Glycolysis Flashcards
What is the purpose of glycolysis?
1 molecule of glucose to 2 molecules of pyruvate and generates 2 molecules of ATP.
Where does glycolysis occur?
Occurs in cells that lack mitochondria (erythrocytes)
Important in overworked muscles that lack O2
Which parts of the body use glycolysis?
- Glucose the only fuel that red blood cells can use
- Glucose the only fuel that brain uses under non-starvation conditions
First part of glycolysis?
transport of glucose into the cell using a transporter
What type of GLUTS are there and what do they do?
- GLUT1: rbcs, high affinity
- GLUT2: liver, low affinity
- GLUT3: neurons, high affinity
- GLUT4 : skeletal tissue, heart, adipose tissue: insulin dependent
What is the significance of GLUT4?
- Insulin signaling, fusion of vesicles, insertion of transporter in membrane
- increases GLUT4 uptake
What are the three phases of Glycolysis?
- Investment
- Splitting
- Recoup/Payoff
Explain the three steps of Phase 1, what their enzymes are and what + and - them?
- Glucose to G6P (hexokinase or glucokinase)
* ATP
* Regulatory - G6P to F6P (phopsoglucose isomerase)
- F6P to F1,6BP (PFK1)
* ATP
* Rate limiting
What are the steps of Phase 2, their enzymes and what + and - them?
- F1,6BP forms DHAP and G3P (Aldolase A)
5. Isomerization of DHAP to G3P (Triose phosphate isomerase)
What are the steps of phase 3, what enzymes and what + and - them?
- Phosphorylation of G3P (GAPDH)
* NAD to NADH - 1,3BPG to 3PG (Phosphoglycerate kinase)
* ADP becomes ATP - Pyruvate formed (Pyruvate kinase)
- irreversible
* ADP becomes ATP
Checkpoints of Glycolysis , what are they? what are they influenced by?
- hexokinase/glucokinase
- PFK1
- pyruvate kinase
- influenced by ATP, AMP, Glucose, Insulin, Glucagon
Where is hexokinase present? whats special about it?
-in all cells
–Highaffinity, functional even at low [glucose]–Inhibited by G6P (product)
Where is glucokinase? whats special about it?
-Liver and pancreatic b cells
–Lowaffinity for glucose, Weakly inhibited by G6P
–Most active when high [glucose], i.e. after meal
–F6P promotes translocation to nucleus
–Sequestered by GK-Regulatory Protein
–High glucose promotes dissociation from GK-RP
–Followed by translocation to cytoplasm
–Insulin induces synthesis of GK and glucagon inhibits synthesis.
What is the rate limiting step in glycolysis and what activates it and stops it?
- Activated AMP, F2,6BP
- inhibited by ATP, citrate
What is F2,6BP and what does it do?
- PFK-2/FBPase-2 is a bifunctional enzyme that works as a kinase in its dephospho form and phosphatase in its phospho form.
- dephospho form favored by insulin
- phospho form favored by glucagon
How does F2,6BP work?
- .High insulin/low glucagon: activate protein phosphatases, dephosphorylate PFK-2/FBPase-2 producing F2,6BP which activates PFK-1
- High glucagon/low insulin: induces high [cAMP], activate protein kinase A, phosphorylates PFK-2/FBPase-2 , reduces PFK-1 activity.
What is Tarui Disease
-Deficiency in PFK-1
-–Exercise-induced muscle cramps and weakness
–Hemolytic anemia
–High bilirubin and jaundice
How is pyruvate kinase regulated?
- Activated by F1,6BP and insulin
- Inhibited by ATP, Alanine, and glucagon
- High insulin: stimulates protein phosphatase, dephosphorylation of PK, activate
- High glucagon: cAMP activates PKA, phosphorylation, PK inhibited.
What other pathways is G6P used for ?
- Pentose phosphate pathway
- galactose metabolism
- glycogen synthesis
- uronic acid pathway
What contributes to pyruvate? Where is pyruvate used?
- Glucose, tryptophan, threonine , alanine make pyruvate
- pyruvate makes Acetyl Coa, Alanine, Oxaloacetate, Lactate, Alanine
What is the only fuel that can cross the BBB?
Glucose
During extreme starvation, what does the brain use for fuel?
ketone bodies
What is hemolytic anemia ?
Premature death of RBCs, often due to issues with glycolysis
Clinical markers: elevated lactate dehydrogenase, unconjugated bilirubin
Fanconi-Bickel syndrome what is it?
mutation in GLUT 2 transporter, autosomal recessive
Unable to take up glucose, fructose and galactose
Treatment – vitamin D and phosphate, and uncooked corn starch
What is gluconeogenesis? where does it occur?
