(L3) - Carbohydrate Metabolism Flashcards
What cell types exclusively use glucose as their fuel source?
-RBCs -Brain (non-starvation)
What enzyme is used for cells that lack mitochondria?
LACTATE
- critical for RBC that lack a mitochondria as they can replenish their NAD supply
- also important in cells that are in overworked muscles that lack O2
What is the net yield of glycolysis per molecule of glucose? L3 S13 LO1
-2 ATP -2 NADH -2 Pyruvate
What are the different types of glucose transporters, where are they found and what are their affinities? L3 S10-11 LO1
GLUT1: -found in all cells; especially high in RBCs and brain -high affinity (Km: 1mM)
GLUT2: -found in liver and pancreas -low specificity (Km: 10mM)
GLUT3: -found in neurons -high affinity (Km: 1mM)
-Glut 1-3 do passive transport
GLUT4: -found in skeletal muscle, adipose tissue, and heart -intermediate affinity (Km: 5mM) -INSULIN REGULATED vesicle fusion
Explain glut 4 placement
-insulin signaling causes the fusion of vesciles with the plasma membrane and placement and insertion of the transporter in the membrane.
What is the first phase of glycolysis and what occurs during it?
Investment -use 2 ATP to trap glucose Net reaction: Glucose + 2 ATP -> fructose 1,6-BP
What does hexokinase do? L3 S14 LO1
Converts glucose to glucose 6-P using ATP Found in all cells
Inhibited by glucose-6P (feedback inhibition)
What does glucokinase do? L3 S14 LO1
Converts glucose to glucose 6-P using ATP Found only in the liver and pancreas
No feedback inhibition by glucose 6-P
- stimulated by: Glucose, F-1-P, insulin
- inhibited by: glucagon and F-6-P
What does phosphofructokinase do? L3 S14 Describe its regulation step by F2,6 bp.
RATE LIMITING Converts fructose 6-P to fructose. 1,6-BP Inhibited by ATP and citrate. And stimulated by AMP and F 2,6 BIS phosphate.
What is the second phase of glycolysis and what occurs during it? L3 S15 LO1
Splitting -goes from 1 6C molecule to 2 3C molecules Net reaction: -Fructose 1,6-BP -> 2 Glyceraldehyde 3-P
What does aldolase do? L3 S15 LO1
Splits fructose 1,6-BP into DHAP and G3P
What does triose phosphate isomerase do? L3 S15
Interconverts DHAP and G3P
Glyceraldehyde 3 Phosphate is used in next step of glycolysis
What is the third phase of glycolysis and what occurs during it? L3 S16 LO1
Recoup/payoff -4ATP and 2NADH are generated -pyruvate is generated
Net reaction: -2 G3P + 4 ADP + 2 NAD+ -> 2 Pyruvate + 4 ATP + 2 NADH
What does glyceraldehyde 3-P dehydrogenase do? L3 S16 LO1
Converts G3P to 1,3-BPG Generates NADH
What does phosphoglycerate kinase do? L3 S16 LO1
Converts 1,3-BPG to 3-phosphoglycerate Generates ATP
What does pyruvate kinase do? L3 S16 LO1
Converts phosphoenolpyruvate to pyruvate Generate ATP
- Activated by F1,6BP and insulin
- Inhibited by ATP, Alanine, and glucagon
- Glycolysis is inhibited
- Phosphoenolpyruvate (PEP) enters gluconeogenesis
- High insulin: stimulates protein phosphatase, dephosphorylation of PK to activate it
- High glucagon: cAMP activates PKA, phosphorylation, PK inhibited.
What are the regulated steps of glycolysis? L3 S19 LO1
-hexokinase/glucokinase -phosphofructokinase-1 -pyruvate kinase
How is hexokinase/glucokinase regulated? L3 S20-21 LO1
Hexokinase is inhibited by G6P Glucokinase is minimally affected by G6P
How is PFK-1 regulated? L3 S22-23 LO1
- High insulin/low glucagon: activate protein phosphatases –> dephosphorylate PFK-2/FBPase-2 (triggers kinase activity), producing F2,6BP which activates PFK-1
- High glucagon/low insulin: induces high [cAMP], activate protein kinase A, phosphorylates PFK-2/FBPase-2 (triggers phosphorylation activity), reduces PFK-1 activity.
