Carbohydrate metabolism. Topics in Metabolism Overview of glucose homeostasis Glucose metabolic...

Carbohydrate metabolism

Topics in Metabolism

• Overview of glucose homeostasis • Glucose metabolic pathways and their regulation

• Glycolysis• Citric acid cycle• Gluconeogenesis• Glycogen metabolism• Pentose phosphate pathway

• Carbohydrate metabolism

GlucoseGlucose

Insulin

GlycogenLactate

Lactate CO2 + H2O

Fat

Glucose

Carbohydrates

•Carbohydrates are called carbohydrates because they are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of (CH2O)n.

•The major nutritional role of carbohydrates is to provide energy and digestible carbohydrates provide 4 kilocalories per gram. No single carbohydrate is essential, but carbohydrates do participate in many required functions in the body.

Clinical example• R.D., a 6-week-old girl, was born after a normal

pregnancy and weighed 3.2 kg at birth . Her parents and two older siblings were in good health. She was breast fed for 4 wk, and her weight gain had been normal. At 4 wk of age, breastfeeding was discontinued and a common baby formula was substituted. As a result of poor initial formula preparation, the child develop a viral gastroenteritis and after several days exhibited fussiness, watery diarrhea, and vomiting. At age of 6 wk she was admitted to the hospital. Urinalysis yielded a +1 reaction for reducing substance.

Pediatric gastroenteritis

• Was there any significant difference between the breast milk and the baby formulas?

• How did the gastroenteritis affect the digestion of carbohydrates?

• was the gastroenteritis related to diarrhea? What might have caused the explosive, acid, watery stool containing reducing substances?

Digestion

• Pre-stomach – Salivary amylase : 1-4 endoglycosidase

GG

GG

G

GG

G 1-4 linkG

G

GG 1-6 link

GG

G

GGG G G G

GG

G

G G

G

maltose

G

GG

isomaltose

amylase

maltotriose

G

G

G

G

Limit dextrins

Stomach

• Not much carbohydrate digestion• Acid and pepsin to unfold proteins• Ruminants have forestomachs with

extensive

microbial populations to breakdown and

anaerobically ferment feed

Small Intestine• Pancreatic enzymes-amylase

G G GG G

G

G G GG G GG

GG G

amylose

amylopectin

G G G G G

amylase

+

G

G G

G G

maltotriose maltose

Limit dextrins

G

Oligosaccharide digestion..cont

G

G G

G G

G

G

G

G G

G

G

Glucoamylase (maltase) or

-dextrinase

G G

G

G

G

-dextrinase

G GG

G

G G

Gmaltase

sucrase

Limit dextrins G

Small intestinePortal for transport of virtually all nutrients

Water and electrolyte balance

Enzymes associated with intestinal surface membranesi. Sucraseii. dextrinaseiii.Glucoamylase (maltase)iv.Lactasev. peptidases

Carbohydrate absorption

Hexose transporter

apical basolateral

Carbohydrate malabsorption– Lactose intolerance

(hypolactasia).– Decline lactase with age– Lactose fermented in LI –

• Gas and volatile FA• Water retention –

diarrhea/bloating

– Not all populations• Northern European – low

incidence• Asian/African Americans – High

1-4 linkage

Glucose metabolism: Breakfast

Eat cereal, bread, skimmed milk, fruit - mixture of monosaccharides (glucose, fructose), disaccharides (lactose, sucrose), complex carbohydrates (starch). Carbohydrates are broken down to monosaccharides for absorption in the small intestine. Glucose enters circulation through portal vein and increased blood glucose is detected 15 min after and peaking at 30-60 min after meal

-1 1 2 3 4 5 6 hrs

Efficiency of glucose disposition after a meal

The amount of glucose in a meal (~100 g) is enough to raise the blood glucose level 8-fold, but in a healthy person, glucose level rises only 60%!

