Title: Chapter 14 Glycolysis, gluconeogenesis, and the pentose phosphate pathway
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- Chapter 14 Glycolysis, gluconeogenesis, and the
pentose phosphate pathway - Chapter 15 Principles of metabolic regulation
Glucose and glycogen - Chapter 16 The citric acid cycle
- Chapter 19 Oxidative phosphorylation
2To remind you..
3GlycolysisGluconeogenesisPentose phosphate
pathway
4Metabolism Catabolism Anabolism
- Catabolic pathways converge to a few end products
- Anabolic pathways diverge to synthesize many
biomolecules - Some pathways serve both in catabolism and
anabolism, such pathways are amphibolic
5Outline
- 14.1 Glycolysis
- First Phase of Glycolysis (preparatory phase)
- Second Phase of Glycolysis (lysis phase)
- 14.2 Feeder pathways for glycolysis
- 14.3 Fates of pyruvate under anaerobic
conditions Fermentation - 14.4 Glucogenesis
- 14.5 Pentose phosphate pathway of glucose
oxidation
614.1 Glycolysis
Why glucose? 1. Complete oxidation yields 2,840
KJ/mol 2. low cytosolic osmolarity 3. capable of
supplying a huge array of metabolic intermediates
for biosynthetic reactions
7Glycolysis ?? The Embden-Meyerhof (Warburg)
Pathway
1. Consume 2 ATP in 1st phase, produce 4 ATP in
2nd phase net gain 4 - 2 2 ATP 2.
Glucose(6C) ? 23C (pyrurate) 3. Pyruvate is
used in 3 ways
8Alcohol fermentation
Lactate fermentation
O2
9Hexokinase
Phosphohexose isomerase
Phospho- Fructosekinase-1
Aldolase
Triose phosphate isomerase
10Glyceraldehyde 3-phosphate Dehydrogenase Phospho
- Glycerate Kinase Phospho- Glycerate Mutase Eno
lase Pyruvate kinase
11 Glycolytic enzyme ?? (1)
Kinase??ATP??or ?????enzyme (2) Mutase(
??)?Transfer functional group from one position
to another in the same molecule, ?P?C3???C2??
(3) Isomerase??Aldose
Ketose (4) Aldolase??????-Aldose???ketose?? (5)
Enolase???enol form ??
-CC-
alcohol
?Mutase is a subclass of isomerase
12First Phase of Glycolysis
- The first reaction - phosphorylation of glucose
- Hexokinase or glucokinase
- This is a priming reaction - ATP is consumed here
in order to get more later - ATP makes the phosphorylation of glucose
spontaneous
13Reaction 1 Phosphorylation of glucose
spontaneous and irreversible
14Hexokinase is allosterically inhibited by its
reaction product
- Hexokinase
- Km 0.1 mM
- So hexokinase is normally active!
- Inhibited by G6P reversibly
- Glucose, mannose.
- Glucokinase
- Km 10 mM
- only turns on when cell is rich in glucose
- Not inhibited by G6P
- Exist in liver
- Specific for glucose
15Reaction 2 Phosphohexose isomerase
(phosphoglucose isomerase) Reaction 3
Phosphofructose kinase (PFK-1)---- 2nd priming
step
Irreversible
16Reaction 4 Fructose 1,6-bisphosphate Aldolase
In erythrocyte 0.23kJ/mol
17Reaction 5 DHAP?Glyceraldehyde 3-P by
triose-phosphate isomerase
18Glycolysis - Second Phase
- Metabolic energy produces 4 ATP
- Net ATP yield for glycolysis is two ATP
- Second phase involves two very high energy
phosphate intermediates - .
- 1,3 BPG
- Phosphoenolpyruvate
19Reaction 6 Glyceraldehyde-3-phosphate
dehydrogenase
1.Addition of a phosphate group to G3P 2.e-
transfer from G3P to NAD (hydride H-) 3.No ATP
or ADP is involved
20Reaction 7 Phosphoryl transfer from 1,3
bisphosphoglycerate to ADP
Pay off
21Substrate-level phosphorylation
- The formation of ATP by phosphoryl transfer from
a substrate such as 1,3 BPG is referred as a
substrate-level phosphorylation. It involves
soluble proteins and chemical intermediates.
Respiration-linked (oxidative) phosphorylation (??
???) involves membrane-bound enzymes and
transmembrane gradients of protons.
22Reaction 8 Conversion of 3-phosphoglycerate to 2
phosphoglycerate Reaction 9 Dehydration of 3-
to 2- phosphoenolpyruvate by enolase(????)