- How is pyruvate kinase regulated? L3 S25 LO1
Activated by F1,6BP and insulin (activation of phosphatases)
Inhibited by ATP, alanine and glucagon (activation of PKA)
What is Tarui disease? L3 S24 LO1
GSD VII Deficiency in PFK-1 (rate limiting enzyme of glycolysis). Causes hemolytic anemia and high bilirubin and jaundice.
What are the possible fates of G6P? L3 S27-28
- Glycolysis (pyruvate)
- Glycogen synthesis (glycogen (convert to G-1-P) )
- galatose matabolism ((convert to G-1-P))
- Pentosephosphate pathway (ribose)
Glycolysis steps
1) Glucose + ATP –> G-6-P + ADP + H+ via hexokinase or glucokinase
IRR
3) f-6-p + atp –> fructose 1,6 bisohspate + ADP + H+ via PFK1
RLS IRR
6) Glyceraldehyde 3 phosphate + Pi + NAD –> 1,3 bisphosphoglycerate + NADH + H+ via glycerate 3 phosphate dehydrogenase
7) 1,3 bisphosphoglycerate + ADP –> 3 phosphoglycerate + ATP
via phosphoglycerate kinase
10 PEP + ADP –> pyruvate + ATP
by pyruvate kinase
IRR
Hexokinase vs Glucokinase
hexokinase is (↓ Km) but ↓ capacity (↓ Vmax) in almost all tissues, substrate is glucose and other sugars, inhibited by g-6-p
glucokinase is ↓ affinity but ↑ Vmax, pancreatic β-cells and liver, substrate is glucose only, weakly inhibited by G-6-P
How do defects in glycolysis affect RBCs? L3 S31-32 LO1
Glycolysis is only means of ATP production for RBCs Failure of glycolysis leads to hemolytic anemia.
What is Fanconi-Bickel syndrome? L3 S37 LO1
Mutation in GLUT2 transporter (liver and pancreatic β cells
- Unable to take up glucose, fructose and galactose
- autosomal recessive
gluconeogenesis steps
1) Pyruvate + 2 CO2 + 2 ATP + 2 H2O –> OAA + ADP + Pi + 2H+ via pyruvate carboxylase
- in mitochondria
3) OAA + GTP <–> PEP + GDP + CO2 via Phosphoenolpyruvate carboxykinase (PEPCK)
(THESE STEPS BYPASS PK)
5) 3 PHOSPHOGLYCERATE + ATP <–> 1,3 bisphosphglycerate + ADP via phosphoglycerate kianse
6) 1,3 bisphosphoglycerate + NADH + H+ <—> G-3-P + NAD+ VIA glyceraldehyde dehydrogenase
9) fructose 1,6 bisphosphate + H20 –> F-6-P via fructose 1,6 bisphosphatase
(bypass step 3)
10) g-6-p to glucose via glucose 6 phosphatase
What factors regulate glycolysis and gluconeogenesis? L3 S44 LO2
- High energy signals (ATP, glucagon) inhibit glycolysis and stimulate gluconeogenensis
- Low energy signals (ADP/AMP, insulin) stimulate glycolysis and inhibit gluconeogenesis
What does pyruvate carboxylase do? How is it regulated, and what cofator is needed with it?
Converts pyruvate to oxaloacetate
- stimulated by acetyl CoA and cortisol
-inhibted by ADP
- it is a mitochondrial enzyme and is ATP and Co2 dependent
- requires a Biotin cofactor
How do you get OAA out of the mitochondria?
–Mitochondrial membrane impermeable to oxaloacetate (OAA)
–OAA reduced to malate by mitochondrial malate dehydrogenase
–Malate transported to cytoplasm via malate shuttle
–Re-oxidized to OAA by cytosolic malate dehydrogenase
What does phsophoenolpyruvate carboxykinase do? L3 S47 LO2 What is it regulated by?
Converts oxaloacetate to PEP using GTP
regulated by: trancription activated by cortisol, glucagon, and thyronine
What does fructose 1,6-bisphosphatase do? L3 S47 LO2
RATE LIMITING Converts fructose 1,6-bisphosphatase to fructose 6-P
- stimulated by citrate and cortisol
- inhibited by AMP F,2,6BP
What does glucose 6-phosphatase do? Where is it found? What regulates it?
Dephosphorylates G6P to glucose ONLY found in liver, kidneys, SI, and pancreas.
