Glycolysis & Gluconeogenesis Flashcards
Glycolysis def
Breakdown of Glucose to 2 Pyruvate
Function of glycolysis
Produce energy in the form form of ATP
NADH in the cytoplasm is worth how many ATP
2
NADH in the mitochondria is worth how many ATP
3
Where does glycolysis occur?
Common in Prokaryotic and Eukaryotic Cells:
1) Occurs in cytosol of cytoplasm
- anaerobic conditions->fermentation producing lactic acid or ethanol
- aerobic conditions-> aerobic respiration in mitochondria (Krebs cycle)
2) ALL TISSUE
Net Production of ATP in Glycolysis
6 ATP
Hexokinase
Glycolysis
Glucose-> Glucose 6 Phosphate
-Phosphoryl transfer at the expense of ATP
-phosphorylation of glucose traps inside cell, because there aren’t any transporters that can transport phosphorylate glucose
-cofactor- Divalent Cation (Mg2+ or Mn2+)
-Exergonic
-Irreversible Rxn
Regulation:
Allosteric regulated by: Glucose 6-Phosphate inhibits(feedback inhibition)
Hormonal Regulation:
Stimulated in the fed state by hormone insulin
Inhibited in the fasting state by hormone glucagon
Broad Substrate Specificity-phosphorylates many hexoses
When Hexokinase binds to Glucose
causes conformation change (cleft closing) in hexokinase
- Active site around glucose becomes more nonpolar which favors donation of gamma phosphate
- Excludes water from active site, which prevents hydrolysis of gamma phosphate by H20
Example of Induced Fit
Substrate Induced Fit Cleft closing is a general feature of kinases
Hexokinase vs Glucokinase
Hexokinase:
Low Km=High Affinity for glucose
Km<0.1 mM which permits efficient metabolism of glucose
Glucokinase:
- AKA hexokinase D or type IV
- found in adult kidney and liver and senses glucose levels
- High Km=Low affinity for glucose, allowing brain and muscles to have first call on glucose
Phosphohexose Isomerase
Glycolysis
Or Phosphoglucose Isomerase
Glucose 6-Phosphate-> Fructose 6-Phosphate
1) Reaction Type: Isomerization- conversion of aldose C-1 to Ketose C-2
2) Helper Molecules: NONE
3) Exergonic
4) Reversible Reaction
5) No regulation
Phosphofructose Kinase-1
Glycolysis
Fructose 6-Phosphate-> Frucose 1,6-Bisphosphate
MOST IMPORTANT CONTROL POINT OF METABOLISM
1) Reaction Type: Phosphoryl Transfer At the expense of ATP
2) No helper molecules
3) Exergonic
4) Irreversible
REGULATED:
Allosteric: regulated by energy charge
-Stimulated by Fructose 2,6 Bisphosphate and AMP
-Inhibited by ATP, citrate, and H+
Phosphofructose Kinase-2 (PFK-2)
synthesis of Fructose 2,6 Bisphosphate which stimulates PFK1 in glycolysis and inactivates gluconeogenesis
Adenylate Kinase
ADP +ADP -> ATP + AMP
salvages ATP from two ADP molecules
-primary reason AMP represents “low energy” charge
Aldolase A
Glycolysis
Fructose 1,6-Bisphosphate-> DHAP and Glyceraldehyde 3-Phosphate
1) Reaction Type: Aldol Cleavage
2) Helper molecules: NONE
3) Exergonic
4) Reversible
5) NOT REGULATED
Triose Phosphate Isomerase
Glycolysis **
Dihyroxyacetone Phosphate Glyceraldehye 3-Phosphate
at equilibrium 96% exist in DHAP 4% GAP
1) Reaction Type: Isomerization
2) Helper Molecules: NONE
3) ENDERGONIC
4) Reversible
NOT REGULATED
Phosphoglyceraldehyde Dehdyrogenase
Glycolysis
OR Glyceraldehyde 3-Phosphate Dehydrogenase
Glyceraldehyde 3-Phosphate-> 1,3-Bisphosphoglycerate
1) Reaction type: Phosphorylation coupled to oxidation of aldehyde to carboxylic acid at the expense of NAD+ 2) Helper molecule-coenzyme-NAD+ 3)Exergonic 4) Reversible 5) NOT REGULATED
3-Bisphosphoglycerate Kinase
Glycolysis **
OR Phosphoglycerate Kinase
1,3 Bisphosphoglycerate -> 3-Phosphoglycerate
