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