-stimulated by cortisol
What is GLUT7? L3 S48 LO2
- Removes glucose from the ER after being produced by glucose 6-phosphatase
- Explained:
- glucose 6-phosphatase, is located in the lumen of the endoplasmic reticulum
- -Glucose 6-phosphatase has 3 subunits: a catalytic unit, a glucose 6-phosphate and Pi antiporter, and a glucose transporter (GLUT7)
- -Glucose 6-phosphate cleaved to form glucose. Then it is is transported into the ER by the G6P transporter, and glucose transported back to the cytoplasm by the GLUT 7
What is the Cori cycle? L2 S49 LO2
Links the lactate produced from anaerobic glycolysis in RBC and exercising muscle to gluconeogenesis in liver.
It does two things:
- Prevents lactate accumulation
- Regenerates glucose
What is Von Gierke disease? L3 S54 L2
GSD1a Deficiency in glucose 6-phosphatase
-Results in inefficient release of glucose from the liver and build of of stored glycogen in liver
What is the significance of fructose metabolism on glycolysis? L3 S63 LO3
Fructose enters glycolysis as G3P, after rate limiting step. High consumption of fructose is linked to obesity because of this.
What is the significance of galactosemia? L3 S65 LO3
Deficiency in glucose 1P uridyltransferase (GALT): classical
- Leads to accumulation of galactitol
- Classic galactosemia: Failure to thrive,
Deficiency in Galactokinase
•Nonclassical variant (Type II): Leads to accumulation of galactose and galactitol in blood and urine
-accumulation of galactitol in lens of eyes leads to cataracts
What is the rate limiting step of the PPP? L3 S69-70 LO4
Glucose 6-phosphate dehydrognase
What is the first step of the PPP? L3 S68 LO4
Oxidative phase Contains rate limiting step Produces NADPH and ribulose 6-phosphate
What is the second step of the PPP? L3 S68 LO4
Non-oxidative Products shunt to glycolysis/gluconeogenesis or nucleotide synthesis
What are the main products of PPP and what are they used for? L3 S73 LO4
Ribose 5-P: -used in nucleotide synthesis NADPH: -Used as a reducing agent in anabolic reactions
How do fed and fasting states regulate glycogenesis and glycogenolysis? L3 S87 LO5
Fed state: -high insulin and glucose -trigger dephosphorylation of glycogen synthase (activating) and glycogen phosphorylase (inactivating) Starving state: -high glucagon, low glucose -triggers phosphorylation of glycogen synthase (inactivating) and glycogen phosphorylase (activating)
Explain how gluokinase is regulated
- F6P production promotes it to be translocated to the nucleus and is sequestered by GK regulatory protein
- glucose and fuctose 1 phosphate will promote disociation and then it is transolcated back into the cytoplasm
- insulin induces synthesis of GK and glucagon inhibits synthesis.
Defects in glycolytic enzymes cause what?
Ineffecctive glycoysis, most defects will cause hemolytic anemia bc RBC do not have mitochondrias.
-failure of Glycolysis will disrupt ion gradients powered by ATP which reduces cell viability that will edstroy and then cause hemolytic anemia.
Hemolytic anemia
Decribe the effect of the brain and glucose
- Brain cells are particularly dependent on glucose because glucose is the preferred fuel for neurons and one of the only fuel molecule that can cross the blood brain barrier (BBB).
- during fasting and mild starvation you will start gluconeogensis
- during extreme starvation brain cells will start making ketone bodies for fuel (beta hydroxybutyrate)
what are clinical markers for hemolytic anemia?
- elevated lactate dehyrogenase in plasma and unconjugated bilirubin
- caused as ATP dependent ion pumps are not working thus Na builds up which causes swelling and lysis
Gluconeogenesis occurs where?
occurs in liver, kidney, and SI
-precursors are lactate, AA, and glycerol
List the precursors of gluconeogenesis
- fructose
- galactose
- glycogen
- glycerol
- propionate
- lactate
- alanine
- amino acids except leucine and lysine
(FGGGPLA)
F1,6 bisphosphatase deficiency
- Similar to Tarui disease in glycolysis
- Presents in infancy or early childhood
- Hypoglycemia, lactic acidosis, ketosis, apnea, hyperventilation (after fructose, glycerol or sorbitol)
Describe the absorption of fructose, galactose and glucose
- Fructose uptake is by GLUT 5
- glucose and galactose uptake is by SGLT 1 VIA SECONDARY ACTIVE TRANSPORT
-glut 2 will then transport all three into the blood and glut 5 will just transfer fructose again
Polyol Pathway
2 step pathway that produces glucose to fructose (how sperm makes fructose because it relies on that)
- Glucose reduced to sorbitol by Aldose reductase (NADPH –> NADP+)
- Sorbitol oxidized to fructose by Sorbitol dehydrogenase (NAD –> NADH)
Decribe Sx of Sorbitol accumulation
- Cells that lack sorbitol dehydrogenase (kidneys, retina, Schwann cells) can accumulate sorbitol, which triggers water influx and causes swelling.
