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Chapter 4
Carbohydrate Metabolism

Glucose transport

Metabolism…..
Time to put some LIFE into the subject
What is Life?
What are the properties of life?
Movement
Turnover of components
Reproduction of one’s kind
Energy Transformations
Chemical Energy is the Dominant Energy
Form in a Living System
Metabolism:
The process by which a living system
derives or uses energy through
chemical change
Energy
A
B
Anabolism: Synthesis. Putting free
energy to work
Endergonic
Catabolism: Degradation. Deriving free energy
Exergonic
ATP: Energy currency. The standard that is used
to gauge all energy compounds
The 5 Rules of Energy Metabolism
Rule: Living system are able to conserve energy
Rule: Heat is wasted energy
Heat is energy that cannot be conserved
Rule: Living systems will do their utmost to prevent
lost of free energy as heat
Rule: Exergonic biochemical transformations channel
a large part of the free energy into chemical
bonds of the product.
Rule: Catabolic reactions drive anabolic reactions
Anaerobic
Aerobic
Oxidized
cofactors
(recycle back
Reduced
cofactors
(drive Ox Phos)
The Glycolysis Pathway






Major anaerobic pathway in all cells
NAD+ is the major oxidant
Requires PO4
Generates 2 ATP’s per glucose oxidized
End product is lactate (mammals) or
ethanol (yeast)
Connects with Krebs cycle via pyruvate
Glycolysis
a-D-Glucose
CH2OH
O
OH
Hexokinase
Glucose-6-Phosphate
ATP
CH2OPO3
O
OH
Phosphoglucoisomerase
Fructose-6-Phosphate
CH2OPO3
O CH2OH
OH
CH2OPO3
O CH2OH
Fructose-6-Phosphate
OH
Phosphofructokinase-I
Fructose 1,6-Bisphosphate
ATP
CH2OPO3
O CH2OPO3
OH
CHO
CH2OPO3
C=O
Aldolase
CH2OH
Dihydroxyacetone-Phosphate
H-C-OH
CH2OPO3
Glyceraldehyde-3Phosphate
ALDOLASE
Fructose 1,6bisphosphate
CH2OP
C=O
HO-C-H
..
H-C-OH
C-OH
CH2OP
CH2OP
C=O
HO-C-H
..
H
+
CHO
H-C-OH
Dihydroxy
Acetone
Phosphate
(DHAP)
C-OH
CH2OP
Glyceraldehyde-3-P
Triose Stage
Dihydroxy
acetone
phosphate
(DHAP)
CH2OPO3
CHO
Glyceraldehyde
3-phosphate
H-C-OH
C=O
CH2OH
CH2OPO3
Triose phosphate isomerase
CHO
PO4
H-C-OH
CH2OPO3 NAD+
Glyceraldehyde-3-P
Dehydrogenase
O
C ~OPO3
COO
H-C-OH
ADP
H-C-OH
ATP CH OPO
CH2OPO3 Phosphoglycerate 2
3
NADH
+ H+ Glycerate 1,3-Kinase
Glycerate 3bisphosphate
phosphate
COO
COO
H-C-OH
H-C-OPO3
CH2OPO3
3-PGA
-H2O
C~OPO3 PEP
CH2OH
2-PGA
Phosphoglyceromutase
COO
CH2
ADP
Enolase
Pyruvate kinase
ATP
Back to Glycolysis
COO NADH + H+ COO
C=O
HO-C-H
CH3
L-lactate
NAD+
CH3
Pyruvate
Regulation of Glycolysis


6-phosphofructokinase-1
Allosteric enzyme
negative allosteric effectors
Citrate , ATP
Positive allosteric effectors
AMP, fructose1,6-bisphosphate, fructose2,6-bisphosphate

Changes in energy state of the cell (ATP
and AMP)
Regulation of Glycolysis
fig.6-4

Regulation of Glycolysis


Pyruvate Kinase
Allosteric enzyme
Isoenzyme in liver
Hexokinase
Different isoenzymes
Hexokinase IV
activated by fructose 1,6
bisphosphate
glucose 6-phosphate is an
allosteric inhibitor
inhibited by alanine
promote biosynthesis
Inhibited by ATP.


