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Chapter 8
Metabolism and Jet Engines Section 8.1: Section 8.1: Glycolysis Section 8.2: Section 8.2: Gluconeogenesis Section 8.3: Section 8.3: The Pentose Phosphate Pathway Section 8.4:Section 8.4: Metabolism of Other Important
Sugars Section 8.5:Section 8.5: Glycogen Metabolism
Carbohydrate Metabolism
Overview
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Metabolism and Jet Engines
Catabolic pathways with a turbo step are optimized and efficient
Energy is fed back into the system to accelerate the fuel input step
Figure 8.1 Glycolysis and the Turbo Jet Engine
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Chapter 8: Overview
Energy transforming pathways of carbohydrate metabolism include glycolysis, glycogenesis, glycogenolysis, gluconeogenesis, and pentose phosphate pathway
Figure 8.2 Major Pathways in Carbohydrate Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Glycolysis (anaerobic process) occurs in almost every living cell
Ancient process central to all life Splits glucose into two three-carbon pyruvate unitsCatabolic process that captures some energy as 2 ATP and 2 NADH
Figure 8.2 Major Pathways in Carbohydrate Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Following the pathway
Carbons H/electrons Phosphate
Section 8.1: Glycolysis
Glycolysis is an anaerobic processTwo stages (stage 1 and 2): energy investment and energy producing
Glycolytic Pathway: D-Glucose + 2 ADP + 2 Pi + 2 NAD+ 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
In eukaryotes, the enzymes for this pathway are in the cytosol. They are all homodimers or homotetramers.
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Figure 8.3 Glycolytic Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Figure 8.3 Glycolytic
Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Table 17-1 Standard Free Energy Changes (G°¢), and Physiological Free Energy Changes (G) in Heart Muscle, of the Reactions of Glycolysisa.
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Section 8.1: Glycolysis
Reactions of the Reactions of the Glycolytic PathwayGlycolytic Pathway
1. Synthesis of glucose-6-phosphate
Phosphorylation of glucose (kinase) prevents transport out of the cell and increases reactivity
2. Conversion of glucose-6-phosphate to fructose-6-phosphate
Conversion of aldose to ketose
Figure 8.3a Glycolytic Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
The nucleophilic attack of the C6—OH group of glucose on the phosphate of an Mg2+–ATP complex.
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The phosphoryl-transfer reaction catalyzed by hexokinase.
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Conformation changes in yeast hexokinase on binding glucose. (a) Space-filling model of a subunit of free hexokinase. (b) Space-filling model of a subunit of free hexokinase in complex with glucose (purple).
\
This same change in conformation
is observed for ALL kinases!
It also accounts for the fact that water cannot be
used for hydrolysis of ATP unless we fool the enzyme
by using xylose instead of glc.
Phosphoglucose isomerase (PGI)
pKs for active site: 6.7 and 9.3(determined by rate vs. pH)
Which aa’s??
Actually Glu (!!!) and His with stabilization of His+ by a Glu (remember the ser protease mechanism!)
Works exactly like HK.
Activated by [AMP] even in the presence of hi [ATP].
Inhibited by hi [ATP]or citrate
Section 8.1: Glycolysis
Reactions of the Reactions of the Glycolytic Pathway Glycolytic Pathway ContinuedContinued
3. Phosphorylation of fructose-6-phosphate
This step is irreversible due to a large decrease in free energy and commits the molecule to glycolysis
4. Cleavage of fructose-1,6-bisphosphate
Aldol cleavage giving an aldose and ketose product
Figure 8.3a Glycolytic Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Reactions of the Reactions of the Glycolytic Pathway Glycolytic Pathway ContinuedContinued
5. Interconversion of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate
Conversion of aldose to ketose enables all carbons to continue through glycolysisFigure 8.3a Glycolytic
Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Figure 17-10 Proposed enzymatic mechanism of the TIM reaction: General Acid Catalysis.
pKs = 6.5 and 9.5Like PGIBut pK1 is for GLU! Normal pk?
GluAsp activity by 1000!
Reaction rate is diffusion limited!!