Insulin level exhibits a much greater increase, from 60 to 400-500 pmol/l (6-8-fold!).

By the end of the post-absorptive period (~5 hrs), about 25 g of the carbohydrate ingested will have been stored as glycogen, and 75 g oxidized.

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Glucose

Insulinmeals

Blood glucose levels are relatively constant

Breakfast: Action of glucose in the -cell

Insulin secretion is stimulated as the glucose concentration rises above 5 mmol/l (the normal baseline concentration of glucose in the plasma).

before meal

after meal

Breakfast: Fate of glucose in muscle

GLUT4Glucose Glucose

Glucose-6-P

Hexokinase

Glycogensynthesis

Glycolysis

Insulin

+

Breakfast: Fate of glucose in adipocytes

GLUT4Glucose Glucose

Insulin

+

Glucose-6-P

Hexokinase

LPL

Insulin+

Glycerol-3-P

Triglycerides

Fatty acids

Insulin

-

Lipoproteins

Intracellular pool of GLUT4 in membranous vesicles translocate to the cell membrane when insulin binds to its receptor. The presence of more receptors increases the Vmax for glucose uptake (does not affect Km). When insulin signal is withdrawn, GLUT4 proteins return to their intracellular pool. GLUT4 is present in muscle and adipose tissue.

GLUT4 activity is regulated by insulin-dependent translocation

Gluconeogenesis

Glucose

Glycogen

adipocytesliver

muscle

Food consumption

Control of blood glucose requires cooperation between organs

liverliver

************************************************************

Definitions:Definitions:

Catabolism = the breakdown ofcomplex substances. Anabolism = the synthesis of complex substances from simpler ones. ***********************************************************

Glucose

Glucose-6-P

Pyruvate

Hexokinase

PentosePhosphateShunt

glycolysis

Carbohydrates• Serve as primary source of energy in the cell• Central to all metabolic processes

Glc-1- phosphate

glycogen

Cytosol - anaerobic

Pyruvatecytosol

Aceytl CoAmitochondria (aerobic)

Krebscycle

Reducingequivalents

OxidativePhosphorylation(ATP)

AMINOACIDS

FATTY ACIDS

No mitochondriaGlucoseGlucoseGlucose

The FullMonty

GlucoseGlycogenLactate

Carbohydrate Metabolism/ Utilization- Tissue Specificity

• Muscle – cardiac and skeletal– Oxidize glucose/produce and store glycogen (fed)– Breakdown glycogen (fasted state)– Shift to other fuels in fasting state (fatty acids)

• Adipose and liver– Glucose acetyl CoA– Glucose to glycerol for triglyceride synthesis– Liver releases glucose for other tissues

• Nervous system– Always use glucose except during extreme fasts

• Reproductive tract/mammary– Glucose required by fetus– Lactose major milk carbohydrate

• Red blood cells– No mitochondria– Oxidize glucose to lactate– Lactate returned to liver for Gluconeogenesis

Breakfast: Fate of glucose in the liver

GLUT2

Glucose

Glucose

Glucose-6-P

Glucokinase

Glycogensynthesis

Pentose phosphate

Glycolysis

Breakfast: Fate of glucose in muscle

GLUT4Glucose Glucose

Glucose-6-P

Hexokinase

Glycogensynthesis

Glycolysis

Insulin

+

Breakfast: Fate of glucose in adipocytes

GLUT4Glucose Glucose

Insulin

+

Glucose-6-P

Hexokinase

LPL

Insulin+

Glycerol-3-P

Triglycerides

Fatty acids

Insulin

-

Lipoproteins

Glucokinase vs. Hexokinase

Glucokinase: Km = 10 mM, not inhibited by glucose 6-phosphate. Present in liver and in pancreas cells.

Hexokinase: Km= 0.2 mM, inhibited by glucose 6-phosphate. Present in most cells.

