Glycogen Metabolism

Chapter-15 Takusagawa’s Note© 1

Chapter-15: Glycogen Metabolism - Glycogen is the animal storage form of branched poly(glucose). - Processes of glycogen breakdown is: (glucose)n → glucose-1-phosphate + (glucose)n-1 - Processes of glycogen synthesis is: (glucose)n-1 + UDP-glucose → (glucose)n - Regulation of glycogen breakdown and synthesis: is controlled by two key enzyme (glycogen phosphorylase and glycogen synthase) activities

which are activated/inactivated by allosteric regulation and phosphorylation / dephosphorylation.

1. Glycogen breakdown Glycogen structure - Glucose molecules in the main chains are connected by α(1→4) glycosidic bonds. - The branches are attached by α(1→6) glycosidic linkages. - A glucose unit on the non-reducing ends is cleaved or attached one by one.

OOH

OH

CH2

O

OOH

OH

CH2OHO

OH

OH

CH2OH

HO O O

CH2OH

OH

OHO

O

CH2OH

OHOH

OHO

O

OOH

HO

CH2OH

O

OOH

OH

CH2OHO

OH

OH

CH2OH

HO O

Non-reducing ends

Reducing end

α(1 4) linkage

α(1 6) linkage

Branchpoint

Different pathways of glycogen breakdown - In muscle: Glycogen → glucose-6-phosphate (G6P) → glycolysis - In liver: Glycogen → G6P → glucose → bloodstream → various cells → glycolysis - Because the muscle cells mainly consume glucose molecules whereas the liver cells mainly

store the glucose molecules.

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Glycogen breakdown requires three enzymes 1. Glycogen phosphorylase (simply call it phosphorylase) (Glycogen)n + Pi ↔ (glycogen)n-1 + G1P (n residues) (n-1 residues) This enzyme releases a glucose unit one by one until it reaches ~ five units (limit branch)

from a branch point. - The enzyme has a crevice where 4-5 units of a left-handed helical glycogen can fit, but it is

too narrow to fit a branch point.

O

- Enzyme-catalyzed modification/demodification process yields two forms of phosphorylase Phosphorylase a --- E-O-PO3

2- (attachment at Ser-14). Phosphorylase b --- No phosphate attachment. - Both forms of enzyme are allosterically activated or inactivated. - Allosteric inhibitors: ATP, G6P and glucose. - Allosteric activator: AMP. Pyridoxal phosphate (PLP) is an essential cofactor for phosphorylase - PLP covalently bound to phosphorylase via a Schiff base to Lys-679. - PLP’s phosphate group probably functions as an acid-base catalyst.

PLP bound to Lys 679 via Shiff base

Pyridoxal phosphate (PLP)

Phosphorylase

N

CH

OH

CH3

H

COPO

O-

O-

H

H

N

(CH2)4

++N

COH

OH

CH3

H

COPO

O-

O-

H

H

2


Reaction mechanism 1. Formation of an [E-PL · Pi · glycogen] ternary complex. 2. Cleavage of the glycosidic bond by an acid catalysis (the Pi donates H to the bridge O

because the Pi receives H from PL), and formation of oxonium ion intermediate (half-chair conformation).

3. Reaction of Pi with the oxonium ion to form G1P.

Half-chair oxonium ion

O+

OHHO

HOCH2OH

H

O-

PO-O

O

PLE

O-

PO O

O-

H

H

OP

O-O

O

PLE

OP

O O-

O

H

HOCH2OH

HOOH O

O

(Glu)n-2

O

OOH

HO

HOCH2OH

H

(Glu)n-2

O

OOHHO

CH2OH O

PO O-

O-

HCH2OH

HO

HOOH

O

OP

O-O

O

PLE

H

Acid catalysis

Glycogen + Pi + Enzyme Ternary complex

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2. Glycogen debranching enzyme - Removes branches so that glycogen phosphorylase can complete reaction. Reaction mechanism 1. Three glucose units of the branch are cleaved and reattached to the non-reducing end of the

main chain. 2. The branch point, the glucose attached to the main chain by the α(1-6) glycosidic bond is

hydrolyzed.