- makes glucose out of pyruvate
- occurs when glucose and glycogen stores are done
- liver, kidney, and small intestine
- Major precursors are lactate, amino acids, and glycerol
Which enzymes are used in gluconeogenesis to bypass glycolysis
- Pyruvate carboxylase
- Phosphoenolpyruvate carboxykinase
- Fructose 1,6-bisphosphatase
- Glucose 6-phosphatase
What are the + and - regulators of gluconeogenesis?
+ are Glucagon, citrate, cortisol, thyroxine, acetyl coa
-are ADP, AMP, Fru 2,6 BP
How is Pyruvate carboxylase regulated?
- Mitochondrial enzyme, mot others are in cytoplasm
- First step of gluconeogenesis, pyruvate forms oxaloacetate
- needs Biotin cofactor, uses CO2 and ATP
- Activated by Acetyl Coa and cortisol
How does the malate shuttle work?
- OAA reduced to malate by mitochondrial malate dehydrogenase
- malate sent to cytoplasm via malate shuttle
- reoxidized to OAA by malate dehydrogenase
What does PEPCK do? how is regulated?
makes oxaloacetate to PEP
activated by cortisol, glucagon, thyroxine
What is Fructose 1,6-Bisphophatase ? what does it do?
- rate limiting step of gluconeogenesis
- F1,6BP to F6P
–Activated: cortisol and citrate
–Inhibited: AMP and F26BP
What is Glucose 6-Phosphatase? what does it do?
-G6P becomes glucose
–Only in liver, kidneys, SI and pancreas
–Activated by cortisol
What is so special about G-6phosphatase?
- has 3 subunits: a catalytic unit, a glucose 6-phosphate and Pi antiporter, and a glucose transporter (GLUT7).
What is the cori cycle? why is it so special?
-Links the lactate produced from anaerobic glycolysis in RBC and exercising muscle to gluconeogenesis in liver
–Prevents lactate accumulation
–Regenerates glucose
What are the precursors of gluconeogenesis?
Carbs, lipids, proteins
What is F1,6BP deficiency? what does it do?
-Hypoglycemia, lactic acidosis, ketosis, apnea, hyperventilation
What is Von Gierke disease? what does it do?
- Deficiency in glucose 6-phosphatase
- Inefficient release of free glucose into the bloodstream by the liver in gluconeogenesis and glycogenolysis
- Patients exhibit marked fasting hypoglycemia, lactic acidosis, hepatomegaly due to buildup of glycogen, hyperlipidemia and potentially retarded growth
What is Glut 5 used to uptake?
Fructose
What is SGLT 1 used to uptake?
glucose/galactose
What can a lack of sorbitol dehydrogenase cost?
sorbitol won’t become fructose, cataracts
What are the steps in galactose metabolism?
Galactose to Galactose1P via Galactokinase
Galactose1P to Glucose1P via GALT
What is lactose intolerance?
- Drinking milk causes disturbances in GI function
- Caused by deficiency in enzyme lactase
Why can fructose lead to pathological conditions?
- Actions of fructokinase and triose kinase bypass the most important regulatory step in glycolysis, the phosphofructokinase-catalyzed reaction
- Fructose-derived G3P and DHAP are processed by glycolysis to pyruvate and acetyl CoA in an unregulated fashion.
- Excess acetyl CoA converted to fatty acids, which can be transported to adipose tissue to form triacylglycerols, resulting in obesity
- Fatty liver
Deficiency in glucose 1P uridyltransferase (GALT) what is it?
Leads to accumulation of galactitol
Deficiency in Galactokinase what is it ?
Leads to accumulation of galactose and galactitol in blood and urine
What does the Pentose Phosphate pathway do?Where does it occur?
–Produces no energy but metabolizes glucose
–Produces the sugar for DNA and RNA formation
–Produces NADPH
-Occurs in the cytosol
What are the two main things in PPP?What is the rate limiting step? What does it produce?
- oxidation of G6P to Ribulose 5 phosphate
- Reduction of NADP to NADPH
-rate limiting step is G6P to 6phosphogluconolactone (G6PD), makes 2 NADPH and 1 CO2
What does G6PD deficiency do?
Presentation of hemolytic anemia when NADPH need is elevated
in PPP, what is the purpose of gluthianone?
regenerates glutathione
detoxifies H2O2 with glutathione reductase
What is the non oxidative phase of the ppp? what steps are there?
- reversible reactions
- End products shunt to glycolytic, gluconeogenic or nucleotide synthesis pathways
- Carbon transfer via transketolase or transaldolase
How is PPP modified based on cellular demand?