Glucokinase vs. Hexokinase

• Glucokinase is also found in -cells of pancreas

• Glucokinase allows liver to respond to increasing blood glucose levels

• At low blood glucose levels, very little is taken up by liver, so that it is spared for other tissues.

• Glucokinase is not inhibited by glucose 6-phosphate, allowing accumulation in liver for storage as glycogen

• Glucokinase has a high Km, so it does not become saturated till very high levels of glucose are reached

• Hexokinase has a low Km and therefore can efficiently use low levels of glucose. But is quickly saturated.

Clinical example

• D.M., a 24-year-old, complaints were fatigue, weight loss, and increase in appetite, thirst, and frequency of urination. At 6 month before his visit he tired easily and tended to fall asleep in class, he had lost approximately 6.8 kg. His grandfather had had diabetes mellitus and his older sister was obese and had recently been diagnosed as having diabetes.

Diabetes mellitus and obesity

• What is the basis for the symptoms of the patient?

• Glucose tolerance test demonstrated in ability to handle a normal glucose load

• Glocoseuria• Familial history of diabetes• Increased appetite and excessive fluid intake

and fluid loss means his energy stores were being wasted and frequent urination was required for elimination of catabolic end products.

Glucose and insulin response in blood

• How does the response to insulin of the obese diabetic person compare with that of the nonobese diabetic person?

Polyol pathway

• What role does the polyol pathway play in disturbance of carbohydrate metabolism?

• Glucose reduced to sorbitol and can oxidase to fructose

• Sorbotol stay in high concentration in lens epithelium, the Schwann cell in peripheral nerve, the papillae in kidney and the islets of Langerhans in the pancreas make cataract and neuropathy

Summary: Glucose metabolism after a carbohydrate breakfast

Net glycogen storage in liver and muscle

In muscle, insulin enhances glucose uptake.

In adipose tissue, insulin prevents lipolysis, enhances glucose uptake, promotes fat storage

Glycolysis

• Conversion of 6-carbon glucose to 3-carbon pyruvate. Pyruvate is converted to lactate when oxygen is low.

• Glycolysis is anaerobic; aerobic metabolism of pyruvate takes place in the TCA cycle.

• Requires some investment of energy to produce ATP. ATP is produced to a much lesser extent than in oxidative phosphorylation. ATP produced can be important, especially in muscle.

• Occurs in cytosol, so resulting compounds must be transported to mitochondria for subsequent metabolism by TCA cycle.

Glycolysis requires investment of energy

The two phosphorylation steps require 2 ATP.

Allosteric enzyme, plays a critical role in regulation

Phosphorylation traps glucose in the cell

Glycolysis, continued

Two 3-carbon fragments are produced from one 6-carbon sugar.Thus far, 2 ATPs consumed, 0 ATPs produced.

Not directly in the glycolysis pathway; must be salvaged by isomerization to glyceraldehyde 3-P

Glycolysis, continued: generation of ATP

Oxidation of two 3-carbon fragments yields 4 ATP (net = 2ATP)

substrate levelphosphorylation

rearrangement

dehydration

The figure is found at http://www.nd.edu/~aseriann/dpg.html (March 2007)

Control points in glycolysis

hexokinaseGlucose-6-P -

*

Regulation of glycolysis

• Glycolytic flux is controlled by need for ATP and/or for intermediates formed by the pathway (e.g., for fatty acid synthesis).•Control occurs at sites of irreversible reactions

• Hexokinase or glucokinase • Phosphofructokinase- major control point; first enzyme “unique” to glycolysis•Pyruvate kinase

•Phosphofructokinase responds to changes in:• Energy state of the cell (high ATP levels inhibit)• H+ concentration (high lactate levels inhibit)• Availability of alternate fuels such as fatty acids, ketone bodies (high citrate levels inhibit)• Insulin/glucagon ratio in blood (high fructose 2,6-bisphosphate levels activate)

Why is phosphofructokinase, rather than hexokinase, the key

control point of glycolysis?