G1P G

G1P G1P G1P

G1P

G1PG1PG1P G1P

G1PG1P G1P

G1PG1P G1P

Limit branch

G1P

Free glucose

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3. Phosphoglucomutase - G1P produced from the glycogen breakdown must be convert to G6P in order to enter

glycolysis or to produce glucose in liver. - Phosphoglucomutase catalyzes the conversion of G1P to G6P. - The Ser of the enzyme is phosphorylated. Reaction mechanism 1. The O6 of G1P attacks the phosphoenzyme to form a dephosphoenzyme-G1,6P intermediate. 2. The Ser-OH group on the dephosphoenzyme attacks the phosphoryl group at C1 to

regenerate the phosphoenzyme, and the product G6P is released.

G6PG1,6PG1P

O

OHOH

OH

HO

CH2OPO32-

Ser

H2C O PO32-

Enzyme

O

OPO32-

OH

OH

HO

CH2OPO32-

Ser

H2C O H

EnzymeEnzyme

Ser

H2C O PO32-

O

OPO32-

OH

OH

HO

CH2OH

2. GLYCOGEN SYNTHESIS - Biosynthetic and degradative pathways of metabolism are almost always different.

Glucose Glycogen

Glycogen synthesis

Glycogen breakdown

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A. UDP-glucose formation by UDP-glucose pyrophosphorylase - In the glycogen synthesis pathway, at first, the uridine diphosphate (UDP) is attached to

glucose. - This reaction is catalyzed by UDP-glucose pyrophosphorylase. Reaction mechanism 1. The phosphoryl oxygen of G1P attacks the α-phosphorus atom of UTP to form UDP-glucose

(ΔG°’ ≈ 0). Since the ΔG°’ ≈ 0, this reaction is not proceeded without some input energy. 2. The released PPi is rapidly hydrolyzed by inorganic pyrophosphatase (ΔG°’ < 0). Since this hydrolysis reaction is exergonic, the first phosphorylation reaction is pulled. ΔG°’ (kJ/mol) G1P + UTP ↔ UDP-glucose + PPi ~0.0 H2O + PPi → 2Pi -33.5 Overall G1P + UTP → UDP-glucose + 2Pi -33.5

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B. Glycogen synthesis by glycogen synthase Reaction mechanism 1. The glycosidic bond between glucose and UDP in UDP-glucose is hydrolyzed. The cleaved

glucose ion takes the oxonium ion intermediate (half-chair conformation), which is stabilized by the enzyme.

2. The glucose unit of UDP-glucose is transferred to the C4-OH group on one of glycogen’s non-reducing ends to form an α (1→4) glycosidic bond.

- UDP is recycled by conversion to UTP with ATP. UDP + ATP ↔ UTP + ADP Thermodynamics - ΔG of glycogen breakdown by glycogen phosphorylase is < 0 (ΔG ≈ -5 to -8 kJ/mol). (glucose)n + Pi ⎯→ G1P + (glucose)n-1 - ΔG of glycogen synthesis by glycogen synthase is < 0 (ΔG ≈ -14 kJ/mol). (glucose)n-1 + UDP-glucose ⎯→ (glucose)n - Thus both reactions takes place spontaneous under the same physiological condition. - However, glycogen synthesis uses one ATP hydrolysis per G1P for energy source. 1. G1P + UTP ⎯→ UDP-glucose + 2Pi 2. UDP-glucose + (glucose)n-1 ⎯→ (glucose)n + UDP 3. UDP + ATP ⎯→ UTP + ADP _____________________________________________________________________________________

G1P + (glucose)n-1 + ATP ⎯→ (glucose)n + ADP + 2Pi

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C. Glycogen Branching - About 7 units of the non-reducing end of α-amylose chain are removed at α(1→4) linkage,

and reattached to the C6 of other α-amylose chain by α(1→6) linkage. - This transfer is carried out by amylo-(1,4→1,6)-transglycosylase (branching enzyme).

Note: - ΔG of hydrolysis of α(1→4) glycosidic bond is -15.5 kJ/mol. - ΔG of hydrolysis of α(1→6) glycosidic bond is -7.1 kJ/mol. - In the branching process, hydrolysis of the α(1→4) glycosidic bond (ΔG = -15.5 kJ/mol)

derives the formation of the α(1→6) glycosidic bond (ΔG = +7.1 kJ/mol). 1. α(1→4) glycosidic bond hydrolysis ΔG = -15.5 kJ/mol 2. α(1→6) glycosidic bond formation ΔG = +7.1 kJ/mol ΔG = -8.4 kJ/mol - Since the reverse reaction (debranching) is endergonic (ΔG > 0), the debranching pathway

must be different. - In the debranching process, the breaking and reforming the α(1→4) glycosidic bond are

energetically canceling each other, and the α(1→6) bond hydrolysis (ΔG = -7.1 kJ/mol) drives the debranching.

