Lehninger Principles of Biochemistry 5/e

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BCHM221 Metabolism (3)
3. Hexose Catabolism (Ch 14)
A. Introduction and Significance B. Glycolytic
Sowing C. Glycolytic Reeping D. Net
Glycolysis E. Pyruvates Fate F. Pentose
Phosphate Pathway G. Regulation
Positron emission tomography Injection with
18-fluorodeoxyglucose (short-lived radioactive
element fluorine-18 and glucose) to track
glucose metabolism and therefore brain activity
glucose is virtually the only energy source in
the brain
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3A. Hexose Catabolism - Significance

• Diabetes (p.539, 542 p.929-930)
• related to blood glucose concentration
• altered ability to regulate glucose metabolism
• normally when glucose high, insulin is
  released
• some diabetics lack the ability to secrete
  insulin

• Cancer (p.540-541)
• glucose uptake/glycolysis 10x faster in cancer
  cells
• some glycolytic enzymes are overproduced

• Lactose Intolerance (p.545)
• defective lactase (hydrolyses lactose to D-Gal
  D-Glc)
• In small intestine lactose not properly
  digested absorbed
• Passes into large intestine lactose converted
  into toxic products ? abdominal cramps/diarrhea

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BOX 14-2 FIG 3
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3A. Introduction
Mouth salivary amylase hydrolyzes glycosidic
bonds Stomach low pH prevents further starch
digestion by amylase Small Intestine (SI)
Pancreas releases pancreatic amylase to SI to
further hydrolyze glycosidic bonds ? further
hydrolysis of disaccharides ? Sucrase, lactase,
maltase, dextrinase, trahalase, enzymes on brush
border ? Active facilitated diffusion across
enterocyte into epithelial cells and into the
blood for transport to tissue Blood Transport
(portal vein to liver)
1. Release monosaccharides into
bloodstream 2. Deliver sugars for storage as
glycogen (liver, muscle tissue
hepatocytes, myocytes) or fat (adipose tissue -
adipocytes) 3. Excrete sugars into urine (XS in
diabetes)
Lehninger, pp 534-535, 562
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3A. Introduction
Glucose can act as either 1. An energy source
2. A metabolic precursor (a.a., n.a., f.a.
) Fate of Glucose?
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Three possible catabolic fates of the pyruvate
formed in glycolysis. Pyruvate also serves as a
precursor in many anabolic reactions, not shown
here.
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3B. Hexose Catabolism - Glycolytic Sowing
note the cell is spending 2 ATP
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3B. Glycolytic Sowing
Sowing the Seeds with ATP 1. Phosphorylation of
Glucose by Hexokinase
Irreversible under cellular conditions
What might be the mechanism? Biochemical Logic?
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Chemical Logic
Each of the 9 glycolytic intermediates between
glucose and pyruvate is phosphorylated, why 1.
retention/permeability 2. conservation of energy
released by ATP 3. active sites of enzymes
(binding energy specificity) ?Most glycolytic
enzymes require Mg2 for activity See p. 531
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• Conversion of Glucose6-P to Fructose 6-P
• by Phosphoglucose Isomerase

Do you think this reaction would be reversible?
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2. Phosphoglucose Isomerase Mechanism see
Animation
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3. Phosphorylation of F-6-P to Fructose
1,6-Bisphosphate by Phosphofructokinase-1
(PFK-1)
Almost irreversible under cellular conditions
Is this reaction spontaneous or
non-spontaneous? For what function is this
enzyme a good candidate?
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4. Cleavage of Fructose 1,6-bisphosphate by
Aldolase a reversible aldol reaction
In the cell, this reaction can proceed in either
direction. Why?
The quick removal of the products during
glycolysis drives the reaction.
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• Fructose-1,6-bisphosphate aldolase mechanism

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• Interconversion of the Triose Phosphates
• by Triose phosphate isomerase
  (TIM)
• Only glyceraldehyde-3-phosphate can be carried
  through the rest of glycolysis
• Two for every
• one glucose

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Note change in carbon numbering, once written as
G-3-P
Following Triose Phosphate Isomerase Reaction
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3C. Hexose Catabolism - Glycolytic Reeping
net gain of 2 ATP per glucose
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3C. Glycolytic Reeping
6. Oxidation of Glyceraldehyde-3-phosphate to
1,3- Bisphosphoglycerate Glyceraldehyde
3-phosphate dehydrogenase produces the acyl
phosphate counterpart (redox)
Production of acyl phosphate (?G?hydrolysis
?49.3 kJ/mol) and NADH (e- carrier)
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6. Glyceraldehyde-3-phosphate dehydrogenase
mechanism
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7. (High-energy) Phosphoryl transfer from
1,3-Bisphosphoglycerate to ADP by
Phosphoglycerate kinase
?G?T ?49.3 kJ/mol (? 30.5 kJ/mol)
? 18.8 kJ/mol
These two steps (6, 7) are energy coupled Why?
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Glyceraldehyde 3-phosphate Dehydrogenase 3-Phosp
hoglycerate kinase

