Metabolism Flashcards
1
Q
Glucose Transporters
A
- GLUT 2: Found in hepatocytes and pancreatic cells, has a high Km (low affinity for glucose), and follows first-order kinetics (picks up glucose in proportion to its concentration in blood); captures and stores excess glucose from hepatic portal vein after a meal; serves as glucose sensor for insulin release in beta-islet cells.
- GLUT 4: Found in adipose tissue and muscle, Km close to normal glucose levels in blood (high affinity for glucose), and follows zero-order kinetics (permits constant rate of glucose influx); responds to glucose concentration in peripheral blood, and insulin increases glucose transport rates in adipose tissue and muscle by stimulating movement of additional GLUT 4 transporters to membrane.
2
Q
Glycolysis Overview
A
- Occurs in cytoplasm of all cells; represents only energy-yielding pathway available in red blood cells because they lack mitochondria.
- Coverts glucose (6C) into two pyruvate molecules (3C each), generating net two ATP and one NADH.
3
Q
Enzymes of Glycolysis
A
- Glucokinase/Hexokinase (Irreversible): Phosphorylates glucose to G6P using 1 ATP. Glucokinase found in hepatocytes and pancreatic beta-islet cells and induced by insulin (high Km, like GLUT 2). Hexokinase found in all tissues and inhibited by G6P (low Km, like GLUT 4).
- PFK-1/PFK-2 (Irreversible): Phosphorylates F6P to F-1,6-BP using 1 ATP; inhibited by ATP and citrate and activated by AMP. In hepatocytes, insulin activates PFK-2 (converts F6P to F-2,6-BP), and F-2,6-BP activates PFK-1; glucagon inhibits PFK-2, lowering F-2,6-BP and inhibiting PFK-1. PFK-2 allows hepatocytes to override inhibition by ATP when energetically satisfied.
- Glyceraldehyde-3-phosphate dehydrogenase: Oxidizes glyceraldehyde-3-phosphate to high-energy 1,3-bisphosphoglycerate and reduces NAD+ to NADH.
- 3-Phosphoglycerate kinase: Transfers high-energy phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP (via first substrate-level phosphorylation) and 3-phosphoglycerate.
- Pyruvate kinase (Irreversible): Transfers high-energy phosphate from PEP to ADP, forming ATP (via second substrate-level phosphorylation) and pyruvate. Activated by F-1,6-BP from PFK-1 reaction (feed-forward activation).
4
Q
Rate-Limiting Enzymes of Metabolism
A
- Glycolysis: Phosphofructokinase-1.
- Glycogenesis: Glycogen synthase.
- Glycogenolysis: Glycogen phosphorylase.
- Gluconeogenesis: Fructose-1,6-bisphosphatase.
- Fermentation: Lactate dehydrogenase.
- Pentose Phosphate Pathway: Glucose-6-phosphate dehydrogenase.
- Citric Acid Cycle: Isocitrate dehydrogenase.
5
Q
Three Irreversible Glycolytic Enzymes
A
- Hexokinase/Glucokinase.
- PFK-1/PFK-2.
- Pyruvate Kinase.
6
Q
Fermentation
A
- Occurs in absence of oxygen.
- Lactate Dehydrogenase: Key fermentation enzyme that reduces pyruvate to lactate and oxidizes NADH to NAD+, replenishing oxidized coenzyme for gylceraldehyde-3-phosphate dehydrogenase.
- Lactate does not form in significant amounts in aerobic tissue; lactate production increases when oxygenation is poor (in skeletal muscle during exercise, MI, stroke) and most ATP generated by anaerobic glycolysis.
7
Q
Important Intermediates of Glycolysis
A
- Dihydroxyacetone phosphate (DHAP): Used for triacylglycerol synthesis in hepatic and adipose tissue; formed from F-1,6-BP and can be isomerized to glycerol-3-phosphate (which can be converted to glycerol for use as triacylglycerol backbone).
- 1,3-BPG and PEP: High-energy intermediates used to generate ATP via substrate-level phosphorylation.
8
Q
Glycolysis in Erythrocytes
A
- Bisphosphoglycerate mutase in RBCs converts 1,3-BPG to 2,3-BPG by moving phosphate from position-1 to position-2.