- High demand of Ribose 5P: rapidly dividing cells oxidative phase favored to produce Ribulose 5P
- High demand for NADPH: non-oxidative products channeled into gluconeogensis for re-entry into PPP
What is the structure of glycogen?
- Glucose molecules within chain linked together via α-1,4 glycosidic bonds
- Branch points formed via α-1,6 glycosidic
- 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
On which end is glycogen degraded?
degraded and extendedfrom non-reducing end
Where is glycogen stored?
- in the liver, muscle as granules
What is the difference between liver and muscle glycogen?
- Liver glycogen - regulates blood glucose levels
- Muscle glycogen - provides reservoir of fuel (glucose) for physical activity
What are the first step of glycogenesis? Include enzymes and what is produced
- Glucose to G6P (Hexokinase/Glucokinase)
- ATP used
2.Glucose6P to Glucose1P (PGM)
- G1P to UDP Glucose (UDPglucosepyrophosphorylase)
- 2 UTP used
What are the second step of glycogenesis? Include enzymes and what is produced
Glycogen primer is elongated,
Glycogen synthase Catalyzes transfer of glucose from UDP-glucose to non-reducing end of glycogen chain
a1,4 bonds
What is the rate limiting step of glycogenosis?
glycogen synthase
UDP Glucose to Glycogen
What is the third step of glycogen synthase?
- branching of glycogen chains
- a α -1, 4 link and reattached elsewhere via α -1, 6 link by glucosyl (4:6) transferase
What are the two main steps of glycogenolysis? main enzymes and substrates
- Chain shortening :
- Glycogen phosphorylase, cleaves g1p subunits from reducing end of glycogen
- uses vitamin B6 as cofactor
- gets with 4 residues of a1,6 branching 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
Liver vs. muscle glu 1 p
- 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
- in muscle, s keletal and cardiac muscle lack glucose-6-phosphatase and can’t hydrolyze Glu-6-P. Use it to generate energy via glycolysis and TCA cycle
Why is the regulation of glycogen metabolism importnat?
- to maintain blood sugar
- to provide energy to muscle
what is the rate limiting step of glycogen synthesis? degradation?
glycogen synthase, glycogen phosphroylase
How does phosphorylation affect glycogen synthase and glycogen phosphorylase?
glycogen synthase:
–dephospho form active
–phospho form inactive
glycogen phosphorylase:
–dephospho form inactive
–phospho form active
When is glycogenesis favored?
–blood glucose high
–insulin high
–cellular ATP high
When is glycogenolysis favored?
–blood glucose low
–glucagon high
- cellular calcium is high
- AMP is high
How is glycogen synthase/phosphorylase regulated by insulin?
- Insulin binds to RTK
- PKB activated
- GLUT 4 translocated
- PKB phosphorylates PP1( activated) and GSK3( inactivated)
Active PP1 dephosphorylates glycogen synthase (activate) and dephosphorylates glycogen phosphorylase (inactivate)
How is glycogen synthase/phosphorylase regulated by glucagon?
Glucagon binds to GPCR on hepatocytes
- turns on G protein
- Activates AC which forms cAMP
- Activates PKA
- PKA phosphorylates glycogen synthase (inactivates)
- PKA phosphorylates PK (activate)
- PKA phosphorylates an inhibitor which inactivates PP1
- Active PK phosphorylates glycogen phosphorylase (activates)
Regulation by Epinephrine of glycogen?
–Fight or flight response
–Epinephrine released by adrenal glands
–Promotes degradation of glycogen
–Pathway similar to glucagon
What is GSD 0?
- Deficiency in glycogen synthase
- Patients cannot synthesize and store glycogen
- Have muscle cramps due to lack of glycogen in muscle
GSD1a/Von Gierke disease?
- Deficiency in glucose 6-phosphatase
- Inefficient release of free glucose into the bloodstream by the liver following gluconeogenesis and glycogenolysis
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
GSD III/Cori Disease?
- Deficiency in α-1,6,-glucosidase (debranching enzyme)
- Patients possess glycogen molecules with large number of short branches
GSD IV/Andersen Disease
- Deficiency in glucosyl (4:6) transferase (branching enzyme)
- Patients have long chain glycogen with fewer branches
GSD V/McArdle Disease?
- Deficiency in muscle glycogen phosphorylase
- Rate limiting step of glycogen breakdown
- Patients unable to supply muscles with enough glucose
GSD VI/Hers Disease?
- Deficiency in liver glycogen phosphorylase
- prevents glycogen breakdown in liver, hence it accumulates in liver causing hepatomegaly