1. α(1→4) glycosidic bond hydrolysis ΔG = -15.5 kJ/mol 2. α(1→4) glycosidic bond formation ΔG = +15.5 kJ/mol 3. α(1→6) glycosidic bond hydrolysis ΔG = -7.1 kJ/mol ΔG = -7.1 kJ/mol

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3. CONTROL OF GLYCOGEN METABOLISM A. Allosteric control of glycogen phosphorylase and glycogen synthase Glycogen phosphorylase - has two forms (a and b), and each form is activated allosterically from T-form to R-form. - Phosphorylase b is the non-phosphorylated enzyme and its R-form is less active, but its

response is fast since its activation signals (activators) are come from the inside of cell. - Phosphorylase a is the Ser-14 phosphorylated enzyme and its R-form is the most active

enzyme, but its response is slow since its activation signals (effectors) are come from the outside of cell.

- The enzyme is phosphorylated and dephosphorylated by phosphorylase kinase and phosphoprotein phosphatase, respectively. The phosphorylation is called “covalent modification”.

- Phosphorylase kinase and phosphoprotein phosphatase are enzyme modificator and demodificator, respectively.

- Unmodified form (phosphorylase b) is mostly T-form, whereas modified form (phosphorylase a) is mostly in the R-form.

Allosteric regulators

Highly active form

- Glycogen synthase has the same forms of phosphorylase, i.e., glycogen synthase a and

b, and their T- and R-forms. -

The allosteric activators and inhibitors of phosphorylase and synthase are listed below. Enzyme Activators Inhibitors Glycogen phosphorylase AMP ATP, G6P Glycogen synthase G6P

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Let us consider how these enzymes are activated or deactivated. - At high demand of ATP, i.e., low [ATP], low [G6P] and high [AMP]: Glycogen

phosphorylase is stimulated and glycogen synthase is inhibited, so flux through this pathway favors the glycogen breakdown.

- At high [ATP] and [G6P]: Glycogen synthesis is favored. B. Covalent modification of enzymes by cyclic cascades: Effector “Signal” amplification - As described above, two enzymes (phosphorylase kinase and phosphoprotein phosphatase)

are involved in the modification of phosphorylase. - The phosphorylation is the most effective activation process of phosphorylase. - Let us assume that one molecule of phosphorylase kinase can phosphorylate many

phosphorylase molecules (say 106 molecules). Then each phosphorylated phosphorylase molecule catalyzes the glycogen breakdown by 106 times.

- Thus the one activation of the kinase molecule is translated into 1012 glycogen breakdown reactions, i.e., production of 1012 glucose molecules.

- These processes are called enzyme cascade. - There are several cycles of enzyme cascades. Monocycle enzyme cascade is shown below. Monocycle cascade

F + e1 F e1Lessactive

Moreactive

Eb Ea

OH P

Moreactive

Lessactive e2Re2+R

ATP ADP

H2OPi

K1

K2

Moreactive

Lessactive

- F and R are kinase and phosphatase (hydrolase), respectively. - e1 and e2 are allosteric effectors of F and R, respectively, and those quantities are very little. - Activated F·e1 and R·e2 can catalyze the phosphorylation and dephosphorylation reactions in

many times. - Thus, the effector’s effects are amplified very much. - Thus, amplification potential of a “signal” of e1 or e2 is enormous.

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Bicyclic enzyme cascade Activation process: 1. Effector e1 activates F1 by forming a F1·e1 complex. 2. The activated F1·e1 phosphorylates F2b, and converts it to an active F2a. 3. The activated F2a phosphorylates Eb, and converts it to an active Ea. 4. The activated Ea catalyzes the reaction such as glycogen breakdown. Deactivation process: 1. Effector e2 activates R1 by forming an R1·e2 complex. 2. The activated R1·e2 hydrolyzes (dephosphorylates) the active F2a, and converts it to an

inactive F2b. 3. Similarly effector e3 activates R2 by forming an R2·e3 complex. 4. The activated R2·e3 hydrolyzes (dephosphorylates) the active Ea, and converts it an inactive Eb.