- Manifests as retinopathy, cataracts and peripheral neuropathy
Fructose metabolism steps
1) fructose —> F-6-P (ENTERS GLYCOLYSIS) via hexokinase
1. a) fructose + ATP –> F-1-P + adp via fructokinase
2) f-1-p –> Glyceraldehyde and DHAP
via aldose B (found in liver) (deficiency is called heridtary fructose intolerance)
…. feed into triacylglyceol synthesis
Why is fructose metabolism faster?
- Bypasses rate limiting step of glycolysis
- Due to absence of PFK-1
- They feed into glycolysis at an un checked fashion
- the excess a-coa will be convereted into FA causing obesity
- liver will also accumulate FA resulting in fatty liver
Galactose metabolism
1) Galactose can turn into galataciol via aldose reducatse
1. a) Galatose + ATP –> Galatose 1 phosphate + ADP via galatokinase
2) galatose 1-p + udp glucose —> Glucose 1 p + udp galatose via Glucose 1P uridyltransferase (GALT)
What is the RLS of galatose metabolism
glucose 1P uridyltransferase (GALT)
What does a defiecieny of aldose B cause?
hereditary fructose intolerance
What is cateracts caused by?
Cataract is the clouding of the normally clear lens of the eye.
If the transferase is not active in the lens of the eye, the presence of aldose reductase causes the accumulating galactose to be reduced to galactitol.
PPP overview
–Produces no energy
–Produces the sugar for DNA and RNA formation
–Oxidation of G6P to Ribulose 5-P
–Reduction of NADP+ to NADPH
–Irreversible oxidative step (catabolic) and
–reversible non-oxidative step (anabolic)
PPP – Oxidative Phase
–2 enzymatic steps, produce 2 NADPH and 1 CO2
1) G6P dehydrogenase: rate-limiting step, reduces NADP+ to NADPH, oxidizes G6P to 6-phosphoglucono-δ-lactone
(inhibited by NADPH)
- Form ribulose 5P via 6-Phosphogluconate dehydrogenase (decarboxylation):
- Reduces NADP+ to NADPH
- Removes a carbon and forms CO2
G6PD deficiency
affects a significant population, particularly those of African descent. Presentation of hemolytic anemia when NADPH need is elevated (infection, oxidizing medications).
What does NADPH help for
regenerating an antioxidant
–NADPH regenerates glutathione (G-SH), an important antioxidant, detoxifies H2O2 with glutathione reductase
PPP – Non-oxidative Phase
- A series of reversible reactions
- End products shunt to glycolytic, gluconeogenic or nucleotide synthesis pathways
- this is where ribose can be formed
When different metabolites are needed from the PPP what happen?
- high demand of Ribose 5P: ox phase is favored to produce ribulose 5 phosphate
- high demand for NADPH non ox products channeled into gluconeogensis for re entry into PPP
Glycogen structure
.Glucose molecules within chain linked together via α-1,4 glycosidic bonds
- Branch points formed via α-1,6 glycosidic bonds between glucose monomers of separate chains
- Non-reducing ends each contain a terminal glucose with a free hydroxyl group at Carbon 4
- Reducing end consists of glucose monomer connected to a protein called glycogenin
- Glycogenin creates a short glycogen polymer on itself and serves as a primer for glycogen synthesis
- Glycogen is degraded and extended from non-reducing end.
Glycogen is stored in
liver and muslces as granules. The granules contain enzymes needed for glycogen metabolism.
Functions of glycogen
- Liver glycogen - regulates blood glucose levels: In liver Glu-1-P converted to Glu-6-P by an epimerase and then to Glu by glucose-6-phosphatase. Free glucose released into blood stream.
- Muscle glycogen - provides reservoir of fuel (glucose) for physical activity, as there is NO G-6-P phopspatase. Muscles use it to generate energy via glycolysis and the TCA cycle.