• Intermediates Can Be Channeled
• Many soluble enzymes are
• highly organized complexes
• reversible protein association
• allows substrate channeling

Figure 15-8, Lehninger 3rd Ed.
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8. Conversion of 3-Phosphoglycerate to
2-Phosphoglycerate by Phosphoglycerate
mutase This enzyme has an interesting
mechanism
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Mechanism of Phosphoglycerate Mutase (animation)
Catalytic amounts required, at the cost of ATP
Phosphorylation of C-2
Dephosphorylation of C-3
Figure 14-8
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(No Transcript)
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9. Dehydration of 2-Phosphoglycerate to
Phosphoenolpyruvate by Enolase Red
istribution of energy from loss of the water ?
great increase in the standard free energy of
phosphate hydrolysis (?17.5 ? ? 61.9 kJ/mol) See
Fig. 13-13, Table 13-6
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10. Transfer of the Phosphoryl group from PEP to
ADP (requires K and either Mg2 or
Mn2)
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3D. Hexose Catabolism - Glycolysis
Overall Reaction
1. Degradation of the glucose carbon skeleton ?
pyruvate
2. Phosphorylation of ADP ? ATP by high energy
intermediates
3. Transfer of hydride electrons to NAD ? NADH
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3B. Hexose Catabolism - Glycolytic Sowing
note the cell is spending 2 ATP
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3C. Hexose Catabolism - Glycolytic Reeping
net gain of 2 ATP per glucose
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3D. Hexose Catabolism - Glycolysis
Overall Reaction Glucose 2ATP 2NAD 4ADP
2Pi ? 2 pyruvate 2ADP 2NADH 2H 4ATP
2H2O Cancel Terms Glucose 2NAD 2ADP 2Pi ?
2 Pyruvate 2NADH 2H 2ATP
2H2O Ultimately 4 e- from Glycolysis Land in
the Respiratory Chain and Help Synthesize ATP
2NADH 2H O2 ? 2NAD 2H2O The carbons end
up as pyruvate.
Lehninger p.531, 538
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• Exergonic Conversion of Glucose to Pyruvate
• ?G?(kJ/mol)
• Glucose 2NAD ? 2Pyruvate 2NADH 2H
  ? 146
• 2 ADP 2 Pi ? 2 ATP 2 H2O
  61
• Overall ? 85 kJ/mol
• Glycolysis is essentially irreversible, driven
  by completion
• by a large net decrease in free energy)
• ET available to glucose is 2840 kJ/mol (glucose
  ? CO2H2O)
• Glycolysis releases only a small fraction of the
  total available energy of glucose. Where is the
  remainder of the energy stored?

• Most of the remaining energy is stored in
  pyruvate
• TCA cycle oxidative phosphorylation

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Where Does Glycolysis Fit Into the Big Picture?
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3E. Hexose Catabolism Pyruvates Fate
c)
b)
a)
Figure 14-3
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3E. Pyruvates Fate Lehninger, p. 546-551
a) Aerobic conditions pyruvate CoA ?
acetylCoA CO2
NADH produced ? electrons go to
O2 in respiration
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b) Lactic Acid Fermentation (Anaerobic) Thes
e reactions dominate when there is
insufficient O2 to oxidize pyruvate (TCA)
and to regenerate NAD by oxidative
phosphorylation This is a
balanced process (What do
athletes and alligators have
in common? Box 14-2, p.
548)

½ Glucose
liver
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c) Alcohol (Ethanol) Fermentation
(Anaerobic)
This is the case for yeast, where glycolysis
still converts glucose to pyruvate Again,
NAD is regenerated (Box 14-3, p. 549)
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What Happens When We Drink Alcoholic Beverages?
Alcohol dehydrogenase Ethanol NAD ?
acetaldehyde NADH  Note Reverse reaction of
2nd step of alcohol fermentation Aldehyde
dehydrogenase acetaldehyde NAD ? acetic acid
NADH

• Acetaldehyde is extremely toxic
• reactive with amino groups and may interact with
  proteins.
• competes for the plasma carrier of pyridoxal
  (vitamin B6)
• ? vitamin deficiency

What if there is a mutation in your aldehyde
dehydrogenase? What if alcohol dehydrogenase
works too quickly?
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3F. Pentose Phosphate Pathway

• What is the Role of this Pathway?
• Production of specialized products for
  particular cells
• a) NADPH is for fatty acid and steroid synthesis
• (reductive biosynthesis)
• b) ribose-5-phosphate for nucleic acid synthesis
• in rapidly dividing cells (bone
  marrow, mucosa, tumour)
• Overall Oxidative Reactions
• glucose-6-phosphate 2NADP H2O ?
• ribose-5-phosphate CO2 2NADPH 2H
• For tissues that only require NADP, pentose
  phosphates
• are recycled back to glucose-6-phosphate,
  producing more
• NADPH (liver, adipose, adrenal/mammary glands,
  gonads...)
• The non-oxidative pathway inter-converts
  hexoses/pentoses