4.1
End of Glycolysis Collection Phase
Net result so far?ATPNAD+
Carbon
Section 8.1: Glycolysis
Reactions of the Reactions of the Glycolytic Pathway Glycolytic Pathway ContinuedContinued
In Step 2 (reactions 6-10), each reaction occurs in duplicate6. Oxidation of glyceraldehyde-3-phosphate
Creates high-energy phosphoanhydride bond for ATP formation and NADH
7. Phosphoryl group transfer
Production of ATP via substrate-level phosphorylation
Figure 8.3b Glycolytic Pathway (Stage 2)
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Some reactions employed in elucidating the enzymatic mechanism of GAPDH. (a) The reaction of iodoacetate with an active site Cys residue. (b) Quantitative tritium transfer from substrate to NAD+.
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32Pi also incorporated
Section 8.1: Glycolysis
Oxidation of glyceraldehyde-3-phosphate (G-3-P) is a 2-step process (reaction 6)
G-3-P undergoes oxidation and phosphorylationG-3-P interacts with the sulfhydryl group in the enzyme’s active site
Figure 8.4Glyceraldehyde-3-Phosphate Dehydrogenase Reaction
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Figure 8.4 Glyceraldehyde-3-Phosphate Dehydrogenase Reaction
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Oxidation of glyceraldehyde-3-phosphate (G-3-P) is a complex process (reaction 6)
Substrate oxidized after interaction with sulfhydrylBound NADH exchanged for NAD+
Enzyme displaced by addition of inorganic phosphate
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Space-filling model of yeast phosphoglycerate kinase showing its deeply clefted bilobal structure.
1,3 BPG + ADP →3 PGA + ATP
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Section 8.1: Glycolysis
Reactions of the Reactions of the Glycolytic Pathway Glycolytic Pathway ContinuedContinued
8. Interconversion of 3-phosphoglycerate and 2-phosphoglycerate
First step in formation of phosphoenolpyruvate (PEP)
9. Dehydration of 2-phosphoglycerate
Production of PEP, which has a high phosphoryl group transfer potential (tautomerization), locks it into the highest energy formFigure 8.4b Glycolytic
Pathway (Stage 2)From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
The pathway for the synthesis and degradation of 2,3-BPG in erythrocytes is a detour from the glycolytic pathway.
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Figure 17-21 Proposed reaction mechanism of enolase.
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F- binds Pi + Mg+2
Potent inhibitor
Section 8.1: Glycolysis
Reactions of the Glycolytic Reactions of the Glycolytic Pathway ContinuedPathway Continued
10. Synthesis of pyruvateFormation of pyruvate and ATP
Produces a net of 2 ATP, 2 NADH, and 2 pyruvate
Figure 8.3b Glycolytic Pathway (Stage 2)
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Figure 17-22 Mechanism of the reaction catalyzed by pyruvate kinase.
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Let's sing!!
http://www.csulb.edu/~cohlberg/songbook.html
Lyrics
http://books.google.com/books?id=oq9ENyL_d9YC&lpg=PP1&pg=PA1#v=onepage&q&f=false
The product’s composition, 3-phosphoglycerateFrom 3 to 2 position can readily mutate
And now 2 phosphoglycerage does something rather strange—Electrons on C@ and 3 proceed to rearrange.
Thus, redox-dehydration, catalysed by enolaseGives PEP formation and bond energy raiseSo phosphoenolphruvate reacts with ADP
The kinase making ATP but NOT reversibly.
In anaerobiosis, pyruvate’s not the end;The problem we suppose is not hard to comprehend’
The dehydrogenation to phosphoglycerateWould grind to halt if NAD+ could not regenerate.
The answer is quite subtle, pryruvayte is reduced,Instead of malate shuttle, L-lactate is produced;Lactate dehydrogenase performs that noble feat,
NADH is oxidised; the pathway is complete.
The balance sheet you’ll see shows transfer of energy,Two ATPs from glucose, and three from G1P
That’s good, but oh to use the way where pyruvate’s reduced—With decarboxylation first, then ethanol produced!!!!!