Glucose-6-phosphate has many functions. It is the start of

• glycolysis • glycogen synthesis• pentose phosphate pathway.

Phosphofructokinase (PFK-1) catalyzes the first unique and irreversible reaction in glycolysis.

The switch: Allosteric inhibition

Allosteric means “other site”

E

Active site

Allosteric site

© 2008 Paul Billiet ODWS

Switching off

• These enzymes have two receptor sites

• One site fits the substrate like other enzymes

• The other site fits an inhibitor molecule

Inhibitor fits into allosteric site

Substratecannot fit into the active site

Inhibitor molecule

© 2008 Paul Billiet ODWS

The allosteric site the enzyme “on-off” switch

E

Active site

Allosteric site empty

Substratefits into the active site

The inhibitor molecule is

absent

Conformational change

Inhibitor fits into allosteric

site

Substratecannot fit into the active site

Inhibitor molecule is present

E

© 2008 Paul Billiet ODWS

Phosphofructokinase

• This enzyme an active site for fructose-6-phosphate molecules to bind with another phosphate group

• It has an allosteric site for ATP molecules, the inhibitor

• When the cell consumes a lot of ATP the level of ATP in the cell falls

• No ATP binds to the allosteric site of phosphofructokinase

• The enzyme’s conformation (shape) changes and the active site accepts substrate molecules

© 2008 Paul Billiet ODWS

Maintaining redox balance

Since the cytosol has a limited amount of NAD+, newly formed NADH must be oxidized to regenerate NAD+ for glycolysis to continue.

Anaerobic - NADH is oxidized by conversion of pyruvate to lactate, catalyzed by lactate dehydrogenase

Aerobic - NADH is oxidized in mitochondria through further metabolism of acetyl CoA in the TCA cycle

pyruvate

lactate acetylCoA

Anaero

bic Aerobic

Fates of Pyruvate under AnaerobicConditions: Fermentation

Formation of acetyl CoAUnder aerobic conditions, pyruvate is not reduced to lactate, but decarboxylated to acetate, which links to Coenzyme A.

• Catalyzed by pyruvate dehydrogenase (PDH) multi-enzyme complex consisting of 3 catalytic subunits and several cofactors. • PDH is directly inhibited by NADH, acetyl CoA, and ATP. • PDH exists in phosphorylated (inactive) and dephosphorylated (active) states. Insulin stimulates dephosphorylation.

PDH PDH-PO4 (active) (inactive)

Protein kinase

Phosphatase

Insulin +

Figure 17–5. Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex. Lipoic acid is joined by an amide link to a lysine residue of the transacetylase component of the enzyme complex. It forms a long flexible arm, allowing the lipoic acid prosthetic group to rotate sequentially between the active sites of each of the enzymes of the complex. (NAD+, nicotinamide adenine dinucleotide; FAD, flavin adenine dinucleotide; TDP, thiamindiphosphate.)

Clinical example

• A full-term male infant failed to gain weight, had episodes of vomiting and showed metabolic acidosis in the neonatal period. A physical examination at 8 mo showed failure to thrive, hypotonia, small muscle mass, severe head leg, and a persistent acidosis, pH 7 to 7.2. Blood lactate (9mmol/L), pyruvate (2.4 mmol/L), and alanin(1.36 mmol/L) were greatly elevated.

Genetic defect in pyruvate dehydrogenase complex

• Why were the plasma concentration of pyruvate, lactate, and alanine abnormally high?

• Enzyme activity of the PDH complex, α- Ketodehydrogenase complex, and dihydrolipoyl dehydrogenase from sonicated fibroblasts grown in culture were are low when compared with enzymes from normal fibroblasts. Explain how these finding happening?

Overview of citric acid cycle(TCA or Krebs cycle)

Oxidation of two-carbon units, producing 2 CO2, 1 GTP, and high-energy electrons in the form of NADH and FADH2.