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See Box14-4 for G6PDH deficiency and Oxidative
damage
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3F. Pentose Phosphate Pathway
Note the end product and the production of
reducing equivalents



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Nonoxidative reactions of the pentose phosphate
pathway (See also Ch 4 5 of Cornish-Bowden)
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Opposing pathways of glycolysis and
gluconeogenesis in rat liver Fig 14-16
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3. What is Regulation? (from Handout 1)
Q What is the result if both catabolic and
anabolic enzyme reactions happen at the same
time? The cell prevents the waste of energy
through regulation 1. Concentration of
substrate, intermediate, enzyme and regulator -
can control metabolic rate by mass action and
enzymatic rate 2. Reciprocal regulation (one
off, other on) where at least one favorable
(irreversible) step of anabolism and catabolism
are catalyzed by a different enzyme ? sites of
regulation 3. Compartmentalization the cell can
keep the concentration of intermediates and
enzymes at different levels in each compartment
(i.e. cytosol versus mitochondria)
A The net reaction would be zero.
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4. Mechanisms of Regulation (from Handout 1)

• Levels of Regulation in Order of Immediacy
• 1. Substrate availability if the intracellular
  S lt Km, the enzyme is below Vmax and rate is
  determined by the S
• 2. Allosteric (feedback) regulation signal of
  cells internal state by a biomolecule to turn
  on/off an enzyme (?s-ms)
• 3. Second messenger signaling metabolism of
  entire being is regulated and integrated by
  growth factors and hormones that act from outside
  the cell (ms ? hrs.)
• - modify activity of existing enzymes, or enzyme
  synthesis/degradation

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3G. Glucose Regulation

• Why Do We Require Regulation? (p. 570-571)
• 1. Cell requires constant ATP and
  precursors for biosynthesis (may have to alter
  fuel source)
• 2. Anabolism and catabolism use same enzymes
  reversibly in addition to the reciprocally
  regulated enzymes
• - this prevents futile cycling
• Where are Regulatory Enzymes Found?
• Enzymes that catalyze irreversible reactions
• Often at metabolic branch points
• Which Enzymes are good candidates for glycolytic
  regulation?

hexokinase, phosphofrucokinase-1, pyruvate
kinase AND glycogen phosphorylase (later)
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3G. Glucose Regulation - Glycolysis


1. Substrate-limited (at or near
equilibrium) 2. Enzyme-limited (far from
equilibrium) - targets for allosteric
regulation

ADP,







Acetyl-CoA, long chain fatty acids
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3G. Glucose Regulation (p.582-594)

• a) Co-ordinated and Allosteric Regulation
• gluconeogenesis (glucose synthesis) and
  glycolysis share many enzymes (equilibrium
  reactions)
• the regulated enzymes are in co-ordination
  (reciprocal)
• Example The hormone glucagon is released by the
  pancreas when blood sugar levels are low. What
  is its likely effect?

-ve
ve
Figure 15-16c
Glucagon reduces the rate of glycolysis and
increases the rate of gluconeogenesis (?
Fructose-2,6-bisphosphate)
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3G. Glucose Regulation

• b) Allosteric Regulation of Hexokinases
• Muscle
• hexokinase tends to consume glucose (muscle
  activity)
• works at maximal activity with normal blood
  glucose levels
• allosteric inhibition by product if it exceeds
  normal levels
• Liver
• produces/distributes glucose
• has an isozyme for hexokinase hexokinase D
• hexokinase D is regulated by blood glucose
  since it has a higher Km than normal blood
  glucose
• hexokinase D is inhibited by fructose-6-phosphate
  
• (since G6P ? F6P)

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3G. Glucose Regulation

• b) Allosteric Regulation of PFK-1
• PFK-1 commits the cell to glycolysis (alternate
  pathways)
• Regulatory sites
• AMP ADP combined can overcome the inhibition
  by ATP
• Citrate (CAC/TCA) can enhance the inhibition by
  ATP as a general intracellular signal that the
  cell is meeting its needs
• CAC citric acid cycle TCA tricarboxylic
  acid cycle Krebs cycle

i) activators (AMP, ADP, Fructose-2,6-bisphosphat
e) ii) inhibitors (ATP)
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3G. Glucose Regulation

• b) Allosteric Regulation of Pyruvate kinase
• An allosteric inhibitor binds to the enzyme,
  changing its
• shape, often changing substrate affinity
• High ATP inhibits pyruvate kinase by reducing
  substrate (PEP) affinity for the enzyme
• Acetyl-CoA and long chain fatty acids (fuels for
  CAC) indicate sufficient energy source from the
  breakdown of protein and fats, therefore
  inhibiting
• the enzyme pyruvate kinase
• More to come when we discuss gluconeogenesis!