Section 8.1: Glycolysis
The Fates of PyruvateThe Fates of PyruvatePyruvate is an energy-rich moleculeUnder aerobic conditions, pyruvate is converted to acetyl-CoA for use in the citric acid cycle and electron transport chain
Figure 8.6 The Fates of Pyruvate
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
The Fates of Pyruvate The Fates of Pyruvate ContinuedContinued
Under anaerobic conditions pyruvate can undergo fermentation: alcoholic or homolactic
Regenerates NAD+ so glycolysis can continue
Figure 8.7 Recycling NADH during Anaerobic Glycolysis
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Energetics of GlycolysisEnergetics of GlycolysisIn red blood cells, only three reactions have significantly negative G values
Figure 8.8 Free Energy Changes during Glycolysis in Red Blood Cells
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Regulation of GlycolysisRegulation of GlycolysisThe rate of the glycolytic pathway in a cell is controlled by the allosteric enzymes:
Hexokinases I, II, and IIIPFK-1Pyruvate kinase
Allosteric enzymes are sensitive indicators of a cell’s metabolic state regulated locally by effector moleculesThe peptide hormones glucagon and insulin also regulate glycolysis
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Regulation of Glycolysis ContinuedRegulation of Glycolysis ContinuedHigh AMP concentrations activate pyruvate kinaseFructose-2,6-bisphosphate, produced via hormone- induced covalent modification of PFK-2, activates PFK-1Accumulation of fructose-1,6-bisphosphate activates PFK-1 providing a feed-forward mechanism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Figure 8.9 Fructose-2,6-Bisphosphate Level Regulation
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.1: Glycolysis
Section 8.2: Gluconeogenesis
Gluconeogenesis is the formation of new glucose molecules from precursors in the liver
Precursor molecules include lactate, pyruvate, and -keto acids
Gluconeogenesis ReactionsGluconeogenesis ReactionsReverse of glycolysis except the three irreversible reactions
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.2: Gluconeogenesis
Figure 8.10 Carbohydrate Metabolism: Gluconeogenesis and Glycolysis
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.2: Gluconeogenesis
Figure 8.10 Carbohydrate Metabolism: Gluconeogenesis and Glycolysis
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.2: Gluconeogenesis
Gluconeogenesis Reactions ContinuedGluconeogenesis Reactions ContinuedThree bypass reactions:
1. Synthesis of phosphoenolpyruvate (PEP) via the enzymes pyruvate carboxylase and pyruvate carboxykinase2. Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate via the enzyme fructose-1,6-bisphosphatase3. Formation of glucose from glucose-6-phosphate via the liver and kidney-specific enzyme glucose-6-phosphatase
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.2: Gluconeogenesis
Gluconeogenesis Gluconeogenesis SubstratesSubstrates
Three of the most important substrates for gluconeogenesis are:
1. Lactate—released by skeletal muscle from the Cori cycle
After transfer to the liver lactate is converted to pyruvate, then to glucose
2. Glycerol—a product of fat metabolism
Figure 8.11 Cori Cycle From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.2: Gluconeogenesis
Gluconeogenesis Substrates ContinuedGluconeogenesis Substrates Continued3. Alanine—generated from pyruvate in exercising muscle
Alanine is converted to pyruvate and then glucose in the liver
Figure 8.12 The Glucose Alanine Cycle
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.2: Gluconeogenesis
Gluconeogenesis Gluconeogenesis RegulationRegulation
Substrate availability Hormones (e.g., cortisol and insulin)
Figure 8.13 Allosteric Regulation of Glycolysis and Gluconeogenesis
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
+
Section 8.2: Gluconeogenesis
Gluconeogenesis Gluconeogenesis Regulation Regulation ContinuedContinued
Allosteric enzymes (pyruvate carboxylase, pyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase)Figure 8.13 Allosteric
Regulation of Glycolysis and GluconeogenesisFrom McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.3: Pentose Phosphate Pathway
Pentose Pentose Phosphate Phosphate PathwayPathway
Alternate glucose metabolic pathwayProducts are NADPH and ribose-5-phosphateTwo phases: oxidative and nonoxidative
Figure 8.14a The Pentose Phosphate Pathway (oxidative)
Glucose-6-phosphate dehydrogenase
Gluconolactonase
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Pentose Pentose Phosphate Phosphate Pathway: Pathway: OxidativeOxidative
Three reactionsResults in ribulose-5-phosphate and two NADPHNADPH is a reducing agent used in anabolic processes
Figure 8.14a The Pentose Phosphate Pathway (oxidative)