1) Reaction Type: Transfer of Phosphoryl group from 1,3-BPG to ADP to regenerate ATP since 1,3-BPG has higher phosphoryl potential than ATP
* SUBSTRATE LEVEL PHOSPHORYLATION
2) Helper Molecules-NONE
3) ENDERGONIC
4) REVERSIBLE-unusal for kinases
5) NOT REGULATED
Phosphoglyceromutase
Glycolysis **
OR Phosphoglycerate Mutase
3-Phosphoglycerate-> 2-phosphoglycerate
1) Reaction type-Phosphoryl shift from C3 to C2
2) Helper Molecules-NONE
3) Endergonic
4) Reversible
5) NOT REGULATED
Enolase
Glycolysis
2-phosphoglycerate-> Phosphoenolpyruvate
1) Reaction type-dehydration
2) Helper molecules; NONE
3) Exergonic
4) reversible
5) NOT REGULATED
Pyruvate Kinase
Glycolysis (LAST STEP)
PEP-> Pyruvate
1) Reaction Type: Phosphoryl Transfer from PEP to ADP to regenerate to ATP since PEP has higher phosphorylation potential than ATP
2) Helper Molecules-NONE
3) Exergonic
4) Irreversible
REGULATED:
Allosterically regulated:
-Stimulated by Fructose 1,6-BP in feedforward stimulation
-Inhibited by ATP and Alanine
IN LIVER:
-inactivated by cAMP dependent Protein Kinase A; low blood glucose=increase glucagon which pyruvate kinase is phosphorylated and inactivated
Pyruvate Kinase Deficiency in RBCs
RBC lack mitochondria thus lack Pyruvate oxidation, Krebs cycle, and Electron Transport chain Thus depends on glycolysis for ATP production
PK deficiency causes change in shape of RBC due to insufficient energy production causing chronic hemolytic fever
3 fates of pyruvate post glycolysis
Fermentation:
- anaerobic conditions
- cytoplasm
1) Lactic acid-higher eukaryotes
2) ethanol-microorganisms
Pyruvate Oxidation
- aerobic conditions
- matrix of mitochondria
Pyruvate: Ethanol Fate
Fermentation-anerobic conditions
-occurs in cytoplasm
Pyruvate-> Acetaldehyde + CO2 -Enzyme: Pyruvate Decarboxylase Reaction type: decarboxylation -Helper Molecule: Prosthetic group-Thiamine pyrophosphate -Reversible -Exergonic
Acetaldehyde-> Ethanol Enzyme: Alcohol dehydrogenase Reaction type: oxidation Helper molecule- Zn2+-cofactor; NADH coenzyme exergonic REGENERATE NAD+
Pyruvate: Lactate Fate
Pyruvate-> Lactate
Enzyme: Lactate dehydrogenase
Lactate + Exercise
formation of lactate reduces pH potentially leading to cramps
-lactate diffuses into blood and can be used to make glucose in liver
Lactate + Heart
Heart gathers lactate from blood and converts to pyruvate
Rossmann Folds
NAD+ binding site are similar in:
- Glyceraldehdye 3-phosphate dehydrogenase
- Lactate Dehydrogenase
- Alcohol Dehydrogenase
Composed of:
- 4 alpha helixes
- 6 parallel beta strands
In the liver, Glycolysis is involved in:
- maintenance of blood glucose levels
- synthesizes glycogen to store glucose when glucose concentration is high
- generates intermediates for biosynthesis
Glycolysis Regulation: resting muscles
Glycolysis is inhibited by High Energy
Resting muscles have High Energy Charge (ATP/AMP)
- ATP binds Allosterically to PFK-1 and PK and inhibiting enzymes function
- inhibition of PFK-1 and PK leads to increase in G 6-P concentration and binds to allosteric site on hexokinase inhibiting it’s function
Glycolysis Regulation: Contracting Muscles
Glycolysis is stimulated by Low Energy
Contracting Muscles utilize existing ATP (converting it to ADP and AMP) reducing the energy charge of surrounding tissue
- AMP displaces ATP from allosteric site of PFK-1 stimulating PFK-1 to produce Fructose 1,6-BP which feedforward stimulation of PK
- as PFK-1 is activated G 6-P conc is reduced thus activating Hexokinase
Glycolysis Regulation: Liver
