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Stack #4571750
| Question | Answer |
|---|---|
| Hexokinase IV (glucokinase) differs from Hexokinase I–III because: | It is not inhibited by G6P, it has a high Km (low binding affinity), and acts as a glucostat |
| Hexokinase IV's ability to act as a glucostat | Its low binding affinity and positive cooperativity for glucose, allows its activity to be directly proportional to blood glucose levels, rather than being saturated at low concentrations |
| Stage 1 Glycolysis | Hexokinase Reaction (Glucose → Glucose-6-phosphate); Mechanism: ATP donates a phosphate to glucose (nucleophilic substitution), forming glucose-6-phosphate. This traps glucose inside the cell. |
| Stage 1 Glycolysis thermodynamics | Exothermic |
| Stage 2 Glycolysis | Phosphoglucose Isomerase (Glucose-6-phosphate → Fructose-6-phosphate); Mechanism: Rearranges the aldose (glucose) to a ketose (fructose). |
| Stage 3 Glycolysis | Phosphofructokinase-1 (Fructose-6-phosphate → Fructose-1,6-bisphosphate); Mechanism: Another ATP donates a phosphate (nucleophilic substitution rxn), making the molecule more reactive. |
| Stage 3 Glycolysis thermodynamics | Exothermic |
| Stage 4 Glycolysis | Aldolase (Fructose-1,6-bisphosphate → DHAP + G3P); Mechanism: The opposite of a carbonyl condensation reaction; Cleaves the 6-carbon sugar into two 3-carbon sugars: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). |
| Stage 4 Glycolysis thermodynamics | Under standard conditions reaction is endothermic, but under cell conditions the reaction is exothermic and proceeds forwards |
| Aldolase mechanism | 1. Active-site Lys attacks carbonyl of fructose bi phosphate→ Schiff base. 2. Protonation prepares for cleavage. 3. Cleavage → releases GAP. 4. Enamine intermediate forms. 5. Hydrolysis releases DHAP. Enzyme restored. |
| Aldolase step 1 | Protonation and Nucleophilic Attack: The carbonyl oxygen of FBP is protonated by an aspartate residue in the enzyme's active site. This facilitates a nucleophilic attack by lysine on the keto carbon, forming a carbinolamine intermediate. |
| Aldolase step 2 | Dehydration: The carbinolamine undergoes dehydration to form a protonated Schiff base (iminium ion). |
| Aldolase step 3 | Retro-Aldol Reaction: The Schiff base is cleaved into an enamine and GAP. |
| Aldolase step 4 | Protonation: The enamine is protonated to form another iminium ion. |
| Aldolase step 5 | Hydrolysis: The iminium ion is hydrolyzed, releasing DHAP and regenerating the enzyme. |
| Stage 5 Glycolysis | Triose Phosphate Isomerase (DHAP ↔ G3P) Mechanism: Converts DHAP to G3P, so both molecules can continue through glycolysis. |
| Stage 5 Glycolysis thermodynamics | Under standard conditions reaction is endothermic, but under cell conditions the reaction is exothermic and proceeds forwards |
| Stage 6 Glycolysis | Glyceraldehyde-3-phosphate Dehydrogenase (G3P → 1,3-Bisphosphoglycerate) Mechanism: Oxidizes G3P, reduces NAD+ to NADH, and adds a phosphate. |
| Stage 6 Glycolysis thermodynamics | Exothermic |
| Stage 7 Glycolysis | Phosphoglycerate Kinase (1,3-BPG → 3-Phosphoglycerate); Mechanism: Transfers a phosphate to ADP, forming ATP (substrate-level phosphorylation). |
| Stage 7 Glycolysis thermodynamics | Exothermic |
| Stage 8 Glycolysis | Phosphoglycerate Mutase (3-Phosphoglycerate → 2-Phosphoglycerate); Mechanism: Moves the phosphate group from the 3rd to the 2nd carbon. |
| Stage 9 Glycolysis | Enolase (2-Phosphoglycerate → Phosphoenolpyruvate); Mechanism: Removes water to form a high-energy enol phosphate. |
| Stage 9 Glycolysis thermodynamics | Endothermic- The enolase reaction continues forward because the next step (pyruvate kinase) rapidly removes PEP, and the cell maintains concentrations that favor the reaction’s progress! |