Regulated by
phosphorylation and
dephosphorylation


The Significance of Glycolysis


Glycolysis is the emergency energyyielding pathway
Main way to produce ATP in some tissues
red blood cells, retina, testis, skin, medulla of kidney

In clinical practice
Aerobic Oxidation of Glucose

1.
2.
3.
Glucose oxidation
Oxidation of glucose to pyruvate in
cytosol
Oxidation of pyruvate to acetylCoA in
mitochondria
Tricarboxylic acid cycle and oxidative
phosphorylation
Mechanism of pyruvate dehydrogenase complex
Fig.6-6

O2
O2
O2
O2
O2
METABOLISM OF
PYRUVATE
O2
O2
O2
Its time to get aerobic
Pyruvate Structure
Look for one NAD+ for each
glyceraldehyde-3-PO4 oxidized to pyruvate
Carboxyl group (acid)
Ketone group
(carbonyl)
a ketoacid
COO
-2
0
-OH
C=O
CH3
Methyl group
–2
+2
Net = –02
Oxidation of Carbon
–2
CH3
–2
C-OH
+2
H-C-OH
P
CH2OH
Glyceraldehyde
3-Phosphate
–2
–2
O
C
+2
C=O
O
+2
C=O
+2
CHO
O
Decarboxylation Reactions
:C
O
O
O
+C
C
O
O
O
Two Types: non-oxidative and oxidative
Non-oxidative
CO2
H3C-C:COOO
NAD+
NADH
Oxidative
H3C-C:
O
H+ H3C-C:H
O
No change in
oxidation state
of carbonyl C
CO2
H3C-C+
O
H2 O
H3C-C-OH
O
Oxidized
carbonyl C
The Energy Story of Glycolysis
Overall ANAEROBIC (no O2)
Glucose + 2ADP + 2Pi
Yeast
Glucose + 2ADP + 2Pi
2 Lactate + 2ATP + 2H2O
2 Ethanol + 2CO2 +2ATP + 2H2O
Overall AEROBIC
Glucose + 2ADP + 2Pi + 2NAD+
2 Pyruvate + 2ATP + 2NADH + 2H+ + 2H2O
5 ATPs
Aerobic
C6H12O6 + 6O2
6CO2 + 6H2O
Anaerobic
CHO
H-C-OH
OH-C-H
H-C-OH
H-C-OH
CH2OH
D-Glucose
Go’= -2,840 kJ/mol
COOC=O
CH3
COO-
146
100 =
2,840
Energy used
C=O
CH3
2 Pyruvates
C6H12O6
2 C3H4O3
Glucose
2 Pyruvate
Go’= -146 kJ/mol
5.2%
Anaerobic
Lactate
Glycolysis
Glucose
Galactose
Fructose
Mannose
pyruvate
Fatty Acids
Acetyl-Coenzyme A
Aerobic
Amino Acids
Krebs Complex
Cycle
Pyruvate dehydrogenase
1Pyruvate
FADH2 dehydrogenase
Dihydrolipoyl transacetylase
Thiamin pyrophosphate
3 NADH
Lipoic acid
Dihydrolipoyl dehydrogenase
Oxidative
Coenzyme A
phosphorylation
FAD
NAD
O2
H2
O
+
CH2 N
N
CH3
N
CH2 CH2 O P O P O
CH3
Vitamin B-1
NH2
O
O
..
O
S
Carbanion
CH3 C : COO
+ CO2
O
Pyruvate
Thiamin pyrophosphate
COENZYME A
Acetyl-Coenzyme A
NH2
N
N
N
O O N
O
H
CH
3
HO
H
HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-P-O CH2
O
O
O
HO CH3
Pantothenate
O OH
B-vitamin
O P O
CH3C
Acetyl Group
O
Thioester bond
O
Adenosine-3’phosphate
Dihydrolipoate
Long hydrocarbon chain
CH2
CH2
CH CH2 CH2 CH2 CH2 COO
HS
SH
6,8 Dithiooctonoate
(Reduced, gained 2 electrons)
CH2
CH2
S
CH CH2 CH2 CH2 CH2 COO
S
Disulfide bond
(Oxidized, lost 2 electrons)
Pyruvate Dehydrogenase Complex
Acetyl-CoA
..
HS-CoA
C-CH3
O
..
CH3-C
O
..
CH3-C
NAD+
TPP
E1
E2
E3
FAD H2
..
NADH ..
OH
Pyruvate Dehydrogenase
Dihydrolipoyl
Transacetylase
Dihydrolipoyl
dehydrogenase
Tricarboxylic Acid Cycle