Mitochondrialmatrix

citrate

Citric acid cycle

Citrate

Succinyl-CoAsynthetase

AconitaseCitrate synthase

Succinatedehydrogenase GTP GDP

Oxaloacetate

Pyruvate

Aconitase

Isocitratedehydrogenase

Isocitratedehydrogenase

-Ketoglutaratedehydrogenase

Succinyl-CoA

-Ketoglutarate

Malatedehydrogenase

Pyruvatedehydrogenase

Fumarase

cis-Aconitate

Isocitrate

Oxalosuccinate

Succinate

Fumarate

Malate

FAD

FADH2

NADNADH

CO2

NADNADH

CO2

NAD

NADH

H2O

H2O

H2ONAD NADHCO2

For reference only

Control points in the citric acid cycle

Rate is adjusted to meet the cell’s need for ATP. Three allosteric enzyme control points:

PDH - inhibited by NADH, acetyl CoA, and ATP.

Isocitrate dehydrogenase - stimulated by ADP; inhibited by ATP and NADH

a-ketoglutarate dehydrogenase—inhibited by NADH, succinyl CoA, high energy charge.

Citric Acid Cycle and Oxidative

PhosphorylationGlycolysis harvests

only a fraction of the ATP available from glucose. Complete oxidation to CO2 takes place in the citric acid cycle.

In oxidative phosphorylation, electrons removed in oxidation reduce O2 to generate a proton gradient and synthesize large amounts of ATP.

Anaerobic

Aerobic

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Insulin

Glucose metabolism: Lunch

Glycogen synthesis in liver and muscle continue with little lag; storage in adipose tissue will continue. Changes are rapid due to previous induction of glucose- and insulin-regulated genes.

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Post-absorptive state

Glucose metabolism: Post-absorptive state

Glucose metabolism: Post-absorptive state

Post-absorptive state—the last meal has been absorbed from the intestinal tract, as after an overnight fast

Glucose levels ~ 5 mmol/lInsulin levels ~ 60 pmol/lGlucagon levels ~ 20 pmol/l

Glucose enters blood almost exclusively from the liver—about one-third from glycogen breakdown, and two-thirds from gluconeogenesis.

Insulin/glucagon ratio

Glucose

Glycolysis

Post-absorptive state: glucose utilization by muscle

Pyruvate

Alanine

Lactateto Liver

from Liver

Gluconeogenesis

Gluconeogenesis• Mechanism to maintain adequate glucose levels in tissues, especially in brain (brain uses 120 g of the 160g of glucose needed daily). Erythrocytes also require glucose.

• Occurs mostly in liver (90%) and kidney (10%)

• Glucose is synthesized from non-carbohydrate precursors derived from muscle, adipose tissue: pyruvate and lactate (60%), amino acids (20%), glycerol (20%)

Gluconeogenesis takes energy and is regulatedConverts pyruvate to glucose

Gluconeogenesis is NOT simply the reverse of glycolysis; it utilizes unique enzymes (pyruvate carboxylase, PEPCK, fructose-1,6-bisphosphatase, and glucose-6-phosphatase) for irreversible reactions.

6 ATP equivalents are consumed in synthesizing 1 glucose from pyruvate in this pathway

hexokinaseGlucose-6-P - Glucose 6-phosphatase

Irreversible steps in gluconeogenesis

• First step by a gluconeogenic-specific enzyme occurs in mitochondria

pyruvate oxaloacetate

Pyruvate carboxylase

• Oxaloacetate is reduced to malate so that it can be transported to the cytosol. In the cytosol, oxaloacetate is then decarboxylated/phosphorylated by PEPCK (phosphoenolpyruvate carboxykinase), a second enzyme unique to gluconeogenesis.

