Chapter 11 - Glycolysis 11.1 Glycolysis Is a Ubiquitous Pathway 1
L5-3 Brenowitz Regulation of Glycolysis Color Ppt PDF
Transcript of L5-3 Brenowitz Regulation of Glycolysis Color Ppt PDF
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• Introduction• Basic mechanisms – intersections of pathways• Physiological questions
• Major sites of regulation• Regulation in liver• Regulation in cardiac muscle
• Alcohol metabolism• Pathways• Clinical correlations • Clinical scenario: Chronic alcohol consumption
Lecture 5-3: Regulation of GlycolysisGlycolysis is a central pathway of glucose metabolism and therefore of carbohydrate metabolism. Its regulation is central to energy management.
All images are from “Marks et al., 2nd Edition, Copyright © 2005 Lippincott & Williams, A Wolters Kluwer Co., All rights reserved” unless otherwise noted.
NADPH
(storage)
Complexcarbohydrates
ATP
TCA Cycle
Glycolysis is central to both catabolic and anabolic metabolism
Glucose-6-P
Glycolysis
Figure 2 of the Section Five Introduction, Mark’s et al.
Note: G6P is not transported back across the plasma membrane
• Catabolic reactions: •Source of ATP
• Anabolic reactions: •Pyruvate is precursor for fatty acid biosynthesis and alanine
•Pentose phosphates precursor of nucleotides.
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Major pathways of glucose metabolism
NADPH
(storage)
Complexcarbohydrates
ATP
TCA Cycle
Glucose-6-P
Glycolysis
Figure 2 of the Section Five Introduction, Mark’s et al.
Glucose
Lactate
AnaerobicGlycolysis
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Alternate fates of pyruvate
Figure 22.5: glycolysis occurs exclusively in the cytosol
Glucose + 2 ADP + 2 Pi →2 Lacate + 2 ATP + 2 H2O + 2 H+
The overall ATP yield for the complete oxidation of one mole of glucose to CO2 is 36 – 38 moles
x2
x2
Fig. 22.9
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A brief note about ‘shuttles’
Fig. 22.8: Malate - aspartate shuttle Fig. 22.7: Glycerol 3-phosphate shuttle
Review of Unit 4…
Regulation of enzymatic activity – A pathway is only as fast as its slowest step
• Enzymes may be activated (+) or inhibited (-)• The concentration of enzymes may be increased or decreased
through the regulation of transcription or translationInduction (+) or repression (-)
• Mechanisms of regulation includeAllostericHormonal
Physiological roles• Tissue specific responses in glucose homeostasis
Liver is a “maintainer organ” – its principal role is to maintain homeostasisMuscle is a “consumer organ” – its principal role is to convert chemical to mechanical energy
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PiGlucose 6-
phosphatase
The major sites of glycolysis regulation in the liver
Figure 36.1: Regulation of glucokinase, PFK-1, and pyruvate kinase in the liver.
↑ Insulin : glucagon ratio –positively regulates PFK-1 and PK
Synthesis of Glucokinase is induced by insulin
The major sites of glycolysis regulation in the skeletal muscle
Figure 22.12: Regulation of hexokinaseand PFK-1 in skeletal muscle.
Product inhibition
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Regulatory considerations with regard to glucokinase• Km (S0.5) ≈ 7 - 10 mM - physiological blood glucose is
5 - 10 mM (80-160 mg/dl)• Glucokinase enzyme induced by increased ratio of insulin : glucagon• Balance of glucokinase and glucose 6-phosphatase activities
(G 6-P → glucose + Pi)
Fig. 31.14 -Differences in Kmbetween the liver enzyme glucokinaseand hexokinase, the enzyme that is found in other tissues and catalyzes the same reaction
PiGlucose 6-
phosphatase
The major sites of glycolysis regulation in the liver
Figure 36.1: Regulation of glucokinase, PFK-1, and pyruvate kinase in the liver.
↑ Insulin : glucagon ratio –positively regulates PFK-1 and PK
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Regulation of Phosphofructokinase-1 (PFK-1)PFK-1 catalyzes Fructose 6-Phosphate → Fructose 1,6 Bisphosphate
• Allosteric regulation of PFK-1 by AMP to a lower Km (higher affinity)
Fig. 22.14 Regulation of PFK-1 by AMP, ATP and fructose 2,6-bisphosphate.
+AMP
Regulation of glycolysis in cardiac muscle by AMP
Marks Figure 22.13: Changes in ATP, ADP and AMP concentrations in skeletal muscle during exercise.
Rest
Exercise~ 20%
~ 300%2 ADP ←⎯⎯→ AMP + ATP
adenylate kinase
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Regulation of Phosphofructokinase-1 (PFK-1)
• Allosteric regulation of PFK-1 by F2,6-BP to a lower Km (higher affinity)
• F2,6-BP is more effective activator of PFK-1, mole for mole, than AMP
Fig. 22.14 Regulation of PFK-1 by AMP, ATP and fructose 2,6-bisphosphate.
Fructose 2,6-bisphosphate (F2,6-BP) regulates PFK-1• Fuctose 2,6-BP is synthesized by Phosphofructokinase-2 (PFK-2)• The level of F 2,6-BP is controlled by phosphorylation of PFK-2
• F2,6-BP is not an glycolysis intermediate
• F2,6-BP regulates PFK-1 in both liver and adipose tissue
• [F2,6-BP] increases when the insulin : glucagon ratio increases since glycolysis in these tissues is the carbon source for the synthesis of triacylglycerols
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PFK-2 has both of these activities
Phosphofructokinase-2 (PFK-2) is a bifunctional enzyme. The ratio of activities is changed by phosphorylation and dephosphorylation of PFK-2.