All Mean the Same
Features
Acetyl-CoA enters forming citrate
Citrate is oxidized and decarboxylated
3 NADH, 1 FADH2, and 1 GTP are formed
Oxaloacetate returns to form citrate
CO2
6
Citrate
CO2
5
a-ketoglutarate
cis-Aconotate
Isocitrate
4
Succinyl-CoA
Succinate
Cycle
Intermediates
Fumarate
Malate
Oxaloacetate
More Reduced
More Oxidized
CH3C ~ S-CoA
CARBON BALANCE
O
4 Oxaloacetate
4 Malate
Citrate 6
2 carbons in
2 carbons out
4 Fumarate
Isocitrate 6
CO2
a-ketoglutarate 5
CO2
4 Succinate
Succinyl-CoA 4
Reactions of Acetyl-CoA
Split here
H
H C-C~S-CoA
..
H
O
CH3-C~S-CoA
O
Acetylations
or
Acylations
HS-CoA
S-CoA
O
C=O COO
H2C
C-OH
COO
C=O
CH2
COO
OAA
Carbanion
Citrate Synthase
(a lyase)
CH2
COO
Citroyl-CoA
Citrate
CH3-C~SCoA
O
Citrate Synthase
COO-
COOC=O
CH2
HS-CoA
-OOC-CH
2- C-OH
CH2 COO-
COO-
Oxaloacetate
(OAA)
CH2COOHO-C-COO-
Acetyl-CoA
CH2COO-
Citric Acid or Citrate
Isocitrate Formation
CH2COO-H2O
HO-C-COOH-C-COOH
Citrate
CH2COOC-COO-
+H2O
H C-COO-
cis-Aconitate
Aconitase
CH2COOH-C-COO-
HO-C-COOH
Isocitrate
CH2COOH-C-COO-
CO2
HO-C-COOH
NAD+ NADH + H+
Isocitrate
COOCH2
CH2
C=O
COO-
a-Ketoglutarate
Isocitrate Dehydrogenase
COOCH2
CH2
C=O
NAD+
FAD
Lipoic acid
HS-CoA
TPP
COO-
a-Ketoglutarate
CO2
COOCH2
CH2
C~SCoA
O
Succinyl-CoA
a-Ketoglutarate
dehydrogenase
Complex
Thioester bond energy conserved as GTP
COOCH2
CH2
C~SCoA
O
Pi
+
GDP GTP
Succinyl-CoA
HS-CoA
COOCH2
CH2
COO-
Succinate
Succinyl-CoA Synthetase
FAD
FADH2
NAD+
NADH + H+
H2O
COOH
COOH
C
C
C
H C
COOH
Succinate
H
COOH
COOH
C OH
C=O
H C
COOH
Fumarate
COOH
Malate
C
COOH
Oxaloacetate
ATP Generated in the Aerobic
Oxidation of Glucose

There are two ways for producing ATP
Substrate level phosphorylation
G1,3-BP to G-3-P, PEP to Pyruvate, SCoA to succinate
Oxidative phosphorylation
ATP Generated in the Aerobic
Oxidation of Glucose

