Chapter 4Carbohydrate Metabolism
Glucose transport
Metabolism…..
Time to put some LIFE into the subject
What is Life? What are the properties of life?
MovementTurnover of componentsReproduction of one’s kind
Energy Transformations
Chemical Energy is the Dominant Energy Form in a Living System
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
A BEnergy
Anabolism: Synthesis. Putting freeenergy to work
Catabolism: Degradation. Deriving free energy
ATP: Energy currency. The standard that is usedto gauge all energy compounds
Endergonic
Exergonic
Rule: Living system are able to conserve energy
Rule: Heat is wasted energy
Heat is energy that cannot be conserved
Rule: Exergonic biochemical transformations channela large part of the free energy into chemical bonds of the product.
Rule: Catabolic reactions drive anabolic reactions
Living systems will do their utmost to preventlost of free energy as heat
Rule:
The 5 Rules of Energy Metabolism
Anaerobic
Aerobic
Reduced cofactors(drive Ox Phos)
Reduced cofactors(drive Ox Phos)
Oxidized cofactors(recycle back
Oxidized cofactors(recycle back
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
-D-Glucose
Glucose-6-Phosphate
Fructose-6-Phosphate
CH2OPO3
O CH2OH
OH
ATPHexokinase
Phosphogluco- isomerase
O
CH2OH
OH
O
CH2OPO3
OH
Fructose-6-Phosphate
CH2OPO3
O CH2OH
OH
CH2OPO3
O CH2OPO3
OH
Fructose 1,6-Bisphosphate
ATPPhosphofructo- kinase-I
CH2OPO3
C=O
CH2OH
CHO
H-C-OH
CH2OPO3
Dihydroxyacetone-Phosphate Glyceraldehyde-3-Phosphate
Aldolase
CH2OP
C=O
HO-C-H
H-C-OH
C-OH
CH2OP
..
CH2OP
C=O
HO-C-H..
H-C-OH
C-OH
CH2OP
+ CHO
ALDOLASE
DihydroxyAcetonePhosphate (DHAP)
Glyceraldehyde-3-P
H
Fructose 1,6- bisphosphate
CH2OPO3
C=O
CH2OH
CHO
H-C-OH
CH2OPO3
Triose Stage
Triose phosphate isomerase
Dihydroxy acetone phosphate (DHAP)
CHO
H-C-OH
CH2OPO3
C
H-C-OH
CH2OPO3
~OPO3
O
PO4
NAD+
NADH+ H+
ADP ATP
Glyceraldehyde-3-PDehydrogenase
COO
H-C-OH
CH2OPO3
Glyceraldehyde 3-phosphate
PhosphoglycerateKinase
Glycerate 1,3-bisphosphate
Glycerate 3-phosphate
COO
H-C-OH
CH2OPO3
COO
C=O
CH3
COO
H-C-OPO3
CH2OH
COO
C~
CH2
OPO3
Pyruvate
3-PGA 2-PGA
PEP
Phosphoglycero- mutase
Enolase
-H2O
ADP
ATPPyruvate kinase
L-lactate
NADH + H+
NAD+
COO
HO-C-H
CH3
Back to Glycolysis
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 Glycolysisfig.6-4
Regulation of Glycolysis Pyruvate Kinase Allosteric enzyme Inhibited by ATP. Isoenzyme in liver activated by fructose 1,6
bisphosphate inhibited by alanine Regulated by
phosphorylation and dephosphorylation
Hexokinase Different isoenzymes
Hexokinase IV glucose 6-phosphate is an
allosteric inhibitor
promote biosynthesis
The Significance of Glycolysis Glycolysis is the emergency energy-
yielding 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 Glucose oxidation
1. Oxidation of glucose to pyruvate in cytosol
