.
Functions include degradation of proteins, nucleic acids, carbohydrates, and lipids; recycling of breakdown products (amino acids, nucleotides).
Lysosomal pathways:
- Phagocytosis: engulfment of large particles (e.g., bacteria) into phagosomes that fuse with lysosomes.
- Autophagy: degradation of cellular organelles via autophagosomes that fuse with lysosomes.
- Receptor-mediated endocytosis: selective uptake of materials via receptor-ligand coated vesicles.

ROS are produced in normal metabolism but require detoxification to prevent damage.
Antioxidants (enzymatic and non-enzymatic) and NADPH (via PPP) are essential for redox balance.
G6PD deficiency compromises NADPH production, increasing susceptibility to oxidative stress and favism after fava bean exposure.
Peroxisomes handle initial fatty acid oxidation and detoxify H₂O₂ with catalase.
Lysosomes and peroxisomes contribute to cellular recycling and detoxification, while lysosomes degrade macromolecules and damaged organelles via multiple pathways.