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Metabolic Pathways and Energy Production
Category: Bioquímica médica; Medicina geral
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Which enzyme catalyzes the irreversible decarboxylation of pyruvate to acetyl‑CoA?
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In β‑oxidation of a 16‑carbon fatty acid, how many acetyl‑CoA molecules are produced?
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What is the primary regulatory mechanism controlling glycogen phosphorylase activity in liver cells?
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Which of the following statements correctly describes the P/O ratio for NADH‑derived electrons in oxidative phosphorylation?
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During anaerobic fermentation in yeast, which end‑product is formed from pyruvate?
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Which transport protein facilitates the entry of activated fatty acids into the mitochondrial matrix for β‑oxidation?
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In the Cori cycle, what is the primary purpose of converting lactate back to glucose in the liver?
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Which enzyme directly converts glycerol into the glycolytic intermediate dihydroxyacetone phosphate (DHAP)?
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What is the net ATP yield from complete oxidation of one glucose molecule according to the provided data?
Metabolic Pathways and Energy Production
Category: Bioquímica médica; Medicina geral
Learning Objectives
- Understand why lactate formation is crucial during intense skeletal muscle activity.
- Identify the key enzymes that control the entry of pyruvate into the citric acid cycle.
- Calculate the yield of acetyl‑CoA from β‑oxidation of long‑chain fatty acids.
- Explain the primary regulatory mechanisms of glycogen phosphorylase in the liver.
- Describe the P/O ratio for NADH‑derived electrons in oxidative phosphorylation.
- Recognize the end‑products of anaerobic fermentation in yeast.
- Know the transport proteins that shuttle activated fatty acids into mitochondria.
- Summarize the physiological purpose of the Cori cycle.
1. Lactate Production and Continued Glycolysis
During high‑intensity exercise, skeletal muscle fibers rely heavily on anaerobic glycolysis to generate ATP quickly. The rapid conversion of glucose to pyruvate produces NADH, but the mitochondrial electron‑transport chain cannot re‑oxidize NADH fast enough under low‑oxygen conditions. Lactate dehydrogenase (LDH) converts pyruvate to lactate, simultaneously regenerating NAD⁺. This regenerated NAD⁺ is essential because glycolytic enzymes such as glyceraldehyde‑3‑phosphate dehydrogenase require it to keep the pathway moving.
Key point: Lactate production is essential for the regeneration of NAD⁺, allowing glycolysis to continue and sustain ATP production during intense muscle contraction.
2. Irreversible Decarboxylation of Pyruvate
The transition from glycolysis to the citric acid cycle is mediated by the pyruvate dehydrogenase complex (PDC). This multi‑enzyme complex catalyzes the irreversible decarboxylation of pyruvate, producing acetyl‑CoA, CO₂, and NADH. Because the reaction is highly exergonic and tightly regulated, it serves as a major control point for carbohydrate oxidation.
Remember: Pyruvate dehydrogenase complex = the enzyme that irreversibly converts pyruvate to acetyl‑CoA.
3. β‑Oxidation of a 16‑Carbon Fatty Acid
β‑Oxidation cleaves fatty acids two carbons at a time, generating one acetyl‑CoA per cycle. A 16‑carbon fatty acid (palmitic acid) undergoes seven complete cycles, each yielding one acetyl‑CoA, and the final round produces a second acetyl‑CoA from the remaining four‑carbon fragment. Thus, the total number of acetyl‑CoA molecules produced is eight.
Calculation summary:
- Number of cycles = (n/2) – 1 = (16/2) – 1 = 7 \n
- Acetyl‑CoA per cycle = 1 → 7
- Final cleavage yields 2 more acetyl‑CoA → total 8
4. Regulation of Glycogen Phosphorylase in Liver
Glycogen phosphorylase catalyzes the rate‑limiting step of glycogen breakdown. In hepatic cells, its activity is primarily controlled by its phosphorylation state, which is governed by phosphorylase kinase**. When phosphorylase kinase is activated (often by glucagon‑cAMP signaling), it phosphorylates glycogen phosphorylase, converting it to the active ‘a’ form. Allosteric effectors such as glucose‑6‑phosphate can modulate activity, but the dominant regulatory mechanism is phosphorylation.
5. P/O Ratio for NADH‑Derived Electrons
The P/O ratio expresses the number of ATP molecules synthesized per atom of oxygen reduced (or per pair of electrons transferred). For electrons entering the electron‑transport chain via NADH (Complex I), the modern consensus value is approximately 2.5 ATP per NADH. This reflects the pumping of protons at Complex I, III, and IV and the stoichiometry of ATP synthase (≈4 protons per ATP).
Contrast this with the older textbook value of 3 ATP per NADH; the updated figure improves the accuracy of metabolic calculations and aligns with experimental data.
6. Anaerobic Fermentation in Yeast
Yeast cells perform alcoholic fermentation when oxygen is limited. Pyruvate is decarboxylated to acetaldehyde, which is then reduced to ethanol, releasing carbon dioxide as a by‑product. The overall reaction is:
2 Glucose → 2 Ethanol + 2 CO₂ + 2 ATP
Thus, the correct end‑product from pyruvate in yeast is ethanol and carbon dioxide.
7. Transport of Activated Fatty Acids into Mitochondria
Long‑chain fatty acids are activated to fatty‑acyl‑CoA in the cytosol, but the inner mitochondrial membrane is impermeable to CoA‑esters. The carnitine shuttle solves this problem:
- Carnitine acyl‑transferase I (CPT I) on the outer mitochondrial membrane transfers the acyl group from CoA to carnitine, forming acyl‑carnitine.
- Acyl‑carnitine is translocated across the inner membrane by a carrier protein.
- Carnitine acyl‑transferase II (CPT II) on the matrix side reconverts acyl‑carnitine to fatty‑acyl‑CoA for β‑oxidation.
The key entry step is catalyzed by CPT I.
8. The Cori Cycle: Lactate to Glucose
During vigorous exercise, muscle cells export lactate to the bloodstream. The liver takes up this lactate and, via gluconeogenesis, converts it back to glucose. This glucose can then be released into the blood to replenish muscle glycogen and maintain blood glucose levels. The Cori cycle therefore serves the vital purpose of recycling lactate into glucose to sustain systemic energy balance during and after intense activity.
Summary
Understanding the interplay of glycolysis, the citric acid cycle, β‑oxidation, and their regulatory mechanisms is essential for medical professionals dealing with metabolic disorders, exercise physiology, and critical care nutrition. Mastery of these concepts enables accurate interpretation of laboratory data (e.g., lactate levels, NADH/NAD⁺ ratios) and informs therapeutic decisions such as carbohydrate loading, fatty‑acid supplementation, and management of hypoglycemia.