2. Oxidation of pyruvate to acetylCoA in mitochondria
3. Tricarboxylic acid cycle and oxidative phosphorylation
Mechanism of pyruvate dehydrogenase complexFig.6-6
O2O2
O2
O2
O2 O2
O2 O2
METABOLISM OF
PYRUVATE
METABOLISM OF
PYRUVATE
Its time to get aerobic
COO
C=O
CH3
ketoacid
Carboxyl group (acid)
Ketone group (carbonyl)
Methyl group
Pyruvate Structure
– 2
-2
+ 2
CH3 C-OH C=O C=O
O
–2–2 –2 –2C
O
O
Net = – 2
Oxidation of Carbon
+2+2+2+2
Look for one NAD+ for eachglyceraldehyde-3-PO4 oxidized to pyruvate
-OH0
0
H-C-OH
CHO
CH2OHP
Glyceraldehyde3-Phosphate
O:C
OC
O
O+
O
C
O
Decarboxylation Reactions
Two Types: non-oxidative and oxidative
No change inoxidation stateof carbonyl C
H3C-C:COO-
O
H3C-C+O
NAD+ NADH
H3C-C:O
CO2
CO2
Oxidizedcarbonyl C
Oxidative
Non-oxidativeH3C-C:H
OH+
H3C-C-OHO
H2O
The Energy Story of Glycolysis
Overall ANAEROBIC (no O2)
Glucose + 2ADP + 2Pi 2 Lactate + 2ATP + 2H2O
Glucose + 2ADP + 2Pi 2 Ethanol + 2CO2 +2ATP + 2H2O
Overall AEROBIC
Yeast
Glucose + 2ADP + 2Pi + 2NAD+
2 Pyruvate + 2ATP + 2NADH + 2H+ + 2H2O
5 ATPs
C6H12O6 + 6O2 6CO2 + 6H2O
CHO
CH2OH
H-C-OHOH-C-H
H-C-OHH-C-OH
D-Glucose
Go’= -2,840 kJ/mol
Go’= -146 kJ/molC6H12O6 2 C3H4O3
Glucose 2 Pyruvate
COO-
C=OCH3
COO-
C=OCH3
5.2%Energy used
1462,840
100 =
Anaerobic
Aerobic
2 Pyruvates
Glycolysis
Oxidative phosphorylation
pyruvate
Krebs Cycle
3 NADH
GlucoseGalactoseFructoseMannose
Fatty Acids
1 FADH2
Lactate
Amino Acids
O2
H2O
Anaerobic
AerobicAcetyl-Coenzyme A
Pyruvate dehydrogenase Complex
Pyruvate dehydrogenase
Dihydrolipoyl transacetylase
Dihydrolipoyl dehydrogenase
NAD
Coenzyme A
Lipoic acid
Thiamin pyrophosphate
FAD
N
N NH2CH3
CH2
P PO
O
O
O
O
O
O
Thiamin pyrophosphate
CH3 COOC
O Pyruvate
..SN
CH3CH2 CH2
+
Carbanion
Vitamin B-1
: + CO2
Pantothenate
CH3C
O
Acetyl Group
Thioester bond
COENZYME A
Acetyl-Coenzyme A
OH
CH2O
O
O
O
O P
N
N
NH2
N
N
B-vitamin
HS-CH2-CH2-NH
-P-O-P-O
O O
O O-C-C-C-CH2-OO H
HO CH3
CH3H-C-CH2-CH2-NO
Adenosine-3’- phosphate
Dihydrolipoate
CH CH2CH2CH2CH2CH2
CH2
SHHS
COO
6,8 Dithiooctonoate(Reduced, gained 2 electrons)
SS
CH2
CH2
CH CH2CH2CH2CH2 COO
(Oxidized, lost 2 electrons)
Long hydrocarbon chain
Disulfide bond
E3
Pyruvate Dehydrogenase Complex
FADTPP
SS
E1 E2
Pyruvate Dehydrogenase
Dihydrolipoyl Transacetylase
Dihydrolipoyldehydrogenase
H
H..
CH3-C
OH
SSC-CH3
O
..
H2..
Acetyl-CoA
HS-CoA
NAD+
NADH..
..
CH3-CO
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
Citrate
6 5 4
Isocitrate
cis-Aconotate
-ketoglutarate Succinyl-CoA
Succinate
Fumarate
Malate
Oxaloacetate
More Reduced More Oxidized
CO2 CO2
Cycle Intermediates
CARBON BALANCECH3C
O
~S-CoA
Citrate
Isocitrate
-ketoglutarate
Succinyl-CoASuccinate
Fumarate
Malate
Oxaloacetate
CO2
CO2
2 carbons in2 carbons out
2 carbons in2 carbons out
6
6
5
44
4
4
4
Reactions of Acetyl-CoA
CH3-C~S-CoA
O H..