???
23Reaction 10 Transfer of the phosphoryl group
from phosphoenolpyruvate to ADP
This is referred to as "substrate-level
phosphorylation
Payoff
24SUMMARY 14.1 Glycolysis
- Glycolysis is a near-universal pathway by which a
glucose molecule is oxidized to two molecules of
pyruvate, with energy conserved as ATP and NADH. - 2. All ten glycolytic enzymes are in the cytosol,
and all ten intermediates are phosphorylated
compounds of three or six carbons.
25SUMMARY 14.1 Glycolysis
- 3. In the preparatory phase of glycolysis, ATP is
invested to convert glucose to fructose
1,6-bisphosphate. The bond between C-3 and C-4 is
then broken to yield two molecules of triose
phosphate.
26SUMMARY 14.1 Glycolysis
- 4. In the payoff phase, each of the two molecules
of glyceraldehyde 3-phosphate derived from
glucose undergoes oxidation at C-1 the energy of
this oxidation reaction is conserved in the
formation of one NADH and two ATP per triose
phosphate oxidized. The net equation for the
overall process is
Glucose2NAD2ADP2Pi ? 2pyruvate2NADH2H
2ATP2H2O
27SUMMARY 14.1 Glycolysis
- 5. Glycolysis is tightly regulated in
coordination with other energy-yielding pathways
to assure a steady supply of ATP. Hexokinase,
PFK-1, and pyruvate kinase are all subject to
allosteric regulation that controls the flow of
carbon through the pathway and maintains constant
levels of metabolic intermediates.
2814.2 Feeder pathways for glycolysis
Glycogen and starch Maltose, lactose, trehalose,
sucrose Fructose, mannose, galactose
29(No Transcript)
30Fig. 14-11Conversion of galactose to glucose
1-phosphate
31Galactosemia
Mutation Symptoms
Galactokinase- deficient (high galactose conc. is found in the blood and urine) Cataracts cause by deposition of galactose metabolite galactitol in the lens
Transferase-deficient Poor growth of children, speech abnormality, mental deficiency, and liver damage even when galactose is withheld from the diet (more severe!)
Epimerase-deficient Same as above but is less severe when dietary galactose is carefully controlled.
32Catabolism of glycogen
- Cellular glycogen
- Dietary glycogen
33Catabolism of cellular glycogen
Glycogen phosphorylase catalyzes an attack by Pi
on the (a1?4) glycosidic linkage that joins the
last two glucose residues at a nonreducing end,
generating glucose 1-phosphate and a polymer one
glucose unit shorter.
14-10
34(No Transcript)
35- Glucose 1-phosphate produced by glycogen
phosphorylase is converted to glucose 6-phosphate
by phosphoglucomutase, which catalyzes the
reversible reaction
Glycolysis Pentose phosphate pathway
36Dietary polysaccharides and disaccharides are
hydrolyzed to monosaccharides by various enzymes
a-amylase Salivary---digest to short
polysaccharide fragment or oligosaccharides Pa
ncrease---digest to disaccharides (maltose),
trisaccharides (maltotrioses) and dextrins
37Intestine secrects enzymes to digest dextrin and
disaccharides
38Catabolism of dietary and cellular glycogen
Cellular Dietary (saliva) Dietary (pancrease)
Enzyme Glycogen phosphorylase and debranching enzyme a-amylases a-amylases
End product glucose-1-P Short polysaccharides fragment or oligosaccharides Di-saccharides and dextrins
39Lactose intolerance?????
Lack lactase Diarrhea and discomfort
Undigested lactose and its metabolites increase
the osmolarity of the intestinal contents,
favoring the retention of water in the intestine.
40Anaerobic Metabolism and tooth decay
(??)
Sucrose ?Plaque (polysaccharide) Products of
anaerobic glycolysis carried out by bacteria
?lactate and pyruvate ?tooth decay
41SUMMARY 14.2 Feeder Pathways for Glycolysis
- 1. Glycogen and starch, polymeric storage forms
of glucose, enter glycolysis in a two-step
process. Phosphorolytic cleavage of a glucose
residue from an end of the polymer, forming
glucose-1 phosphate, is catalyzed by glycogen
phosphorylase or starch phosphorylase.
Phosphoglucomutase then converts the glucose
1-phosphate to glucose 6-phosphate, which can
enter glycolysis.
42SUMMARY 14.2 Feeder Pathways for Glycolysis
- 2. Ingested polysaccharides and disaccharides are
converted to monosaccharides by intestinal
hydrolytic enzymes, and the monosaccharides then
enter intestinal cells and are transported to the
liver or other tissues.