| Stage 10 Glycolysis | Pyruvate Kinase (Phosphoenolpyruvate → Pyruvate) Mechanism: Transfers phosphate to ADP, making ATP and pyruvate. |
| Stage 10 Glycolysis thermodynamics | Highly exothermic |
| Substrate-Level Phosphorylation | ATP (or GTP) is formed directly by transferring a phosphate group from a high-energy substrate to ADP |
| Regular Phosphorylation | (Oxidative or Photophosphorylation): ATP is made indirectly using energy from electron transport (oxidative) or light (photo) |
| weakly endergonic glycolysis reactions | - Phosphoglucose Isomerase, Aldolase, Triose Phosphate Isomerase, Phosphoglycerate Mutase, Enolase -These reactions have small positive ΔG°' values (weakly endergonic) but can proceed forward in cells due to substrate/product concentrations |
| What are the high energy compounds formed during glycolysis | 1,3-Bisphosphoglycerate (1,3-BPG) and Phosphoenolpyruvate (PEP) |
| First bipass enzyme in gluconeogenesis | 1. Pyruvate Carboxylase: Converts pyruvate to oxaloacetate (in the mitochondria) 2. Phosphoenolpyruvate Carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate (PEP). |
| Pyruvate Carboxylase | Gluconeogenesis enzyme: Converts pyruvate to oxaloacetate (in the mitochondria) |
| Phosphoenolpyruvate Carboxykinase (PEPCK) | Gluconeogenesis enzyme: Converts oxaloacetate to phosphoenolpyruvate (PEP). |
| Second bipass enzyme in gluconeogenesis | Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate to fructose-6-phosphate. |
| Third bipass enzyme in gluconeogenesis | Glucose-6-phosphatase: Converts glucose-6-phosphate to free glucose (mainly in the liver) |
| Three regulatory steps in glycolysis | Also known as the ireverable steps. 1. Hexokinase, 2. PFK 3. PK. These steps are regulated by allosteric effectors, covalent modification, and energy status (ATP/AMP levels). |
| Regulation of Hexokinase (Glucose → Glucose-6-phosphate) | Inhibited by its product, glucose-6-phosphate (feedback inhibition). |
| Regulation of Glucokinase (Glucose → Glucose-6-phosphate) | (in liver): Regulated by glucokinase regulatory protein and blood glucose levels. |
| Regulation of Phosphofructokinase-1 (PFK-1) (Fructose-6-phosphate → Fructose-1,6-bisphosphate) | Activated by: AMP and fructose-2,6-bisphosphate (signals low energy). Inhibited by: ATP (high energy) and citrate (signals plenty of biosynthetic precursors). |
| Regulation of Pyruvate Kinase (Phosphoenolpyruvate → Pyruvate) | Activated by: Fructose-1,6-bisphosphate (feed-forward activation). Inhibited by: ATP (high energy) and alanine (signals enough building blocks). Liver pyruvate kinase: Also regulated by phosphorylation (inactivation by glucagon). |
| PFK-2/FBPase-2 in glycolysis | A bifunctional enzyme: Regulates the step catalyzed by Phosphofructokinase-1 (PFK-1); controls the levels of fructose-2,6-bisphosphate, which is a strong activator of PFK-1 |
| PFK-2 activity | synthesizes fructose-2,6-bisphosphate (F2,6BP) from fructose-6-phosphate |
| FBPase-2 activity | breaks down F2,6BP to fructose-6-phosphate. |
| Fructose-2,6-bisphosphate (F2,6BP) | A powerful allosteric regulator made by PFK-2; Activates PFK-1 (stimulates glycolysis). Inhibits fructose-1,6-bisphosphatase (inhibits gluconeogenesis). |
| PFK-2/FBPase-2 Regulation Mechanism: | When PFK-2 is active, F2,6BP levels rise → glycolysis is stimulated. When FBPase-2 is active, F2,6BP levels fall → glycolysis is inhibited (and gluconeogenesis is favored). |
| Insulin effects on glycolysis | (fed state): activates PFK-2, increasing F2,6BP and promoting glycolysis. |
| Glucagon effects on glycolysis | Glucagon (fasting state): activates FBPase-2, decreasing F2,6BP and inhibiting glycolysis. |
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