Split here
HS-CoA
C-C~S-CoAH
H
O
Carbanion
COO
C=O
CH2
COO
OAA
Acetylations orAcylations
COO
C-OH
CH2
COO
Citroyl-CoA
H2CC=O
S-CoA
Citrate Synthase
(a lyase)
O-
Citrate
CH3-C~SCoA
O
COO-
C=O
CH2
COO-
COO-
C-OH
CH2 COO-
-CH2--OOCHS-CoA
CH2COO-
HO-C-COO-
CH2COO-
Citric Acid or Citrate
Acetyl-CoA
Citrate Synthase
Oxaloacetate
(OAA)
CH2COO-
HO-C-COO-
H-C-COO-
H
CH2COO-
C-COO-
C-COO-H
CH2COO-
H-C-COO-
HO-C-COO-
H
-H2O +H2O
Citrate cis-Aconitate Isocitrate
Aconitase
Isocitrate Formation
CH2COO-
H-C-COO-
HO-C-COO-
H
Isocitrate
CO2
NAD+ NADH + H+
COO-
CH2
CH2
C=O
COO-
-Ketoglutarate
Isocitrate Dehydrogenase
COO-
CH2
CH2
C=O
COO-
-Ketoglutarate
COO-
CH2
CH2
C~SCoA
O
Succinyl-CoA
-Ketoglutaratedehydrogenase
Complex
HS-CoATPP
Lipoic acidFADNAD+
CO2
COO-
CH2
CH2
C~SCoA
O
COO-
CH2
CH2
COO-
SuccinateSuccinyl-CoA
GTPGDP
Pi
+
HS-CoA
Succinyl-CoA Synthetase
Thioester bond energy conserved as GTPThioester bond energy conserved as GTP
COOH
COOH
C
C
COOH
COOH
C=O
C
COOH
COOH
C
C
H
H
Succinate Fumarate
FAD FADH2
Malate Oxaloacetate
COOH
COOH
C
C
OH
H
H2ONAD+ NADH + H+
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
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- Inhibitors
Regulation of the Kreb’s Cycle
Fatty acids and ATP
Regulators-Activators
and AMP
Key Regulatory Points:
1. Pyruvate dehydrogenase Complex
Inhibited by NADH and Acetyl-CoAInhibited 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:
NADH and acetyl-CoA reverse the pyruvate dehydrogenase reaction by competing with NAD+ and HS-CoA
1. Competitive Inhibition
2. Covalent Modification (second level regulation)E-1 subunits of PDH complex is subject to phosphorylation
Insulin
EpinephrineGlucagon
Cyclic-AMPprotein kinase
E1-OH
E1-OPO3H2O
HPO4= ATP
ADP
PDH kinase
PDH phosphatase
ActiveActive
InactiveInactive
TPP FAD
1 2 3
ATP
Regulation of the Citric Acid Cycle
Primary modes:1. Substrate availability (key enzymes are subsaturated)
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
Allostery is not a primary modeAllostery is not a primary mode
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.
