Carbohydrate metabolism
Dec 27, 2015
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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Glucose
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
y in
tak
e(k
J/m
in)
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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Periods of exercise
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Snack
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.