The resulting phosphoenol pyruvate is metabolized by glycolysis enzymes in reverse, until the next irreversible step

Irreversible steps in gluconeogenesis (continued)

• Fructose 1,6-bisphosphate + H2O

fructose-6-phosphate + Pi

Fructose 1,6-bisphosphatase

• In liver, glucose-6-phosphate can be dephosphorylated to glucose, which is released and transported to other tissues. This reaction occurs in the lumen of the endoplasmic reticulum.

Requires 5 proteins!

2) Ca-binding stabilizing protein (SP)

1) G-6-P transporter

3) G-6-Pase4) Glucose transporter5) Pi transporter

Post-absorptive state: glucose production by liver

Glucose

Glycogenolysis Gluconeogenesis

Glucose

Lactate AlaninePeripheraltissues

Glycerol

Glucose metabolism: Post-absorptive state

Substrate cycles between tissues provide substrates for gluconeogenesis in liver. This requires incomplete oxidation of glucose in tissues such as muscle and blood cells.

Substrates for gluconeogenesis:Lactate—60% (muscle, blood cells)Alanine—20% (muscle)Glycerol—20% (adipose tissue)

Cori Cycle—Lactate released as end product of glycolysis in peripheral tissue is returned to the liver for gluconeogenesis.

Alanine Cycle—Amino groups derived from proteolysis followed by TCA cycle are transferred to pyruvate, giving rise to alanine. Alanine is used for gluconeogenesis in liver.

Cooperation between peripheral tissues and liver to maintain blood glucose level (alanine and Cori cycles)

MovementActive transport

Signal amplificationBiosynthesis

Oxidation of fuel

molecules

High energy charge inhibits catabolic pathways and stimulate anabolic pathways

How is metabolism regulated?

(anabolic)

(catabolic)

How is metabolism regulated?

Fast mechanisms, for immediate changes

Substrate concentrationAllosteric regulation (feedback, feed forward)Phosphorylation-dephosphorylationSignals emanating from hormone action

Slow mechanisms, for long-term changes

Genetic regulationResponse to diet and other environmental variables

long term effects

How is metabolism regulated?

Rapid effect

Rapid effects

Phosphofructokinase (PFK-1) as a regulator of glycolysis

fructose-6-phosphate fructose-1,6-bisphosphatePFK-1

PFK-1 is allosterically inhibited by:

• High ATP: lowers affinity for fructose-6-phosphate by binding to a regulatory site distinct from catalytic site.• High H+: reduces activity to prevent excessive lactic acid formation and drop in blood pH (acidosis).• Citrate: signals ample biosynthetic precursors and availability of fatty acids or ketone bodies for oxidation.

Phosphofructokinase (PFK-1) as a regulator of glycolysis

PFK-1 is also activated by:Fructose-2,6-bisphosphate (F-2,6-P2)

F-6-P

F-1,6-P2

F-2,6-P2

glycolysis

+

PFK-2

PFK-1

Activates PFK-1 by increasing its affinity for fructose-6-phosphate and diminishing the inhibitory effect of ATP.

F-2,6-P2

Phosphofructokinase-2 (PFK-2) is also a phosphatase (bifunctional

enzyme)

fructose-6-phosphate fructose-2,6-bisphosphate

phosphatase

kinase

ATP ADP

Pi

Phosphorylation of bifunctional enzyme • decreases kinase activity • activates phosphatase

Hormonal control of F-2,6-P2 levels and glycolysis

Hormonal regulation of bifunctional enzyme

• Glucagon increases cAMP levels in liver, activates cAMP-dependent protein kinase, which phosphorylates PFK2, decreases F-2,6-P inhibits glycolysis

• Insulin decreases cAMP, increases F-2,6-P stimulates glycolysis.

Phosphorylation of PFK2activates its phosphatase activity

En

erg

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tak

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Daily energy intake vs. output

We need a mechanism to store food energy and release it when it is needed.