- 2,3-BPG binds allosterically to hemoglobin and decreases its affinity for oxygen, shifting oxygen dissociation curve to the right and allowing unloading of oxygen in tissues but still allowing 100% saturation in lungs.
9
Q
Glycogen Storage
A
- Glycogen: Branched polymer of glucose; storage form of glucose.
- Glycogen synthesis and degradation occurs mainly in liver (to mobilize glucose between meals to prevent hypoglycemia) and skeletal muscle (for muscle contractions).
10
Q
Glycogenesis
A
- Glycogenesis: Synthesis of glycogen granules.
- Glucose-6-phosphate converted to glucose-1-phosphate by mutase; G1P + UTP -> UDP-Glucose + PPi; UDP comes off as glucose added to glycogen chain.
- Glycogen Synthase: Catalyzes alpha-1,4 glycosidic bond between glucose in linear chains of granule; stimulated by G6P and insulin, and inhibited by epinephrine and glucagon.
- Branching Enzyme: Hydrolyzes alpha-1,4 bond and forms alpha-1,6 bond to introduce branches into granule.
11
Q
Glycogenolysis
A
- Glycogenolysis: Process of breaking down glycogen to mobilize glucose.
- Glycogen Phosphorylase: Breaks alpha-1,4 glycosidic bonds using inorganic phosphate to release glucose-1-phosphate from granule periphery (cannot break alpha-1,6 bonds); activated by glucagon in liver (so glucose can be provided to body) by AMP and epinephrine in skeletal muscle (which signal that muscle is active and requires more glucose), and inhibited by ATP.
- Debranching Enzyme: Deconstructs branches exposed by glycogen phosphorylase.
- G1P converted to G6P by mutase, and G6P converted to glucose by glucose-6-phosphatase).
12
Q
Gluconeogenesis
A
- Carried out mainly by liver, and kidney to small extent; promoted by glucagon and epinephrine (which act to raise blood glucose levels), and inhibited by insulin (which acts to lower blood glucose levels).
- Substrates for Gluconeogenesis: Glycerol-3-phosphate (from triacylglycerols in adipose tissue), lactate (from anaerobic respiration), and glucogenic amino acids (from muscle proteins, all except leucine and lysine).
- Glycerol-3-phosphate converted to DHAP by glycerol-3-phosphate dehydrogenase; lactate converted to pyruvate by lactate dehydrogenase; alanine converted to pyruvate by alanine aminotransferase.
- Pyruvate to Oxaloacetate to PEP: Pyruvate carboxylase converts pyruvate to OAA in mitochondria using ATP, OAA transported out of mitochondria via malate-aspartate shuttle, PEP carboxykinase converts cytosolic OAA to PEP in cytoplasm using GTP. Pyruvate carboxylase activated by acetyl-CoA, and PEPCK induced by glucagon and cortisol.
- PEP to G3P via three reversible reactions.
- F-1,6-BP to F6P: Fructose-1,6-bisphosphatase dephosphorylates F-1,6-BP to F6P; activated by ATP, and inhibited by AMP and F-2,6-BP.
- F6P to G6P via reversible reaction.
- G6P to Glucose: Glucose-6-phosphatase dephosphorylates G6P to glucose in lumen of ER in hepatocytes.
13
Q
Pentose Phosphate Pathway
A
- Produces NADPH and ribose-5-phosphate for nucleotide synthesis.
- Irreversible enzyme G6PD converts G6P to ribose-5-phosphate, generating NADPH.
- G6PD activated by insulin and NADP+ and inhibited by NADPH.
- NADPH acts as electron donor and potent reducing agent for biosynthesis of fatty acids and cholesterol, in immune system, and to help prevent oxidative damage (as compared to NAD+, which acts as electron carrier and potent oxidizing agent for ATP synthesis).
14
Q
Acetyl-CoA Synthesis
A
- Pyruvate from glycolysis enters mitochondrion via active transport.
- Pyruvate dehydrogenase complex decarboxylates and oxidizes pyruvate (3C) to acetyl-CoA (2C) while reducing NAD+ to NADH.
- PDH activated by insulin in liver, and inhibited by acetyl-CoA, ATP, and NADH.
- PDH Kinase phosphorylates PDH complex, inhibiting acetyl-CoA production when ATP levels rise; PDH Phosphatase dephosphorylates PDH complex, stimulating acetyl-CoA production when ADP levels rise.