In aerobic oxidation of glucose
5 NAD+, 1 FAD
Stoichiometry: 2.5 ATP per NADH
1.5 ATP per FADH
Table 6-1
Regulation of the Kreb’s Cycle
Pyruvate Dehydrogenase complex
Pyruvate + TPP  Acetal-TPP + CO2
Acetal-TPP + S-S  Ac-S ^ SH + TPP
Ac-S ^ SH + HS-CoA  AcS-CoA + HS ^ SH
HS ^ SH + FAD  S-S + FADH2
FADH2 + NAD+  FAD + NADH + H+
Pyruvate + HS-CoA + NAD+  Acetyl-CoA + NADH + H+
Regulators-Activators
Regulators- Inhibitors
and AMP
Fatty acids and ATP
Key Regulatory Points:
1. Pyruvate dehydrogenase Complex
Inhibited by NADH and Acetyl-CoA
NADH
[NAD+]
Acetyl-CoA
HS-CoA
High NADH means that the cell is experiencing a
surplus of oxidative substrates and should not produce
more. Carbon flow should be redirected towards synthesis.
High Acetyl-CoA means that carbon flow into the Krebs
cycle is abundant and should be shut down and rechanneled
towards biosynthesis
Mechanism:
1. Competitive Inhibition
NADH and acetyl-CoA reverse the pyruvate dehydrogenase
reaction by competing with NAD+ and HS-CoA
2. Covalent Modification (second level regulation)
E-1 subunits of PDH complex is subject to phosphorylation
TPP
Active
FAD
HPO4=
1
2
Insulin
3
E1-OH
PDH
phosphatase
H2O
ATP
PDH
kinase
E1-OPO3
Inactive
ADP
Epinephrine
Glucagon
Cyclic-AMP
protein kinase
ATP
Regulation of the Citric Acid Cycle
Primary modes:
1. Substrate availability (key enzymes are subsaturated)
Allostery is not a primary mode
2. Product inhibition
3. Feedback inhibition (competitive)
Key regulators:
1. Acetyl-CoA (controls citrate synthase)
2. OAA (controls citrate synthase, regulated by NADH)
3. NADH (controls citrate synthase, isocitrate dehydrogenase
4. Calcium (stimulates NADH production)
See Fig. 6-9
Pentose Phosphate Pathway

PENTOSE PHOSPHATE
Pathway



Glucose-6-PO4  Ribose-5-PO4
Synthesize NADPH for fatty acid synthesis
Metabolize pentoses
Take Home: The PENTOSE PHOSPHATE pathway is
basically used for the synthesis of NADPH and D-ribose.
It plays only a minor role (compared to GLYCOLYSIS)
in degradation for ATP energy.
1) NADPH (Nicotinamide Adenine Dinucleotide Phosphate, reduced form) is
essentially identical in structure to NADH, with the exception of the phosphate at
the 2’-position of the ribose ring of the adenine nucleotide. Just as NADH, the
molecule consists of two nucleotides (heterocyclic, aromatic base attached to a
ribose sugar at carbon-1 attached to a phosphate at carbon-5) attached to one
another by a phosphoanhydride bond linking their 5’-phosphates. NADPH differs
from NADH physiologically in that its primary use is in the synthesis of metabolic
intermediates (NADPH provides the electrons to reduce them), while NADH is
used to generate ATP by contributing its reducing power to the electron transport
chain
Basic Process