Section 8.3: Pentose Phosphate Pathway
6-phosphogluconate dehydrogenase
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Pentose Phosphate Pentose Phosphate Pathway: NonoxidativePathway: Nonoxidative
Produces important intermediates for nucleotide biosynthesis and glycolysis
Ribose-5-phosphateGlyceraldehyde-3-phosphateFructose-6-phosphate
Figure 8.14b The Pentose Phosphate Pathway (nonoxidative)
Section 8.3: Pentose Phosphate Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Pentose Pentose Phosphate Phosphate PathwayPathway
If the cell requires more NADPH than ribose molecules, products of the nonoxidative phase can be shuttled into glycolysis
Figure 8.15 Carbohydrate Metabolism: Glycolysis and the Phosphate Pathway
Section 8.3: Pentose Phosphate Pathway
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.4: Metabolism of Other Important Sugars
Fructose, mannose, and galactose are also important sugars for vertebrates
Most common sugars found in oligosaccharides besides glucose
Figure 8.16 Carbohydrate Metabolism: Galactose Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.4: Metabolism of Other Important Sugars
Fructose MetabolismFructose MetabolismSecond to glucose in the human dietCan enter the glycolytic pathway in two ways:
Through the liver (multi-enzymatic process) Muscle and adipose tissue (hexokinase)
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.4: Metabolism of Other Important Sugars
Figure 8.16 Carbohydrate Metabolism: Other Important Sugars
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
GlycogenesisGlycogenesisSynthesis of glycogen, the storage form of glucose, occurs after a mealRequires a set of three reactions (1 and 2 are preparatory and 3 is for chain elongation):
1. Synthesis of glucose-1-phosphate (G1P) from glucose-6-phosphate by phosphoglucomutase2. Synthesis of UDP-glucose from G1P by UDP-glucose phosphorylase
Section 8.5: Glycogen Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Glycogenesis ContinuedGlycogenesis Continued3. Synthesis of Glycogen from UDP-glucose requires two enzymes: Glycogen synthase to grow the chain
Figure 8.17a Glycogen Synthesis
Section 8.5: Glycogen Metabolism
Glycogen synthase
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Branching enzyme
Section 8.5: Glycogen Metabolism
Glycogenesis Glycogenesis ContinuedContinued
Branching enzyme amylo-(1,41,6)-glucosyl transferase creates (1,6) linkages for branches
Figure 8.17b Glycogen Synthesis
(1,6) Glycosidic Linkage is formed
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
GlycogenolysisGlycogenolysisGlycogen degradation requires two reactions:
1. Removal of glucose from nonreducing ends (glycogen phosphorylase) within four glucose of a branch point
Section 8.5: Glycogen Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Figure 8.18 Glycogen Degradation
Section 8.5: Glycogen Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.5: Glycogen Metabolism
Figure 8.19 Glycogen Degradation via Debranching Enzyme
Glycogenolysis Glycogenolysis Cont.Cont.
Glycogen degradation requires two reactions:
2. Hydrolysis of the (1,6) glycosidic bonds at branch points by amylo-(1,6)-glucosidase (debranching enzyme)
Amylo-(1,6)-glucosidase
Amylo-(1,6)-glucosidase
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Section 8.5: Glycogen Metabolism
Figure 8.19 Glycogen Degradation via Debranching Enzyme
Amylo-(1,6)-glucosidase
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Regulation of Regulation of Glycogen Glycogen MetabolismMetabolism
Carefully regulated to maintain consistent energy levelsRegulation involves insulin, glucagon, epinephrine, and allosteric effectors
Section 8.5: Glycogen Metabolism
Figure 8.21 Major Factors Affecting Glycogen Metabolism
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Figure 8.21 Major Factors Affecting Glycogen Metabolism
Section 8.5: Glycogen Metabolism
Glucagon activates glycogenolysisInsulin inhibits glycogenolysis and activates glycogenesisEpinephrine release activates glycogenolysis and inhibits glycogenesis
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Biochemistry in Perspective
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Biochemistry in Perspective
The phenomenon, in which glucose represses aerobic metabolism, is the Crabtree effectCrabtree effectRapid production of ethanol has the effect of eliminating microbial competitorsOnce glucose levels are depleted and O2 is available the yeast reabsorbs the ethanol and converts it to acetaldehyde for use as an energy source
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press
Figure 8A Ethanol Metabolism in S. cerevisiae
Biochemistry in Perspective
From McKee and McKee, Biochemistry, 5th Edition, © 2011 Oxford University Press