43SUMMARY 14.2 Feeder Pathways for Glycolysis
- 3. A variety of D-hexoses, including fructose,
galactose, and mannose, can be funneled into
glycolysis. Each is phosphorylated and converted
to either glucose 6-phosphate or fructose
6-phosphate.
44SUMMARY 14.2 Feeder Pathways for Glycolysis
- 4. Conversion of galactose 1-phosphate to glucose
1-phosphate involves two nucleotide derivatives
UDP-galactose and UDP-glucose. Genetic defects in
any of the three enzymes that catalyze conversion
of galactose to glucose 1-phosphate result in
galactosemias of varying severity.
4514.3 Fates of Pyruvate under Anaerobic
Conditions Fermentation
O2
46Anaerobic pathways
??
yeast
muscle
Fetal Alcohol syndrome
47Thiamine Pyrophosphate Carries Active
Acetaldehyde Groups
- Thiamine pyrophosphate (TPP), a coenzyme derived
from vitamin B1. - Lack of vitamin B1 in the human diet leads to the
condition known as beriberi. - Thiamine pyrophosphate plays an important role in
the cleavage of bonds adjacent to a carbonyl
group, such as the decarboxylation of -keto
acids, and in chemical rearrangements in which an
activated acetaldehyde group is transferred from
one carbon atom to another
48Thiamine Pyrophosphate Carries Active
Acetaldehyde Groups
49Replenishment of NAD
1. under anaerobic condition Lactic acid
fermentation 2. under anaerobic condition
Alcoholic fermentation 3. under aerobic
condition Mitochondria oxidation of each
NADH to yield 2.5 (1.5) ATP
50Oxygen debt
Cori cycle
51SUMMARY 14.3 Fates of Pyruvate under anaerobic
conditions Fermentation
- 1. The NADH formed in glycolysis must be recycled
to regenerate NAD, which is required as an
electron acceptor in the first step of the payoff
phase. Under aerobic conditions, electrons pass
from NADH to O2 in mitochondrial respiration.
52SUMMARY 14.3 Fates of Pyruvate under anaerobic
conditions Fermentation
- 2. Under anaerobic or hypoxic conditions, many
organisms regenerate NAD by transferring
electrons from NADH to pyruvate, forming lactate.
Other organisms, such as yeast, regenerate NAD by
reducing pyruvate to ethanol and CO2.
53Regulation of carbohydrate catabolism
Cellular energy level and intermediate
concentration Glycogen phosphorylase,
hexokinase, phosphofructokinase, pyruvate kinase
1. At branching point 2. Large and negative free
energy
54Regulation of PFK-1
FIGURE 1518 Phosphofructokinase-1 (PFK-1) and
its regulation.
55??ATP?PFK-1 ?????inhibitor?
?ATP?,?ADP??Low energy charge?glycolysis ?ATP
?,?ADP??High energy charge?glycolysis
-
ATP??,ADP??allosteric site ?activator,
ATP??active site? substrate ATP??,ATP??alloster
ic site ? inhibitor
FIGURE 626 Subunit interactions in an allosteric
enzyme, and interactions with inhibitors and
activators.
5614.4 gluconeogenesis
- Synthesis of "new glucose" from common
metabolites - Humans consume 160 g of glucose per day
- 75 of that is in the brain
- Body fluids contain only 20 g of glucose
- Glycogen stores yield 180-200 g of glucose
- So the body must be able to make its own glucose
57Fig. 6-29
58- Substrates pyruvate, lactate, glycerol, amino
acids (except Lys and Leu) - Active in liver and kidney supply glucose to
brain and muscle - Seven of ten glycolytic steps in reverse
- Three steps not reversal of glycolysis for two
reasons - energetics
- regulation
59- Occurs mainly in liver and kidneys
- Not the mere reversal of glycolysis for 2
reasons - Energetics must change to make gluconeogenesis
favorable (delta G of glycolysis -74 kJ/mol - Reciprocal regulation must turn one on and the
other off - this requires something new!
60- Something Borrowed, Something New
- Seven steps of glycolysis are retained
- Steps 2 and 4-9
- Three steps are replaced
- Steps 1, 3, and 10 (the regulated steps!)