Found in cytosol
Two phases
Oxidative nonreversible
Nonoxidative reversible
2) The pentose phosphate pathway serves substantially two functions in cells:
to provide ribose (a pentose) and its derivative 2-deoxyribose for nucleic acid
synthesis (ribose is the sugar in RNA, 2-deoxyribose in DNA), and to provide
NADPH as a reducing agent.
The oxidation and decarboxylation of glucose-6-phosphate to ribulose-5phosphate occurs in three steps, accompanied by the generation of two
molecules of NADPH. The first step is the oxidation of the hydroxymethylene
group at position one to a carbonyl group,
yielding a lactone (cyclic ester) and a molecule of NADPH. The second step is
then to hydrolyze the lactone to the free carboxylic acid. The carboxyl group
of the carboxylic acid is then removed by oxidative decarboxylation,
converting the 6-carbon sugar acid to a 5-carbon sugar, with the
accompanying production of another molecule of NADPH.
3) Once glucose-6-phosphate has been oxidized and decarboxylated to
ribulose-5-phosphate, this latter keto-sugar is converted to the
corresponding aldose, ribose-5-phosphate, by the enzyme
phosphopentose isomerase. The ribose-5-phosphate produced in this
way can now be used in the synthesis of nucleotides for incorporation
into nucleic acids. The reaction proceeds through an enediol (C=C
double bond and two hydroxyl groups) intermediate, as the enzyme
takes advantage of the dissociability of the hydrogen on the terminal
hydroxyl group to generate an oxyanion and move the C=O double
bond to the terminal carbon, producing the aldehyde and reducing the
ketone to an alcohol.
4) In order to control ribose synthesis, a mechanism exists to remove this
sugar when it is in excess, by converting it to glycolytic intermediates. A series
of three enzymatic steps are carried out, transferring two- and three-carbon
fragments from one sugar to another, and all of these steps are similar in
mechanism to an aldol condensation (remember that aldolase, the enzyme in
glycolysis which fragments the six-carbon, bisphosphorylated sugar fructose1,6-bisphosphate to two phosphorylated three-carbon fragments, breaks the
carbon-carbon bond through the reverse mechanism of the aldol condensation).
In these cases, however, the enzyme functions by cleaving a fragment from the
donor sugar by a reverse aldol condensation, and then attaches it to the
acceptor sugar using the forward reaction. The enzymes are transketolase,
which transfers a two-carbon fragment terminating on the interior side in a
carbonyl, and transaldolase, which transfers a three-carbon fragment
terminating on the interior side in a hydroxymethylene group.
5) The first reaction which assists in the conversion of ribose-5phosphate to glycolytic intermediates, catalyzed by transketolase, is
the transfer of the 1- and 2-carbons from xylulose-5-phosphate to the
1-carbon of ribose-5-phosphate. This leaves the last three carbons
from xylulose-5-phosphate as glyceraldehyde-3-phosphate, the first
three-carbon fragment encountered in glycolysis, and sedoheptulose7-phosphate, formed from the ribose-5-phosphate, which is a sevencarbon sugar.
6) Xylulose-5-phosphate is an unusual sugar which is
produced from ribulose-5-phosphate, simply by inverting the
configuration at carbon-3. This reaction is carried out by the
enzyme phosphopentose epimerase, and is freely reversible.
Thus, in the first reaction converting ribose-5-phosphate to
glycolytic intermediates, both ribose-5-phosphate and ribulose5-phosphate (the latter in the form of xylulose-5-phosphate) are
being degraded to other species, and ultimately carried off in
glycolysis.
7) The second reaction which leads from intermediates in the pentose
phosphate pathway to glycolytic intermediates is mediated by
transaldolase. This enzyme transfers a three-carbon fragment
(carbons 1, 2 and 3) from the sedoheptulose-7-phosphate just formed
in the first reaction to the glyceraldehyde-3-phosphate just formed in
the first reaction, yielding a four-carbon fragment, erythrose-4phosphate, and a six-carbon fragment, fructose-6-phosphate. The
fructose-6-phosphate is now free to enter the glycolytic pathway.