+ e1 e1

F2b F2a

OH P

e2e2+

ATP ADP

H2OPi

K1

K2

K3

Pi H2O

ADPATP

+ e3 e3

POH

EaEbR1

F1 F1

R1

P

R2 R2

In the glycogen metabolism (bicycle enzyme cascade), the corresponding symbols are: P = PO3

2-; e1 = cAMP; F1 = cAMP-dependent protein kinase; F2 = Phosphorylase kinase; E = Glycogen phosphorylase; [R1·e2 and R2·e3 = Phosphoprotein phosphatase-1].

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C. Glycogen phosphorylase bicyclic cascade - Epinephrine binds its receptor on the cell surface. Its signal is transferred to a G-protein. - The activated G-protein (Gα) activates the adenylate cyclase (AC). - The activated AC catalyzes the cAMP formation from ATP. - cAMP (effector e1) activates cAMP-dependent protein kinase (cAPK). - Insulin activates an insulin-stimulated protein kinase. - The activated insulin-stimulated protein kinase activates phosphoprotein phosphatase-1 (PP-

1). - PP-1 dephosphorylates the phosphorylated proteins to inactivate them except for glycogen synthase

which is activated by PP-1.

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Glycogen breakdown favor pathwayGlycogen synthesis favor pathway

ATP

Otherkinases

Ca2+

P

PPhosphoproteinphosphataseinhibitor-1 a

Phosphoproteinphosphataseinhibitor-1 b

ADPATP

H2OPi

Phosphoproteinphosphataseinhibitor-1 a

Phosphoproteinphosphatase-1 (inactive)

Phosphoproteinphosphatase-1 (active)

H2OPi

ADPATP

Glycogen-Psynthase b

Glycogensynthase a

H2OPi

ADPATP

Glycogenphosphorylase b

Ser 14-CH2O-PGlycogenphosphorylase a

Ser 14-CH2OH

H2OPi

ADPATPPP

(α β γ δ)4Phosphorylase kinase a

Phosphorylase kinase b

(α β γ δ)4

R2C2 2C

+ R2(cAMP)4cAMP-dependent protein kinase (active)

+ 4cAMP cAMP-dependent protein kinase (inactive)

EpinephrinAdenylate cyclase

Insulininsulin-stimulatedprotein kinase

Note: Phosphorylationactivates phosphorylasebut inactivates synthase


Adenosine-3’,5’-cyclic monophosphate (cAMP) - The primary intracellular signal e1 is adenosine-3’,5’-cyclic monophosphate (cAMP) in both

glycogen phosphorylase and glycogen synthase cascades.

N

N N

N

O

OHO

C

P

O

H

NH2

H

HH

O-

O

ATP

PPi

Adenylatecyclase

H2O

phosphodiesteraseAMP

- cAMP is absolutely required for the activity of cAMP-dependent protein kinase (cAPK). - cAPK phosphorylates specific Ser and/or Thr residues of numerous cellular proteins. - These protein have cAPK’s consensus recognition sequence, Arg-Arg-X-Ser/Thr-Y (X:

small residue; Y: Large hydrophobic residue). - cAPK is tetramer composed of two regulatory and two catalytic subunits, R2C2. - cAMP binds to the regulatory subunits → dissociate active catalytic monomers (2C). - Protein kinases play key roles in the signaling pathways by hormones, growth factors,

neurotransmitters and toxins. Phosphorylase kinase - is activated by Ca2+ (as low as 10-7 M) and by covalent modification. - is (αβγδ)4. - The γ subunit has full catalytic activity (ability to convert phosphorylase b to phosphorylase a). - The α, β, and δ subunits are inhibitors of the catalytic reaction. - The phosphorylation of α and β-subunits reduces the inhibitory activities of α and β-

subunits. - The δ subunit is calmodulin (CaM), which is a ubiquitous eukaryotic Ca2+-binding protein. - When Ca2+ binds to CaM’s 4 Ca2+-binding sites, CaM undergoes an extensive

conformational change that activates phosphorylase kinase. Actually, - Ca2+ binds to the Ca2+-binding domain → conformational change exposes a hydrophobic

patch → CaM binds to the CaM-binding domain of phosphorylase kinase γ subunit at the hydrophobic patch section → binding of CaM activates phosphorylase kinase (also other protein kinases).