- The new reactions provide for a spontaneous
pathway (?G negative in the direction of sugar
synthesis), and they provide new mechanisms of
regulation
61Glycolysis
Gluconeogenesis
62Three unique gluconeogenesis steps First bypass
- Pyruvate to PEP
- Pyruvate carboxylase converts pyruvate to
oxaloacetate - Biotin-dependent enzyme
- ATP bicarbonate to carbonylphosphate
intermediate to carboxybiotin intermediate - Allosteric activation by acetyl CoA
- Localized in mitochondria
- Oxaloacetate to malate and back (if PEP
carboxykinase is localized in cytosol) - PEP carboxykinase converts oxaloacetate to PEP
- Decarboxylation drives reactions
- GTP or ATP supplies Pi and drives reaction
- Localized in mitochondria or in cytosol
63Synthesis of PEP from pyruvate
Overall reaction Pyruvate ATP GTP HCO3-
? phosphenolpyruvate ADP GDP Pi CO2
64PEP carboxykinase- Rabbit liver
mitochondria Rat liver cytosolic Human liver
both
65Three unique gluconeogenesis steps Second bypass
- Fructose-1,6 bisphosphate to fructose 6 phosphate
- Hydrolysis
- Fructose-1,6-bisphosphatase
- Acetyl CoA or citrate stimulates
- fructose-2,6-bisphosphate inhibits
- AMP inhibits
66Three unique gluconeogenesis steps Third bypass
- Glucose-6-phosphate to glucose
- Hydrolysis
- Glucose-6-phosphatase
- Release product into ER lumen
- Enzyme expressed in liver and kidney (not in
muscle or brain) - High Km for G6P substrate-level control
67- FIGURE 156 Hydrolysis of glucose 6-phosphate by
glucose 6-phosphatase of the ER. The catalytic
site of glucose 6-phosphatase faces the lumen of
the ER. A glucose 6-phosphate (G6P) transporter
(T1) carries the substrate from the cytosol to
the lumen, and the products glucose and Pi pass
to the cytosol on specific transporters (T2 and
T3). Glucose leaves the cell via the GLUT2
transporter in the plasma membrane.
68Pentose phosphate pathway of glucose oxidation
6914.5 Pentose Phosphate Pathway
- aka phosphogluconate pathway, hexose
monophosphate shunt - Provides NADPH for biosynthesis
- Produces ribose-5-P
- Two oxidative processes followed by five
non-oxidative steps - Operates mostly in cytoplasm of liver and adipose
cells - NADPH is used in cytosol for fatty acid synthesis
70Oxidative Steps
- of the Pentose Phosphate Pathway
- Glucose-6-P Dehydrogenase
- Irreversible 1st step - highly regulated!
- Gluconolactonase
- Uncatalyzed reaction happens too
- 6-Phosphogluconate Dehydrogenase
- An oxidative decarboxylation (in that order!)
- Phosphopentose isomerase
- converts ketose to aldose
71(No Transcript)
72The Nonoxidative Steps
- Phosphopentose Epimerase
- epimerizes at C-3
- Transketolase (TPP-dependent)
- transfer of two-carbon units
- Transaldolase (Schiff base mechanism)
- transfers a three-carbon unit
73 ???
7404172005
75(No Transcript)
76Clinical aspects of PPP --???
- G6P dehydrogenase deficiency
- G-S-S-G NADPH H ? 2G-SH NADP (by
glutathione reductase) - 2G-SH H2O2 ? G-S-S-G 2H2O (by gluthathione
peroxidase) - increase rate of oxidation of hemoglobin
- Susceptible to oxidants (antimalarial primaquine,
aspirin, or sulfonamides), or fava beans
77SUMMARY 14.5 Pentose Phosphate Pathway
ofGlucose Oxidation
- 1. NADPH provides reducing power for biosynthetic
reactions, and ribose 5-phosphate is a precursor
for nucleotide and nucleic acid synthesis.
Rapidly growing tissues and tissues carrying out
active biosynthesis of fatty acids, cholesterol,
or steroid hormones send more glucose 6-phosphate
through the pentose phosphate pathway than do
tissues with less demand
78SUMMARY 14.5 Pentose Phosphate Pathway
ofGlucose Oxidation
- 2. The first phase of the pentose phosphate
pathway consists of two oxidations that convert
glucose 6-phosphate to ribulose 5-phosphate and
reduce NADP to NADPH. The second phase comprises
nonoxidative steps that convert pentose
phosphates to glucose 6-phosphate, which begins
the cycle again.
79SUMMARY 14.5 Pentose Phosphate Pathway
ofGlucose Oxidation
- 3. Entry of glucose 6-phosphate either into
glycolysis or into the pentose phosphate pathway
is largely determined by the relative
concentrations of NADP and NADPH.