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

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


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


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


Following the pathway

Carbons H/electrons Phosphate


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.



Figure 8.3 Glycolytic Pathway



Figure 8.3 Glycolytic

Pathway


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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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


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).

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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


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




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


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



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)


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


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



Figure 8.4 Glyceraldehyde-3-Phosphate Dehydrogenase Reaction



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


Space-filling model of yeast phosphoglycerate kinase showing its deeply clefted bilobal structure.

1,3 BPG + ADP →3 PGA + ATP

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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


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)


Figure 17-22 Mechanism of the reaction catalyzed by pyruvate kinase.

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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!!!!!


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



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



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



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



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


Figure 8.9 Fructose-2,6-Bisphosphate Level Regulation



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



Figure 8.10 Carbohydrate Metabolism: Gluconeogenesis and Glycolysis



Figure 8.10 Carbohydrate Metabolism: Gluconeogenesis and Glycolysis



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



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

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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



Gluconeogenesis Gluconeogenesis RegulationRegulation

Substrate availability Hormones (e.g., cortisol and insulin)

Figure 8.13 Allosteric Regulation of Glycolysis and 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


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)


6-phosphogluconate dehydrogenase


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)



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 8.4: 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



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)



Figure 8.16 Carbohydrate Metabolism: Other Important Sugars


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


Glycogenesis ContinuedGlycogenesis Continued3. Synthesis of Glycogen from UDP-glucose requires two enzymes: Glycogen synthase to grow the chain

Figure 8.17a Glycogen Synthesis


Glycogen synthase


Branching enzyme


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


GlycogenolysisGlycogenolysisGlycogen degradation requires two reactions:

1. Removal of glucose from nonreducing ends (glycogen phosphorylase) within four glucose of a branch point



Figure 8.18 Glycogen Degradation




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




Figure 8.19 Glycogen Degradation via Debranching Enzyme



Regulation of Regulation of Glycogen Glycogen MetabolismMetabolism

Carefully regulated to maintain consistent energy levelsRegulation involves insulin, glucagon, epinephrine, and allosteric effectors


Figure 8.21 Major Factors Affecting Glycogen Metabolism


Figure 8.21 Major Factors Affecting Glycogen Metabolism


Glucagon activates glycogenolysisInsulin inhibits glycogenolysis and activates glycogenesisEpinephrine release activates glycogenolysis and inhibits glycogenesis


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


Figure 8A Ethanol Metabolism in S. cerevisiae



Documents

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