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-5-phosphate 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 fructose-1,6-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-5-phosphate 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 sedoheptulose-7-phosphate, formed from the ribose-5-phosphate, which is a seven-carbon 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 ribulose-5-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-4-phosphate, 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-3-phosphate; 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 ribose-5-phosphate, the conversion of glucose-6-phosphate to ribose-5-phosphate 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 -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-1,4-glucosidic bond
7% of glucosyl residues are joined by a-1,6-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 ENZYMESGLYCOGEN SYNTHESIS ENZYMES
UDP-glucose pyrophosphorylase forms UDP-glucose
Glycogen Synthase major polymerizing enzyme
a1.,4->1,6-glucantransferase
UDP-glucose pyrophosphorylase forms UDP-glucose
Glycogen Synthase major polymerizing enzyme
a1.,4->1,6-glucantransferase
Glycogen SynthesisGlycogen
Glucose-1-PO4 UDP-GlucoseGlucose-6-PO4
Degradation Synthesis
GLYCOGEN SYNTHESISGLYCOGEN SYNTHESIS ACTIVATION OF D-GLUCOSE
GLYCOSYL TRANSFER
BRANCHING
ACTIVATION OF D-GLUCOSE
GLYCOSYL TRANSFER
BRANCHING
ACTIVATION
UDP-GLUCOSE
G-1-P + UTP UDP-GLUCOSE + PPi
2 PiUDP-Glucose pyrophosphorylase
O
CH2OH
H
H
H
OH
OHHO P
O
O
O
O
O
P
O
O
H
O
OH
N
N
O
O
CH2
HOUridine diphosphate Glucose(UDP)
Glucose 1-PO4 UDP-Glucose
Glycogen
Glycogen SynthasePhosphorylase
UDP-glucose pyrophosphorylase
Glucose 1-PO4 + UTP UDP-Glucose + PPi
Go’(kJ mol-1)
H2O + PPi 2 Pi
Glucose 1-PO4 + UTP + H2O UDP-Glucose + 2 Pi
~0
-33.5
-33.5
The hydrolysis of pyrophosphate drives this reactionThe hydrolysis of pyrophosphate drives this reaction
Activated glucoseUTP PPi
O
CH2OH
H
H
H
OH
OH
O
CH2OH
H
H
H
OH
OHHO O O
O
CH2OH
H
H
H
OH
OHHO P
O
O
O
O
O
P
O
O
H
O
OH
N
N
O
O
CH2
HO
GLYCOSYL TRANSFER
NON-REDUCING END
UDP
O
CH2OH
H
H
H
OH
OH O
O
CH2OH
H
H
H
OH
OH
O
CH2OH
H
H
H
OH
OHO OHO
NEW
BRANCHINGGlycogeninCleave
a1.,4->1,6-glucantransferase
Glycogen Degradation (Glycogenolysis)
Glycogenolysis is not the reverse of glycogenesis
Glycogen Breakdown
Glycogen
Glucose-1-Phosphate
Glucose-6-Phosphate
Phosphorylase and Debranching Enzyme
PO4
GlucoseGlucose
GlycolysisGlycolysis
Phosphoglucomutase
Take home: Glycogen contributes glucose to glycolysis andto blood glucose (Liver)
O
O
O
CH2OH
HO-P-OH
O
OO
O
CH2OH
HO
PHOSPHORYLYSISGlucose-1-PO4
Phosphorylase
HO-P-OH
O
O
HO-P-OH
O
O
O
CH2OH
HO
O-P-OH
O
O
O
CH2OH
HO
Glycogen Phosphorylase
N
C
N C
Glycogen Storage Site
Can accommodate on 4-5 sugars
Pyridoxal 5’-PO4 at active sites
Cyclic AMP
PHOS B
Phosphorylase: A Homo Dimer
PHOS A
2 ATP
2 ADP
Phosphorylase B Kinase
* More active
2 H2O
2 PO4
Less Active
CovalentPhosphorylasePhosphatase
PHOS BMore active
+ 2 AMP - 2 AMPAllosteric+
Immediate
Hormonal Regulation
Debranching Enzyme
1,41,4 glucantransferase1,6-gluglucosidase
D-glucose
Limit BranchGlycogen
+
PhosphorylasePhosphorylase
Highly branched core
TAKE HOME:
What activates glycogen degradationinactivates glycogen synthesis.
What activates glycogen synthesis inactivates glycogen degradation
DEGRADATION
SYNTHESIS
RECIPROCAL REGULATION
Glycogen
Glucose-1-PO4 UDP-GlucoseGlucose-6-PO4
Phosphorylase a
Phosphorylase b
PO4
Glycogen synthase a
Glycogen synthase bPO4
ATP
ADP
PO4PO4
H2O H2O
ATP
ADP
Active
Less Active
Less Active
Active
The Significance of Glycogenesis and Glycogenolysis
Liver
maintain blood glucose concentration
Skeletal muscle
fuel reserve for synthesis of ATP
Glycogen Storage Diseases Deficiency ofglucose 6-phosphatase
liver phosphorylase
liver phosphorylase kinase
branching enzyme
debranching enzyme
muscle phosphorylase
Table 6-2
Gluconeogenesis The process of transformation of non-carbohy
drates to glucose or glycogen Principal organs
liver, kidney
Non-carbohydrates
glucogenic amino acids
lactate
glycerol
organic acids
Blood Glucose
Blood Glucose
Ribose 5-PO4Ribose 5-PO4
GlycogenGlycogen
L-lactateL-lactate Pyruvate
PEP
2PGA
3PGA
1,3 bisPGA
Gly-3-P
F1,6bisP
OAA
F6P
G6P
DHAP
Glucose
H2O
PO4
Phosphatase
H2OPO4Phosphatase
Kinase
Kinase
Kinase
Kinase
Gluconeogenesis Synthesis of glucose de novo (from scratch)
An anabolic pathway for the synthesis of glucosefrom L-lactate or smaller precursors.