8) The final reaction leading from intermediates in the pentose phosphate
pathway to glycolytic intermediates is carried out by transketolase, just as was
the first reaction. In this reaction, another molecule of xylulose-5-phosphate is
cleaved, and the two-carbon fragment consisting of carbons 1 and 2 is
transferred to the molecule of erythrose-4-phosphate just formed in the
transaldolase reaction, yielding a molecule of glyceraldehyde-3-phosphate and
another molecule of fructose-6-phosphate. Both of these products are capable
of entering glycolysis directly, and so there are no leftover fragments produced
in this overall conversion. Because another molecule of xylulose-5-phosphate
has entered the reaction, the overall conversion consists of two molecules of
xylulose-5-phosphate and one molecule of ribose-5-phosphate going to two
molecules of fructose-6-phosphate and one molecule of glyceraldehyde-3phosphate; the xylulose-5-phosphate can be produced from ribose-5-phosphate
through ribulose-5-phosphate, and so the net reaction is the removal to
glycolysis of three molecules of ribose-5-phosphate.
9) Because the NADPH and ribose-5-phosphate produced by the
pentose phosphate pathway are used for quite different purposes, it is
sometimes necessary to produce them in different amounts. Therefore,
the cell has different modes in which the pentose phosphate pathway
can function. In the case where much more ribose-5-phosphate is
required than NADPH, the ribose-5-phosphate is produced from
glyceraldehyde-3-phosphate and fructose-6-phosphate by running the
transaldolase and -ketolase reactions in reverse. This allows the cell’s
NADP+ supply to remain essentially unaffected
10) When both NADPH and ribose-5-phosphate are needed in
large amounts, the predominant reaction used by the cell to
generate them is the conversion of glucose-6-phosphate to
ribose-5-phosphate, with the liberation of two molecules of
NADPH for each molecule of glucose-6-phosphate converted.
11) When much larger amounts of NADPH are required than ribose5-phosphate, the conversion of glucose-6-phosphate to ribose-5phosphate is the main reaction used, but the ribose-5-phosphate is
immediately recycled through the transaldolase and -ketolase
reactions, with gluconeogenesis returning the fructose-6-phosphate
and glyceraldehyde-3-phosphate to glucose-6-phosphate for another
round.
12) An alternative use of the pentose phosphate pathway can be implemented
when NADPH is needed in great quantity while ribose-5-phosphate is not.
This use involves not recycling the ribose-5-phosphate to glucose-6-phosphate,
but rather carrying the glycolytic intermediates forward, rather than
backward. The final destination of the ribose-5-phosphate in this case is thus
pyruvate, which can enter the Citric Acid Cycle as acetyl CoA and produce
ATP. This mode is implemented when the cell requires both NADPH and ATP
or NADH, rather than predominantly NADPH.
13) An important use of the NADPH produced in the pentose phosphate pathway is in the
maintenance of a reducing environment in the cell. In order to reduce oxidized
sulfhydryls back to their free states in the laboratory, we use mercaptoethanol or
dithiothreitol, but the cellular equivalent of this reducing agent is glutathione.
Glutathione is a tripeptide, similar in structure to Glu-Cys-Gly, but with the exception
that the glutamate residue is ligated to the cysteine through the R-group carboxyl, rather
than the normal peptide-forming carboxyl (attached to the a-carbon). The sulfhydryl
group of the cysteine R-group functions as the reducing agent, and recombines with
disulfide bonds in a variety of molecules to release as a free sulfhydryl one of those
partners in the disulfide. Another molecule of glutathione carries out the same reaction on
the glutathione-subject molecule disulfide, releasing the other partner and producing an
oxidized glutathione dimer. NADPH is used to reduce both glutathiones back to the
sulfhydryl form, such that they can carry out this reaction again. In this way, the cell
protects its components from the activities of reducing agents, as free sulfhydryls perform
a variety of needed functions in cellular molecules.
Glycogen Formation and
Degradation