Glycolysis CAN NOT FERMENT thus no Lactate, which means H+ (protons) have no effect on regulation ofPFK1
Utilizes Glucokinase isozyme of Hexokinase, with High Km thus lower affinity for glucose-allows Brain and muscles to have first. all
Allosteric Regulation
1) PFK2 Produces Fructose 2,6 bisphosphate which stimulates PFK1 to produce Fructose 1,6-BP to feedforward and stimulates PK (SAME AS MUSCLES)
2) PK is also stimulated by alanine
Inhibition:
High energy Charge (ATP) inhibits PFK-1 and PK
and Citrate from Krebs cycle also inhibits PFK1
Glucose Transport
Sodium monosaccharide cotransport system
- transports glucose AGAINST conc gradient
- requires energy from sodium/potassium ATPase pump
GLUT
Sodium independent facilitate diffusion(transport) of glucose down its concentration gradient across the Plasma membrane -14 different isoforms of GLUTS composed of -single polypeptide-500 amino acids -12 transmembrane alpha helix structres -tissue specific pattern of expression
Glycolysis in cancer cells
In cancer cels, glycolysis often functions anaerobically (Fementation), even in the presence of oxygen
- Called anaerobic glycolysis or “Warburg Effect”
- Medicine-capitiliazes on this with testing for cancer
Glucogneogenesis Def
New synthesis of glucose
2 Types;
Anabolic-synthesis from noncarbohydrate precursors
Conversion-from other carbohydrates (C5 and C6)
Where does gluconeogenesis occur?
LIVER AND KIDNEY
- During an overnight fast, gluconeogenesis occurs in the liver (90%) and kidney (10%)
- extended fast, the kidney takes over a larger percentage of gluconeogenesis (40%)
Glucose Requirements per day
Human Adults= 160g/day (24 hrs)
-brain requires 120g of that 160g per day
Glycogen stores in humans
Glycogen stores= 190g of glucose
-once glycogen stores are depleted, glucose is produced by gluconeogenesis
Potential Substrates for Gluconeogenesis
1) Lactate
2) Pyruvate
3) Glycerol-from catabolism of triacylglycerol
- animals DO NOT convert fatty acids to glucose
4) Alpha Ketoacids-from catabolism of glucogenic amino acids
Gluconeogenesis: Glycerol Substrate
Glycerol Source:
Hydrolysis of Triacylglycerols of adipose tissues, but Glycerol Kinase is not found in adipose tissues. Therefore Glycerol inters bloodstream and travels to liver
Once in liver:
- Glycerol is phosphorylated to Glycerol Phosphate at the expense of ATP Gamma Phosphate; catalyzed by Glycerol Kinase
- Glycerol Phosphate is then oxidized to Dihydroxyacetone Phosphate at the expense of NAD+ to NADH; catalyzed by Glycerol Phosphate dehydrogenase
Gluconeogenesis: Lactate Substrate
CORI CYCLE:
- Lactate is formed in skeletal muscles and tissues lacking mitochondria during strenuous exercise
- lactate diffuses into blood and carried to liver
- Lactate diffuses into liver where it is used to synthesize glucose
Source of Amino Acids in Gluconeogenesis
During a fast, amino acids come from hydrolysis of tissue proteins
Gluconeogenesis: Pyruvate Substrate
ONLY IN LIVER AND KIDNEY:
1) Pyruvate is carboxylated to Oxaloacetate by Pyruvate Carboxylase at the expense of ATP-> ADP
2) Oxaloacetate is unable to leave mitochondria, so is reduced to Malate at the expense of NADH oxidized to NAD+; catalyzed by Malate dehydrogenase (mt)
3) Malate travels to cytoplasm of cell and is oxidized to Oxaloacetate at the expense of NAD+ reduced to NADH; catalyzed by Malate Dehydrogenase (cytosol)
4) Oxaloacetate is then decarboxylated phosphorylated to PEP at the expense of GTP-> GDP;catalyzed by PEP carboxykinase (cytosol)
Pyruvate Carboxylase
- Purpose?