Significance:
Primarily in the liver (80%); kidney (20%)
Maintains blood glucose levels
The anabolic arm of the Cori cycle
L-lactate Pyruvate
PEP
2PGA
3PGA
1,3BPGA
L-alanine
Gly3P DHAP
Stage I Gluconeogenesis F1,6BP
OAA L-malate
L-malateOAA
Mitochondria
Glycerol
L-aspartate
PEP carboxykinase PEPCKPyruvate
Carboxylase
PyruvateCarboxylase
1
2
PentosePhosphate
F1,6BP
F6P
G6P G1P UDP-glucose
Glycogen
Glucose
R5P
Fructose 1,6 -bisphosphatase
PFK-1
Glucose-6-phosphatase
Stage IIGluconeogenesis
Hexokinase
3
4
Problems: 3 irreversible reactionsProblems: 3 irreversible reactions
Go’ = -61.9 kJ per molPEP Pyruvate
F-1,6 bisPO4 F-6-PO4 Go’= -17.2 kJ per mol
Glucose-6-PO4 Glucose Go’= -20.9 kJ per mol
Take home: Gluconeogenesis feature enzymes that bypass 3 irreversible KINASE steps
Take home: Gluconeogenesis feature enzymes that bypass 3 irreversible KINASE steps
CH3CCOOH + HCO3
O
+ ATP + ADP + PO4=HOOC-CH2CCOOH
O
new carboxyl group
Second Entry Point for Pyruvate
CO2 Fixation Reactions
(CH2)4COO
C
N N
S
| |O
HH
Biotin
C
N N
S
| |O
H
CH2(CH2)3CO
N LysH
C
O
O
Biocytin Carboxybiocytin
C
N N
S
| |O
HH
CH2(CH2)3CO
N LysH
Swinging Arm
Biotin’s only function is to fix CO2
ATP + HCO3 COPO3
O
HO
| |
+ ADP
Pyruvate carboxylase
Biocytin
O
O
S
NNC-
O
CH2
CH2CH2
CH2
C=0
Carboxy Biotin
Carboxy group
HN
CH2CH2
CH2CH2
C
Carboxylase Enzyme
Swinging Arm
(the cofactor of biotin)
Attach to Enzymeat lysine -amine group
Attach to Enzymeat lysine -amine group
3 Bypasses in Gluconeogenesis
PEP
Fructose 1,6bisPO4
Glucose-6-PO4 Glucose
Fructose-6-PO4
OAA
GTP GDP
CO2
COO
C=O
CH2
COO
COO
C~O
CH2
PO3
PEP Carboxykinase
PO4
PO4
Fructose 1,6 bisphosphatase
Glucose 6 phosphatase
H2O
H2O
THE CORI CYCLE
Liver is a major anabolic organ
Muscle is a major catabolic tissue
L-lactate D-glucose
D-glucoseL-lactate
BloodGlucose
BloodLactate
Cori Cycle
REGULATION
Rule 2. Kinases in glycolysis; phosphatases in synthesisException: PEPCK in synthesis - cAMP
Rule 1. Allosteric are targets of metabolite regulators (effectors)
Rule 3. ATP, citrate, acetyl-CoA, G6P turn on synthesis
ENZYMES
POSTIVE EFFECTORS
FOCUS ON CARBON FLOWL-lactate Glucose (Synthesis)Glucose Pyruvate (Degradation)
AMP, F2,6BP,turn on degradation
NEGATIVE EFFECTORS
Rule 4. ATP, acetyl-CoA, citrate,G6P turn off degradationAMP, F2,6BP turn off synthesis
(Allosteric, cAMP-dependent, organ-specific isozymes)
RECIPROCAL REGULATION
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