93% of glucose units are joined by a-1,4glucosidic bond
7% of glucosyl residues are joined by a1,6-glucosidic bonds
Fig.6-11
Glycogen Formation and
Degradation



Main Chain: branch
point every 3 units
Branch: 5-12 glucosyl
residues
High Solubility
many terminals
4 hydroxyl groups

More reactive points
for synthesis and
degradation.
GLYCOGEN SYNTHESIS
ENZYMES

UDP-glucose pyrophosphorylase


Glycogen Synthase


forms UDP-glucose
major polymerizing enzyme
a1.,4->1,6-glucantransferase
Glycogen Synthesis
Glycogen
Degradation
Glucose-6-PO4
Synthesis
Glucose-1-PO4
UDP-Glucose
GLYCOGEN SYNTHESIS

ACTIVATION OF D-GLUCOSE

GLYCOSYL TRANSFER

BRANCHING
ACTIVATION
UDP-GLUCOSE
G-1-P + UTP
UDP-GLUCOSE + PPi
UDP-Glucose pyrophosphorylase
2 Pi
O
CH2OH
H
HO
O
OH H
HN
O
O
O
N
O P O P O CH2
O
H OH
O
O
HO
Uridine diphosphate (UDP) Glucose
OH
Glycogen
Glycogen Synthase
Phosphorylase
UTP
Glucose 1-PO4
PPi
Activated glucose
UDP-Glucose
UDP-glucose pyrophosphorylase
Go’(kJ mol-1)
Glucose 1-PO4 + UTP
H2O + PPi
Glucose 1-PO4 + UTP + H2O
UDP-Glucose + PPi
2 Pi
~0
-33.5
UDP-Glucose + 2 Pi -33.5
The hydrolysis of pyrophosphate drives this reaction
O
CH2OH
H
HO
HN
O
O
O
O
OH H
GLYCOSYL TRANSFER
N
O P O P O CH2
O
H OH
O
O
HO
UDP
OH
CH2OH
O
H
HO
CH2OH
O
H
OH H
O
H OH
OH H
O
H OH
NON-REDUCING END
CH2OH
CH2OH
NEW
H
HO
O
OH H
H OH
H
O
CH2OH
O
OH H
H OH
H
O
O
OH H
H OH
O
BRANCHING
Cleave
Glycogenin
a1.,4->1,6-glucantransferase
Glycogen Degradation
(Glycogenolysis)

Glycogenolysis is not the reverse of
glycogenesis
Glycogen Breakdown
Glycogen
Phosphorylase and
PO4
Debranching Enzyme
Glucose-1-Phosphate
Phosphoglucomutase
Glucose-6-Phosphate
Glucose
Glycolysis
Take home: Glycogen contributes glucose to glycolysis and
to blood glucose (Liver)
Phosphorylase
O
O
O
HO-P-OH
HO-P-OH
O
HO
CH2OH
O
CH2OH
O
HO-P-OH
O
O
O
O
CH2OH
CH2OH
O
O
O
HO
O-P-OH
HO
O
Glucose-1-PO4
PHOSPHORYLYSIS
O
Glycogen Phosphorylase
C
N
Glycogen Storage Site
Can accommodate on 4-5 sugars
Pyridoxal 5’-PO4 at active sites
N
C
Phosphorylase: A Homo Dimer
* More active
2 H2O
Phosphorylase
Phosphatase
2 PO4
PHOS A
2 ADP
Phosphorylase B
Kinase
Covalent
2 ATP
PHOS B
Less Active
+ 2 AMP
+
Immediate
- 2 AMP
Allosteric
PHOS B
More active
Cyclic AMP
Hormonal Regulation
Debranching Enzyme
Highly branched core
Phosphorylase
Phosphorylase
Glycogen
Limit Branch
a1,41,4 glucantransferase
a1,6-gluglucosidase
+
D-glucose
TAKE HOME:
DEGRADATION
What activates glycogen degradation
inactivates glycogen synthesis.
SYNTHESIS
What activates glycogen synthesis
inactivates glycogen degradation
H2O
PO4
PO4
Phosphorylase a
Active
Phosphorylase b
Less Active
Glucose-6-PO4
Glycogen
ADP H2O
ATP
PO4
PO4
Glycogen synthase b
Less Active
Glycogen synthase a
Active
Glucose-1-PO4
ADP
ATP
UDP-Glucose
The Significance of Glycogenesis
and Glycogenolysis

Liver
maintain blood glucose concentration

Skeletal muscle
fuel reserve for synthesis of ATP
Glycogen Storage Diseases

Deficiency of
glucose 6-phosphatase
liver phosphorylase
liver phosphorylase kinase
branching enzyme
debranching enzyme
muscle phosphorylase
Table 6-2
Gluconeogenesis


The process of transformation of noncarbohydrates to glucose or glycogen
Principal organs
liver, kidney