- Found?
- Prosthetic Group
- Regulation
Produces Oxaloacetate from Pyruvate for gluconeogenesis and to replenish oxaloacetate as intermediate in Krebs cycle
Found in:
- liver and kidney cells
- Muscle cells only to replenish Oxaloacetate as Krebs cycle Intermediate
Prosthetic Group
-Biotin
Allosterically Regulated
Stimulated by Acetyl CoA
Biotin
- function
- vit
- deficiency
Prosthetic Group of Pyruvate Carboxylase
-covalently attached to E amino group of lysine by amide bond called biotycin at this state
Function:
-Carries activated CO2 for carboxylation and carboxyl group transfer in certain enzymes of gluconeogensis and fatty acid synthesis
Vit-Biotin
Def
-rash about eye brows, muscle pain, fatigue (rare)
PEP Carboxykinase
or PEP CK
Decarboxylates and phosphorylates oxaloacetate to phosphoenolpyruvate (PEP)
Gluconeogenesis: Glycolysis step 3
ONLY IN LIVER AND KIDNEYS-Gluconeogenesis
Fructose 1,6-BP -> Fructose 6-Phosphate
Enzyme: Fructose 1,6-Bisphosphatase
-dephosphorylation of Fructose 1,6-BP
Regulated by:ENERGY CHARGE (OPPOSITE OF Glycolysis)
- inhibited by High AMP (OR LOW ENERGY CHARGE)
- inhibited by Fructose 2,6-BP
Gluconeogenesis: Glycolysis Step 1
ONLY IN LIVER AND KIDNEYS-GLuconeogenesis
Muscles cells lack Glucose 6-P
For glucose 6-Phosphate to reach Glucose 6-Phosphatase must be transported to the Lumen of Endoplasmic Reticulum (ER)
1) Glucose 6-Phosphate (cytoplasm)-> Glucose 6-P (ER)
- enzyme=Glucose 6-Phosphatase translocate
2) Glucose 6-Phosphate (ER)-> Glucose
- once in lumen of ER Glucose 6-Phosphatase dephosphorylates to produce glucose
REGULATION OF Gluconeogenesis
-Glucagon to stimulate gluconeogenesis
Glucagon is secreted by pancreatic islet and stimulates gluconeogenesis:
1) Lowering the conc of Fructose 2,6-BP
- activates fructose 1,6-Bisphosphatase (gluconeogenesis)
- inhibits PFK-1 (glycolysis)
2) Inhibits by Pyruvate Kinase by Phosphorylation
- glucagon elevates cAMP which stimulates cAMP-dependent protein kinase to phosphorylate PYRUVATE KINASE. PEP is then shifted to synthesis of glucose
3) Increasing Transcription of PEP carboxykinase gene
Regulation of Gluconeogenesis
-Substrate Availability
- specific availability of glucogenic amino acids
- decresed conc of insulin favors mobilization of amino acids from muscle protein
Regulation of Gluconeogenesis:
-Allosteric inhibitors/activators
Allosteric Activation of Acetyl-CoA
-fasting stimulates hepatic Pyruvate Carboxylase
Allosteric Inhibition by AMP
Malignant Hyperthermia
Both glucose and glycolysis are running at high rates leading to uncontrolled hydrolysis of ATP which generates HEAT
Substrate Cycles
Changes in rates of two opposing cycles can vastly increase the rate of production of the product “B”
- Allosteric effector
- Net flux increases or decreases
Transcriptional Control by insulin/glucagon
Insulin release (fed state) stimulates the expression of PFK-1, PK, and the bifunctional enzyme (PFK2/FBPase2) stimulating glycolysis
Glucagon Release (fasting) stimulates expression of phosphoenolpyruvate carboxykinase, and fructose 1,6-bisphosphatase
Bifunctional Enzyme of Glycolysis/gluconeogenesis
REGULATION OF GLYCOLYSIS/GLUCONEOGENESIS AT TRANSITION POINT-Fructose 2,6-Bisphosphate
PFK2 and FBPase2 reside on the same polypeptide