Fructose 2,6-Bisphosphate is synthesized and degraded by a single enzyme, PFK-2
Coordinate regulation of liver PFK-2
↑ cAMP
Higher PFK-2 kinase activity
Higher PFK-2 phosphataseactivity
results in more F 2,6-BP
results in lessF 2,6-BP
• The two activities of PFK-2 are controlled by Protein Kinase A and phosphoprotein phosphatase
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Activation of glycolysis by Fructose 2,6-Bisphosphate
• In liver and adipose tissue after a high carbohydrate meal (↑ Insulin : Glucagon)
• PFK-2 is dephosphorylatedby protein phosphorylase
• The kinase activity of PFK-2 increases
• Therefore, levels of F2,6-BP increase
• F2,6-BP allostericallyactivates PFK-1
• Glycolysis is stimulated allowing glucose to be converted to triacylglycerols for storage
P
PFK-2 PFK-2phosphorylase
F-6-PF-2,6-BP
PFK-1F-6-P F-1,6-P
Inhibition of glycolysis in the LIVER by glucagon or epinephrine via a signal transduction cascade
The steps of a signaling cascade…activation of protein kinase A (PKA) →phosphorylation of phosphofructokinase-2 (PFK-2) →increase in the fructose 2,6-bisphosphatase activity →less fructose 2,6-bisphosphate (F 2,6-BP) →phosphofructokinase-1 (PFK-1) activity decreases →
…resulting in less glycolysis in the liver
From Figure 28.8:
↓ Insulin : Glucagon
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The response of PFK-2 to hormones is tissue specific
For example, the hormone epinephrine mobilizes fuels during acute stress -
• Liver: A ‘maintainer’ organ, provides plasma glucose by limiting glycolysis: Epinephrine → elevated cAMP, phosphorylates bifunctional enzyme → increases PFK-2 phosphatase activity → less F 2,6 BP
• Cardiac muscle: A ‘consumer’ organ, increases glycolysis to produce pyruvate for energy production:Epinephrine → elevated cAMP, phosphorylates bifunctional enzyme → increases PFK-2 kinase activity → more F 2,6 BP
LiverPFK-2
Liver phosphoryation favors phosphatase activity
CardiacMusclePFK-2 Muscle phosphoryation favors kinase activity
Regulation of liver pyruvate kinase by phosphorylation status. P = a serine residue phosphorylated by protein kinase A. (Alternative splicing yields different proteins.)
Tissue specific enzyme isozymes
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Liver:Pyruvate kinase
Liver:Pyruvate kinase
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Alcohol (Ethanol) is a fuel
Ethanol is principally metabolized in the liver providing as many as 13 moles of ATP per mole of ethanol
Alcohol (Ethanol) is a fuel
Use of ethanol as a motor fuel on a large scale raises environmental and food supply concerns
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Alcohol metabolismAlcohol dehydrogenase(Cytosolic enzyme)
Aldehyde dehydrogenase(Mitochondrial enzyme)
Fig. 25.1
(Toxic)
(Nontoxic)Fig. 25.2
Alcohol metabolism
TCA cycleFatty Acid Synthesis
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Clinical scenario: Metabolic effects of chronic ethanol consumption
• Summary of metabolic pathways• Ethanol is both lipid and water soluble and thus is
readily absorbed by passive diffusion • Ethanol is principally metabolized in the liver with as
many as 13 ATP synthesized per molecule• Products are acetaldehyde, acetate and acetyl CoA• Acetyl CoA →TCA cycle or fatty acid synthesis in the liver • Acetaldehyde is toxic• Reducing equivalent NADH and H+ produced
• Nutritional Effects• Chronic alcohol consumption results in decreased
adsorption of vitamins B1 (thiamin) and B12
• Behavioral Effects – (Acetaldehyde)
Nutritional effects?
?Source: Carlsberg brewery website
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• The enzymes of alcohol metabolism are a family of isozymes• Variation in the isozyme proportions affects
• rate of alcohol clearance, • degree of inebriation, • side effects of alcohol consumption and• susceptibility to alcohol-induced liver disease
• Points 2 – 4 are directly related to the concentration of the acetaldehyde intermediate
• The common allele of ALDH a low Km resulting in a high affinity for its substrate acetaldehyde
• Since even low concentrations of acetaldehyde (toxic) are converted to acetate (nontoxic), the concentration of the intermediate rarely exceeds 20 μM even during intoxication.
• An ALDH isozyme present with ~ 40% frequency in East Asian populations has a high Km resulting in low affinity for its substrate acetaldehyde
• High acetaldehyde concentrations result from minimal alcohol consumption
• Accumulation of toxic acetaldehyde results in facial flushing, nausea and vomiting
• Possession of this isozyme is associated with protection against alcoholism
Genetic variation of aldehydedehydrogenase (ALDH)
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• Isozymes of alcohol dehydrogenase that have a low Km result in metabolic changes similar to an acetaldehyde dehydrogenase isozyme with high Km and thus low affinity.
• The concentration of a reaction intermediate is the result of its influx and efflux…
Genetic variation of alcohol dehydrogenase (ADH)
Why isn’t ethanol a ‘good’ fuel?
The products of its metabolism alter the flow of metabolites…
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Alcohol metabolism favors ketosis and triglyceride synthesis through the excessive production of NADH