0 4 8 12 16 20 240

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Glycogen

Energy storage forms

Triacylglycerol (fat)

GlucoseFatty acid

Energy Reserves of Humans

(kcal)(g) 280 70LiverGlycogen

24,000 6,000MuscleProtein

135,00015,000AdiposeFat

480 120MuscleGlycogen

Fuel ReservesTissueFuel

Energy Reserves of Humans

kcal/g

4

9

4

~24 hr supply for body (brain)

Not available for export as glucose

Glucose storage as glycogen

• Glycogen is a multi-branched glucose polymer with up to 60,000 glucose residues. Glucose residues linked 1,4 in linear chains and 1,6 at branch points. Molecular weight in the millions.• Stored in liver and muscle as cytoplasmic granules—amount varies depending on time and size of recent meals• Valuable as a storage form because it is a readily mobilized form of glucose• Glycogen breakdown (glycogenolysis) and glycogen synthesis occur by separate pathways.

Schematic representation of glycogen molecule Glycogen granules in liver

Protein

GlycogenolysisGlycogen phosphorylase—sequentially removes glucose from ends of glycogen chains by phosphorolytic cleavage. This produces glucose that is already phosphorylated. Additional enzymes are required for ‘debranching’.

Phosphoglucomutase— catalyzes a shift in the phosphate group from C-1 to C-6. Reaction is reversible.In liver, glucose 6-phosphate can be cleaved to glucose by glucose 6-phosphatase, to be released to blood and transported to other organs. Other pathways are available to all organs.

Glucose Pentose phosphateGlycolysis

Glycogen phosphorylase,phosphate

Phosphoglucomutase

Glycogen

Glucose 1-phosphate

Glucose 6-phosphate

Phosphogluco-mutase

Glycogen

Glucose-6-phosphate

Glucose-1-phosphate

Phosphorylase,phosphate

Glycogen

Glucose-6-phosphatase Glucose

Glycogen degradation: phosphorylase-1,4 linked glucose residues

in linear chains

-1,6 linked glucose residues at branch points

Glycogen

Transferaseα-1,6-glucosidase

Debranching enzyme

Glycogen

Glycogen

Transferaseα-1,6-glucosidase

Debranching enzyme

Glycogen degradation: debranching enzyme1 enzyme with 2 catalytic sites

UDP-glucose pyrophosphorylase

Phosphogluco-mutase

Glycogen

Glucose-6-phosphate

Glucose-1-phosphate

UDP-Glucose

UTP

Glycogen synthesis: glycogen synthase

Glycogenin

Glycogen synthaseH

UDP

Glycogen

Branchingenzyme

Glycogen synthesis: branching enzyme

Reciprocal regulation of glycogen synthesis and breakdown

Active forms of enzymes = ‘a’Inactive forms of enzymes = ‘b’

Glycogen

synthasea

UDP-Glucose

Synthase-Pb

• Activity of glycogen synthase and glycogen phosphorylase are regulated by phosphorylation/dephosphorylation.

• Phosphorylation activates glycogen phosphorylase, inactivates glycogen synthase. Catalyzed by special kinases.

• Dephosphorylation is catalyzed by protein phosphatase 1.

Phosphorylaseb

Glucose-1-phosphate

Phosphorylase-Pa

Reciprocal regulation of glycogen synthesis and breakdown

Mechanisms regulating glycogen synthesis and degradation are complex

Regulation by allosteric effectors• Glucose 6-phoshate activates glycogen synthase,

inhibits glycogen phosphorylase• ATP inhibits phosphorylase• Glucose inhibits phosphorylase (in liver)• Ca++ and AMP activate phosphorylase (in muscle)

Regulation by phosphorylation states • Cascade of reactions, starting with hormonal stimulation• Glucagon/epinephrine activate cAMP-activated protein kinase A, which activates “phosphorylase kinase”, which then phosphorylates glycogen phosphorylase • Effect of insulin opposite to that of glucagon: stimulates phosphatases

Glucose regulates liver glycogen metabolism

• In liver, phosphorylase a is inhibited allosterically by glucose.