ActiveInactive hydrophobic sectionP

P

δ

γβ

αCa2+

ADPATP

cAPK

P

P Hydrophobic patch

Inactive

α

β γ

δ α

β γ

δ

13


X-ray and NMR studies on the structure of CaM indicate that CaM changes its structure significantly when it binds to the specific oligopeptide of the γ-subunit of phosphorylase kinase.

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Muscle contraction processes Initial muscle contraction by nerve impulses → release Ca2+ from reservoir to cytosol → Ca2+

bind to CaM → activate phosphorylase kinase → phosphorylate (activate) glycogen phosphorylase → glycogen breakdown → ATP synthesis → ATP is used for contraction of muscle.

Phosphoprotein phosphatase-1 (PP-1) - catalyzes hydrolytic dephosphorylation (remove the phosphate group by hydrolysis). - is only active when it is bound to glycogen through its glycogen-binding G-subunit. - has two phosphorylation sites. 1. Activation site (P1) by insulin-stimulated protein kinase. 2. Deactivation site (P2) by cAPK. When the P2 site is phosphorylated by cAPK, PP-1 is

released from G-subunit. The PP-1 itself is inactive. Insulin and epinephrine have antagonistic effects on glycogen metabolism.

- PP-1 is also inhibited by phosphoprotein phosphatase inhibitor 1 (protein). - Phosphoprotein phosphatase inhibitor 1 is an active inhibitor protein when it is

phosphorylated by cAPK. - Thus, cAMP concentration controls not only the phosphorylation activity (stimulation) but

also dephosphorylation activity (suppression).

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D. Glycogen synthase - is inactivated by the bicycle enzyme cascade. 1. cAMP activates cAMP-dependent protein kinase. 2. cAMP-dependent protein kinase phosphorylates phosphorylase kinase a. 3. Phosphorylase kinase a phosphorylates glycogen synthase a. The phosphorylation on

glycogen synthase inactivates it, glycogen synthase b is inactive phosphorylated form. - is activated by phosphoprotein phosphatase-1 which dephosphorylates the glycogen synthase

(b → a). E. Integration of glycogen metabolism control mechanisms - The rates of glycogen breakdown and synthesis largely depend on the rate of the

phosphorylation and dephosphorylation reactions of the two bicyclic cascades. - Phosphorylation and dephosphorylation are controlled by hormones. Hormones trigger glycogen metabolism through the intermediacy of second messengers. In liver: Hormone glucagon controls glycogen metabolism (stimulate glycogen breakdown). - Glucagon is a 29 amino acid polypeptide.

In muscle and various tissues: Insulin and epinephrine (adrenaline) and norepinephrine

(noradrenaline) control glycogen metabolism.

- Insulin is a polypeptide hormone. Inactive form, proinsulin is a single peptide of 84 amino acid residues, while the active mature hormone is two chains (30 and 21 amino acid residues) connected by two S-S bonds.

X = CH3 EpinephrinX = H Norepinephrin

HO

HO

CHCH

OH

H

HNH

X

16


- Hormonal stimulation at their plasma membranes occurs through the mediation of transmembrane protein, receptor (review Chapter 21b).

Cytosol

Receptor

Second messenger

Hormone

Cell membrane

- When a hormone binds at the receptor, the second messenger is released into cytosol. - cAMP, Ca2+, inositol-1,4,5-triphosphate (IP3), diacylglycerol (DG), and NO are second

messengers. - Hormonal stimulation by glucagon or epinephrine increases the intracellular [cAMP] →

increase cAPK activity → increases the rate of phosphorylation of many proteins and decreases the rate of dephosphorylation.

- Cyclic cascades amplify the small change of [cAMP] to a large change in the fraction of enzymes in their phosphorylated forms.

- Hormonal stimulation by insulin increases the phosphoprotein phosphatase-1 (PP-1) activity. - PP-1 dephosphorylates various proteins. - Thus epinephrine and insulin are antagonistic relation. F. Maintenance of blood glucose levels - Need to maintain glucose concentration at ~5 mM in blood. When the [glucose] in blood is low: 1. Glucagon is secreted from pancreas into bloodstream. 2. Glucagon binds at glucagon receptor on the liver cells and activates adenylate cyclase,

thereby increasing intracellular [cAMP]. 3. The [cAMP] increase triggers an increase in the rate of glycogen breakdown, leading to

increase intracellular [G6P]. 4. Since G6P cannot pass through the cell membrane and is not major energy source of liver,

G6P is hydrolyzed to glucose by glucose-6-phosphatase. The resulting glucose enters the blood stream.