- Acetyl-CoA can be obtained from metabolism of carbohydrates, fatty acids, and amino acids.
15
Q
Citric Acid Cycle
A
- Oxidizes acetyl-CoA to CO2 and H2O, and generates high-energy NADH and FADH2 electron carriers in mitochondrial matrix.
- Cycle will not occur anaerobically; inhibited by accumulation of NADH and FADH2 due to hypoxia.
- Citrate Synthase: Acetyl-CoA + OAA -> Citrate + CoA-SH.
- Aconitase: Citrate -> Isocitrate.
- Isocitrate Dehydrogenase (RL enzyme): Isocitrate + NAD+ -> alpha-Ketoglutarate + NADH + CO2.
- alpha-Ketoglutarate Dehydrogenase Complex: alpha-Ketoglutarate + CoA-SH + NAD+ -> Succinyl-CoA + NADH + CO2.
- Succinyl-CoA Synthetase: Succinyl-CoA + GDP + Pi -> Succinate + GTP + CoA-SH.
- Succinate Dehydrogenase (complex II of ETC in inner membrane): Succinate + FAD -> Fumarate + FADH2.
- Fumarase: Fumarate -> Malate.
- Malate Dehydrogenase: Malate + NAD+ -> OAA + NADH.
16
Q
Control Points of Citric Acid Cycle
A
- Citrate Synthase: Inhibited by ATP, NADH, succinyl-CoA, citrate.
- Isocitrate Dehydrogenase: Inhibited by ATP and NADH, stimulated by ADP and NAD+.
- alpha-Ketoglutarate Dehydrogenase Complex: Inhibited by ATP, NADH, succinyl-CoA.
17
Q
Electron Transport Chain and Oxidative Phosphorylation
A
- Complex I (NADH-CoQ Oxidoreductase): Oxidizes NADH to NAD+ and reduces CoQ to CoQH2 (via FMN and Fe-S); 2 electrons transferred from NADH to CoQ, and 4 protons pumped.
- Complex II (Succinate-CoQ Oxidoreductase): Oxidizes succinate to fumarate and reduces CoQ to CoQH2 (via FAD and Fe-S); 2 electrons transferred from succinate to CoQ, and no protons pumped.
- Complex III (CoQH2-Cytochrome C Oxidoreductase): Q cycle reduces two cytochrome C molecules by oxidizing two CoQH2 molecules, generates one CoQH2 by taking up two protons from matrix, and pumps 4 protons.
- Complex IV (Cytochrome C Oxidase): Oxidizes cytochrome C and reduces O2 to H2O; 4 electrons transferred in total from 4 cytochrome C proteins to O2, and 2 protons pumped. Cyanide irreversibly binds to cytochrome oxidase and prevents transfer of electrons to oxygen, greatly affecting heart and CNS cells.
- Proton pumping decreases pH of intermembrane space and increases voltage difference across inner membrane, contributing to electrochemical gradient across inner membrane; ATP synthase harnesses energy stored by electrochemical gradient to phosphorylate ADP using inorganic phosphate to ATP via Chemiosmotic Coupling (coupling of proton gradient and ATP synthesis).
- O2 and ADP are key regulators of oxidative phosphorylation. If O2 is limited, the rate of oxidative phosphorylation decreases and the concentration of NADH (and FADH2) increases, and this NADH accumulation inhibits the citric acid cycle (via process called Respiratory Control). If O2 is adequate, the rate of oxidative phosphorylation depends on the availability of ADP; ADP accumulation implies low ATP levels and thus promotes ATP synthesis (ADP allosterically activates isocitrate dehydrogenase of the citric acid cycle).
- Uncouplers dissipate proton gradient and thus prevent ATP synthesis without affecting ETC. ADP accumulates and causes O2 consumption and NADH oxidation to increase; the energy produced from the transport of electrons is released as heat (often perceived as fever).
19
Q
NADH Shuttles
A
- Net ATP yield per glucose molecules ranges from 30 to 32, due to Glycerol-3-Phosphate Shuttle (via FAD-dependent GPDH, generating 1.5 ATP) and Malate-Aspartate Shuttle (via mitochondrial malate dehydrogenase, generating 2.5 ATP).
- See Gluconeogenesis diagram.