-regulation is by phosphorylation of serine in regulatory domain
Phosphorylation turns off the kinase activity/turning on the phosphatase activity (FBPase2) which reduced the concentration of Fructose 2,6-BP
Dephosphorylation turns on the kinase activity(PFK2)/turning off the phosphatase activity which increases the concentration of Fructose 2,6-BP
Bifunctional Enzyme at Low glucose levels
1) glucagon triggers production of cAMP by adenylate cyclase
2) cAMP stimulates Protein Kinase A to phosphorylate the bifunctional enzyme
3) Phosphorylation stimulates FBPase2 activity and inhibits PFK2 activity
4) FBPase2 dephosphorylates C-2 of Fructose 2,6-Bisphosphate producing Fructose 6-Phosphate
5) Low Fructose 2,6-Bisphosphate concentration inhibits PFK-1 and favors gluconeogenesis
Bifunctional enzyme at High Glucose Levels
1) insulin stimulates Phosphoprotein Phosphatase
2) Phosphoprotein Phosphatase dephosphorylates the bifunctional enzyme activating PFK2
3) PFK2 phosphorylates Fructose 6-Phosphate to Fructose 2,6Bisphosphate
4) Increase in Fructose 2,6-BP concentration stimulates PFK1 and favors glycolysis
What other carbohydrates can be used in glycolysis
Fructose (2 entry points-depending on from other tissues or liver)
Galactose (1 entry point)
Glycolysis: Fructose
Pathway In Liver:
1) Fructose-> Fructose 1-Phosphate at the expense of ATP-> ADP catalyzed by Fructose Kinase
2) Fructose 1-Phosphate-> DHAP + Glyceraldehyde; catalyzed by Fructose 1-Phosphate Aldolase
* *Glyceraldehyde needs to be phosphorylated
3) Glyceraldehye-> Glyceraldehyde 3-Phosphate at the expense of ATP-ADP; catalyzed by TRIOSE KINASE
Pathway in other tissues:
Fructose-> Fructose 6-Phosphate catalyzed by Hexokinase at the expense of ATP-> ADP
Galactitol
Causes Cateracts
- galatosemia causes increased galactose concentration in the blood which can leads to increased concentration in some tissues
- In the eye its converted to Galactitol which causes cateracts
Lactase Deficiency
Can lead to Lactose Intolerance
-Deficiency (reduction)
Microorganisms ferment lactose into lactic acid and methane and hydrogen gas
Mechanism of Triose Phosphate Isomerase
DHAP GAP *want to form GAP
-contain amino acids Glu and His
enzyme approaches Kinetic Perfection
1) Glu (general base) abstracts proton from DHAP
- His (general acid) donates proton to C=O of DHAP (or C2)
2) Glu (general acid) donates to C2 (old C=O of DHAP)
- His (general base) abstracts proton from Hydroxyl of C1
3) Glu (negative) and His (positive) returns to original form
Alcohol Dehydrogenase Active site
Zn2+ linked to:
- 2 S of Cyst
- N of His
- O of aldehyde of Acetaldehyde, which polarizes carbonyl group
NADH-transfers hydride to C of acetaldehyde
Mechanism of Glyceraldehyde 3-phosphate dehydrogenase
Two processes:
- oxidation of aldehyde to carboxylic acid by NAD+ (-G)
- Joining of Carboxylic acid and orthophosphate (Pi) to form acyl phosphate (+G)
- steps are tightly coupled by covalent attachment of acid intermediate to enzyme by thioester bond
1) Cys reacts with aldehyde of GAP forming hemethioacetal
2) Hydride transferred from substrate (oxidized) to NAD+ reduced to NADH
- concomitantly a thioester is formed between substrate and enzyme
- His accepts proton
3) NADH is exchanged to NAD+
4) Orthophosphate (Pi) attaches thioester forming 1,3-BPG