• Glucose activates glycogen synthase (indirectly)

Hormonal stimulation of glycogenolysisGlucagon Epinephrine

Glucose 1-P

+

-

-Glycogen

+

cAMP

UDP-glucose

Glucose 6-P

ATPATP

Glucose

Pyruvate

Fat

+

liver

Pyruvate

muscle

muscle weakness; enlarged heart

Pompe disease (glycogenosis type II) is a lysosomal storage

disease

Lysosomal accumulation of glycogen(not epinephrine responsive because sequestered by lysosomal membrane)

How would you treat it?

Pompe disease (type II glycogen storage disease) is caused by deficiency of lysosomal a-glucosidase, which normally degrades glycogen in lysosomes.

Von Gierke disease (type I glycogen storage disease) is caused by inability to generate glucose from glucose 6-phosphate. That enzyme is key to keeping blood glucose level up during the overnight fast.

How would you treat it?

Glycogen storage diseasesFor reference only

Alternative fates of glucose in the cell

Glucose 6-phosphate

Glycogen 6-phosphogluconatepyruvate

Ribose 5-phosphate

Pentose phosphate pathway

• AKA “pentose shunt” or “hexose monophosphate” shunt

• Major Functions:• Synthesis of pentose sugars for DNA, RNA, ATP, NADH, FAD• Generate NADPH from NADP+ for biosynthetic reactions

• Minor Functions:• Interconversion of 3,4,5,6, and 7 carbon sugars•Generate glycolytic intermediates

• Rate is controlled by levels of NADP+

Glucose-6-P dehydrogenase

Clinical example

• Fauvism

Glucose 6-phosphate + NADP+

6-phosphoglucono--lactone + NADPH + H+

Glucose 6-phosphateDehydrogenase

Glucose 6-phosphate dehydrogenaseFirst step in pentose phosphate pathway:

Required for generation of NADPH in erythrocytes; deficiency leads to hemolytic anemia induced by drugs or infection. Cells cannot maintain reduced glutathione. G6PD deficiency affects over 200 million people. High incidence in some parts of the world suggests that it confers a selective advantage against the malaria parasite.

Heinz bodies in red cells represent denatured proteins (including hemoglobin)

lactonaseH20

6-phosphogluconate

Roles of NADPH

•Biosynthesis•Fatty acids•Cholesterol •Neurotransmitters•Nucleotides

-Glu—Cys—Gly

-Glu—Cys—Gly

S

S

-Glu—Cys—Gly

SHNADPH + NADP+

OxidizedGlutathione (GSH)

Reducedglutathione

• Detoxification• Reduction of oxidized GSH in erythrocytes:

Keeps hemoglobin iron in a ferrous state

Stabilizes erythrocyte membrane

+ H++

(disulfide form) (sulfhydryl form)

Pentose phosphate pathway

• AKA “pentose shunt” or “hexose monophosphate” shunt

• Major Functions:• Synthesis of pentose sugars for DNA, RNA, ATP, NADH, FAD• Generate NADPH from NADP+ for biosynthetic reactions

• Minor Functions:• Interconversion of 3,4,5,6, and 7 carbon sugars•Generate glycolytic intermediates

• Rate is controlled by levels of NADP+

Glucose-6-P dehydrogenase

Clinical example

• A child had nausea, vomiting, and symptoms of hypoglycemia: sweating, dizziness, and trembling. It was reported that these attacks occurred shortly after eating fruit or cane sugar. This child was below normal weight, had cirrhosis of liver, a normal glucose tolerance test, and reducing substances in the urine that did not glucose.

Clinical example • A boy with normal weight was born. From the

third day of life the child developed an increasing degree of jaundice and at the same time become indolent and difficult to feed. Between the 7th and 9th days, exchange blood transfusion was performed three times, but the serum bilirubin concentration still remained high. A positive test for reducing sugars was in urine. Hereditary galactosemia was then suspected and special tests was performed.