When blood glucose level is high (immediate after meal) 1. Glucagon level decrease. 2. Insulin is secreted from pancreatic β cells. - In muscle, - Insulin stimulates the glucose transporter on the muscle cells. Thus, glucose in bloodstream

enters into the cells. - Insulin stimulates insulin-stimulated protein kinase which phosphorylates G-subunit of

phosphoprotein phosphatase-1 (PP-1). - The phosphorylated PP-1 dephosphorylates glycogen synthase b to increase glycogen

synthesis, and also dephosphorylates glycogen phosphorylase a to decrease glycogen breakdown.

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- In liver, - Glucose in blood enters freely into the liver cells when the [glucose] is high in bloodstream. - Glucose itself inhibits glycogen phosphorylase a (R state) and shifts equilibrium to glycogen

phosphorylase b (T state). [Phosphorylase A → Phosphorylase B] - Phosphoprotein phosphatase inhibitor-1 is dephosphorylated and is released from PP-1. - PP-1 activates glycogen synthase and deactivates glycogen phosphorylase a. Therefore, the

liver can store the excess of glucose as glycogen.

Phosphorylase a

Glycogen synthase a Enzyme activity in mouse liver

Time after glucose infusion (min)

0 2 4 6 8

How does liver work as a buffer of blood [glucose]? - Liver does not have hexokinase, but glucokinase. - Hexokinase (that is in tissues other than in liver) obeys Michaelis-Menten kinetics, and is

inhibited by product G6P. - Glucokinase (that is in liver) displays sigmoidal kinetics, and is not inhibited by G6P at

physiological concentration. - The KM of hexokinase is much smaller than K0.5 of glucokinase, indicating that hexokinase

has stronger glucose affinity than glucokinase, i.e., hexokinase is good for glycolysis. KM (Hexokinase) << K0.5 (Glucokinase), i.e., 0.2 mM << 5 mM.

[Glucose] in blood < 5 mM: Liver releases glucose (G6P→ Glucose) [Glucose] in blood > 5 mM: Liver stores glucose (Glucose → G6P)

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Liver cells are freely permeable to glucose, but other cells are not - Thus, at low [glucose] in bloodstream, liver releases glucose whereas at high [glucose] in

bloodstream, liver cells receive glucose and convert it to glycogen since glucose freely permeable to liver.

Other [glucose] regulator, β-D-fructose-2,6-bisphosphate (F2,6P) - F2,6P is an extremely potent allosteric activator of PFK and inhibitor of FBPase. - Thus, the [glucose] is regulated by the [F2,6P].

β-D-Fructose-2,6-bisphosphate (F2,6P)

OO

CH2OHHO

OH

CH2O-2O3P PO32-

- F2,6P is formed from fructose-6-phosphate (F6P) catalyzed by phosphofructokinase-2 (PFK-2)

and is hydrolyzed to F6P by fructose bisphosphatase-2 (FBPase-2). Phosphofructokinase-2 (PFK-2) and Fructose bisphosphatase-2 (FBPase-2) - The 100kD homodimeric protein carries both PFK-2 and FBPase-2 activities which are

located on different domains.

PFK-2 FBPase-2

Phosphorylation sites

- Isozymes in liver and heart muscle have the phosphorylation site, whereas the isozyme in

skeletal muscle does not have the phosphorylation site.

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Liver isozyme - Phosphorylation by cAPK effects: - Inhibition of PFK-2 activity. ⎬ F2,6P is hydrolyzed → [F2,6P]↓ - Activation of FBPase-2 activity. ⎭ - Thus, increase FBPase activity and decrease PFK since the allosteric activator F2,6P is

not available to activate PFK and to inhibit FBPase. - Therefore, the glycolysis is inversed, and G6P is produced. Then G6P hydrolysis

produces free glucose that is secreted into bloodstream.