Non-carbohydrates
glucogenic amino acids
lactate
glycerol
organic acids
Phosphatase
Blood
Glucose
PO4
H2O
Glucose
Kinase
G6P
Ribose 5-PO4
Glycogen
F6P
Kinase
F1,6bisP
PO4
Phosphatase
H2O
Gly-3-P
DHAP
1,3 bisPGA
Kinase
3PGA
2PGA
PEP
Kinase
L-lactate
Pyruvate
OAA
Gluconeogenesis
Synthesis of glucose de novo (from scratch)
An anabolic pathway for the synthesis of glucose
from L-lactate or smaller precursors.
Significance:
Primarily in the liver (80%); kidney (20%)
Maintains blood glucose levels
The anabolic arm of the Cori cycle
Stage I
Gluconeogenesis
F1,6BP
Gly3P
DHAP
Glycerol
1,3BPGA
3PGA
Pyruvate
Carboxylase
2PGA
L-aspartate
2
PEP
PEP carboxykinase
PEPCK
OAA
L-malate
1
L-lactate
Pyruvate
L-alanine
OAA
L-malate
Mitochondria
R5P
Pentose
Phosphate
4
Glycogen
Glucose-6-phosphatase
Glucose
G6P
G1P
UDP-glucose
Hexokinase
F6P
3
Fructose 1,6 -
PFK-1
bisphosphatase
F1,6BP
Stage II
Gluconeogenesis
Problems: 3 irreversible reactions
PEP
Go’ = -61.9 kJ per mol
Pyruvate
F-1,6 bisPO4
F-6-PO4
Glucose-6-PO4
Glucose
Go’= -17.2 kJ per mol
Go’= -20.9 kJ per mol
Take home: Gluconeogenesis feature enzymes
that bypass 3 irreversible KINASE steps
Second Entry Point for Pyruvate
new carboxyl group
CH3CCOOH + HCO3 + ATP
O
HOOC-CH2CCOOH+ ADP + PO=
4
O
Pyruvate carboxylase O
||
ATP + HCO3
C
HO
+ ADP
OPO3
CO2 Fixation Reactions
O
O
O
||
||
||
C
HN
Swinging Arm
NH
HN
C
O
C
C
NH
O
N
NH
O
O
(CH2)4 COO
CH2(CH2)3C
CH2(CH2)3C
S
S
Biotin
Biocytin
N
H
Lys
S
Carboxybiocytin
Biotin’s only function is to fix CO2
N
H
Lys
Biocytin
(the cofactor of biotin)
O
O
C- N
Carboxy
Biotin
N
O
Carboxy group
S
CH2
CH2
CH2
CH2
C=0
Swinging
Arm
HN
CH2
CH2
CH2
CH2
Attach to Enzyme
at lysine -amine
group
C
Carboxylase Enzyme
3 Bypasses in Gluconeogenesis
COO
C=O
COO
GTP
GDP
CH2
COO
OAA
C~OPO3
CH2
CO2
PEP
PEP Carboxykinase
H2O
PO4
Fructose 1,6bisPO4
Fructose 1,6 bisphosphatase
PO4
H2O
Glucose-6-PO4
Glucose 6 phosphatase
Fructose-6-PO4
Glucose
Liver is a major anabolic organ
L-lactate
Blood
Lactate
D-glucose
THE CORI CYCLE
L-lactate
Blood
Glucose
D-glucose
Muscle is a major catabolic tissue
Cori Cycle

REGULATION
FOCUS ON CARBON FLOW
L-lactate
Glucose (Synthesis)
Glucose
Pyruvate (Degradation)
ENZYMES (Allosteric, cAMP-dependent, organ-specific isozymes)
Rule
1. Allosteric are targets
of metabolite regulators (effectors)
RECIPROCAL
REGULATION
Rule 2. Kinases in glycolysis; phosphatases in synthesis
Exception: PEPCK in synthesis - cAMP
POSTIVE EFFECTORS
Rule 3. ATP, citrate, acetyl-CoA, G6P turn on synthesis
AMP, F2,6BP,turn on degradation
NEGATIVE EFFECTORS
Rule 4. ATP, acetyl-CoA, citrate,G6P turn off degradation
AMP, F2,6BP turn off synthesis
The Significance of
Gluconeogenesis


Replenishment of glucose and maintaining
normal blood sugar level
Replenishment of liver glycogen
“three carbon” compounds


Regulation of Acid-Base Balance
Clearing the products
lactate, glycerol

Glucogenic amino acids to glucose
Blood Sugar and Its Regulation

Blood sugar level
3.89-6.11mmol/l

Major source of blood glucose
digestion and absorption of glucose from intestine


Glycogenolysis and gluconeogenesis
Fig.6-18
Regulation of Blood Glucose
Concentration

Insulin
decreasing blood sugar levels

Glucagon, epinephrine glucocorticoid
increasing blood sugar levels