Glycogenesis steps
- Trapping and Activation of Glucose: Glucokinase/hexokinase in cytosol of hepatocytes and muscle makes glucose to glucose-6-phosphate
- Phosphoglucomutase then reversibly isomerizes glucose-6-phosphate to glucose-1-phosphate
- Uridine diphosphate(UDP)-glucose pyrophosphorylase then transfers the glucose-1-phosphate to uridine triphosphate (UTP) which generates UDP-glucose (active form of glucose)
2) elongation: Glycogen synthase (rate limiting enzyme). Catalyzes transfer of glucose from UDP-glucose to non-reducing end of glycogen chain. Forms α-1,4 glycosidic bonds between glucose molecules
3) Branching of glycogen chains:
When glycogen chain reaches 11 residues, a fragment of the chain (about 7 residues long) is broken off at an α -1, 4 link and reattached elsewhere via α -1, 6 link by glucosyl (4:6) transferase.
Why is glycogen branched?
•Branching increases solubility of glycogen and increases number of terminal non-reducing ends.
Glycogenolysis steps
- Chain shortening (release of Glu-1-P)
- Glycogen phosphorylase (GP) (rate limiting enzyme) catalyzes cleavage of glucose residues as a glucose-1-phosphate from non-reducing end of glycogen
–GP uses pyridoxal phosphate (vitamin B6) as cofactor
–Phosphorolysis continues till GP gets within 4 residues of α-1,6 linkage of a branch point.
- Branch transfer and release of glucose: debranching enzyme uses its transferase (4:4) activity to transfer a block of 3 of the remaining 4 glucose to the non-reducing end of the main chain forming an α-1,4 bond.
Then the enzymes cleaves the α-1,6 bond of the single remaining glucoseresidue to release free glucose.
NET: Generates Glu-1-P and free Glu in a ratio of 10:1
RLS for Glycogen metabolism
Glycogen synthase - the rate limiting step of synthes
–dephospho form active
–phospho form inactive
Glycogen phosphorylase - the rate limiting step of degradation
–dephospho form inactive
–phospho form active
When is glycogenesis favored?
When is glycogenolysis favored?
Glycogenesis:
-High blood glucose, insulin, and cellular ATP levels
Glucogenolysis:
-during fasting state (low blood gluose and glucagon is high) or during exersice (high cellular calcium levels and aMP levels).
Blood glucose levels
- normal: 70-100mg (fasting)
- < 140 fed
- pre dm (100-125 mg) fasting, >140 after eating
- DM 126 mg/dl, > 199mg (fed)
Glycogen Storage Diseases
0, Vigra, pills, cause, a, massive, hardon
-VON GIERKES, POMPE, CORI, ANDERSONS, MCARDLES, HERS
GSD 0:
- Deficiency in glycogen synthase
- Patients cannot synthesize and store glycogen
- Rely on glucose in diet
- Vulnerable to hypoglycemia when fasting (e.g., during sleep)
- Have muscle cramps due to lack of glycogen in muscle
- Need to eat frequently
GSD1a/Von Gierke disease
-Deficiency in glucose 6-phosphatase
- Inefficient release of free glucose into the bloodstream by the liver following gluconeogenesis and glycogenolysis.
- Patients exhibit marked fasting hypoglycemia, lactic acidosis, hepatomegaly due to buildup of glycogen, hyperlipidemia.
- Mutations in catalytic site of glucose 6-phosphatase
GSD II/Pompe Disease
Deficiency in Acid Maltase aka acid α -glucosidase
- Impairs lysosomal glycogenolysis resulting in accumulation of glycogen in lysosomes.
- Disrupts normal functioning of muscle and liver cells.
- Progressive muscle weakness (myopathy) in body including heart and skeletal muscle.
GSD III/Cori Disease
Deficiency in α-1,6,-glucosidase (debranching enzyme).
- Patients possess glycogen molecules with large number of short branches.
- Light hypoglycemia and hepatomegaly.
GSD IV/Andersen Disease
Deficiency in glucosyl (4:6) transferase (branching enzyme)
- Patients have long chain glycogen with fewer branches.
- Causes enlargement of liver and spleen, scarring of liver tissue (cirrhosis).
- Death by 5 years of age.
GSD V/McArdle Disease
Deficiency in muscle glycogen phosphorylase
- Rate limiting step of glycogen breakdown
- Patients unable to supply muscles with enough glucose
- Exercise intolerance
- Patients recommended to reduce strenuous exercise
Hers Disease
Deficiency in liver glycogen phosphorylase
- Prevents glycogen breakdown in liver, hence it accumulates in liver causing hepatomegaly.
- Low blood glucose levels.
PPP is high in which cells?
- lactating mammary glands
- lung and liver
- phagocytic cells