FBPase-2FBPase

PFK-2PFK

[F1,6P]↓ [F6P]↑ [F2,6P]↓

G6P G1P

Glycogen synthesis pathway

GlycogenGlucoseBloodstream Heart muscle isozyme - Phosphorylation by cAPK effects: - Activation of PFK-2 activity ⎬ F2,6P is produced → [F2,6P]↑ - Inhibition of FBPase-2 activity ⎭ - Thus, increase PFK activity and decrease FBPase since F2,6P activates PFK and inhibits

FBPase. - Therefore, the glycolysis pathway is activated.

Glycogen G1P G6P

[F2,6P]↑[F6P]↓[F1,6P]↑PFK PFK-2FBPase FBPase-2

Glycolysis pathway

Glycolysis

- The phosphorylation of PFK-2/FBPase-2 in heart muscle activates PFK-2 rather than inhibits

PFK-2 as seen in liver. Increase glycogen breakdown effects: - In liver: increase glucose secretion into bloodstream, and decrease glycolysis. - In muscle: increase glycolysis. In skeletal muscle isozyme, there is no phosphorylation site. Thus, there is no subject to cAMP-

dependent phosphorylation control.

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G. Response to stress - Epinephrine and norepinephrine are released into the bloodstream by adrenal glands in

response to stress. - also stimulate the pancreatic α cells to secrete glucagon. - These hormones bind to: - β-adrenergic receptors, stimulates adenylate cyclase, and produces cAMP. - α-adrenergic receptors, stimulates phospholipase C, and produces inositol-1,4,5-triphosphate

(IP3), diacylglycerol (DG), and Ca2+. These second messengers act as reinforce the cells’ response to cAMP.

Muscle cells do not have glucagons receptors.

← Liver Cell

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4. GLYCOGEN STORAGE DISEASE

Enzyme deficiency Reaction Symptoms Glucose-6-phosphatase G6P → glucose [G6P]↑ in liver, [glucose]↓ in blood α-1,4-Glucosidase maltose → 2 glucose Large accumulation of glycogen Amylo-1,6-glucosidase branch glycogen → glucose Accumulation of abnormal short

chain glycogen Amylo-(1,4→1,6)-transglycosylase

glucose → branch glycogen Very long unbranched glycogen

Glycogen phosphorylase (muscle):

glycogen → G6P Inability of glycogen breakdown

Glycogen phosphorylase (liver):

glycogen → G6P

Inability of glycogen breakdown

Phosphofructokinase F6P → FBP (F1,6P)

Accumulation of G6P and F6P

Phosphorylase kinase glycogen breakdown Inability to convert phosphorylase b to a

Glycogen synthase UDP-glucose → glycogen Inability to synthesize glycogen in liver

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D. Mechanism of passive-mediated glucose transport - Erythrocyte glucose transporter is a 55 KD protein, which is composed of 12 transmembrane helices. - The helices form a bundle whose interior is hydrophilic and exterior is hydrophobic. - Glucose is transport through the channel.

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Glucose transport occurs via a gated pore mechanism (Erythrocytes, Liver cells) - When glucose transports across the erythrocyte membrane, no other molecule or ion are accompanied. - Simply glucose moves from high concentration side to low concentration side.

Insulin-sensitive exocytosis/endocytosis controls the glucose transport (Fat and muscle cells) - Cellular glucose uptake is regulated through the insulin-

sensitive exocytosis/endocytosis of the vesicles containing the glucose transports.

- When the [glucose] in blood is high, insulin is secreted from pancreas.

- Insulin stimulates the exocytosis activity of the vesicle that contains the glucose transporters.

- The glucose transporters are moved on the membrane. - Glucose in blood is taken up and stored in cells. - When the [glucose] in blood is low, the glucose

transporters are moved into the cell by endocytosis so that glucose in cells cannot flow out.

- Thus glucose uptake is regulated by population of glucose

transporters.

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Chapter-15 Takusagawa’s Note©

25

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A. Na+-Glucose symport (Small intestine) - Glucose is absorbed from lumen of small intestine into brush border cells by Na+ concentration gradient. - Actually, the Na+ is pumped out by (Na+-K+)-ATPase from the brush border cell to create the Na+ gradient.

- Na+-glucose symport system is a random Bi Bi kinetic mechanism. Note: most antiport systems are ordered sequential kinetic mechanism.

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Glycogen Metabolism