Glycolysis And PPP Flashcards
What is the fate of absorbed glucose?
GLUT (facilitated diffusion)
– GLUT 2 – liver
– GLUT 1 and 3 – neurons and brain – GLUT 1 - erythrocytes
– GLUT 4 - adipose tissue and muscle (Insulin- responsive)
How is glucose 6 phosphate formed?
• Glucokinase: liver and pancreatic -cells of pancreas (high Km)
– More active when blood glucose is elevated; ‘Glucose sensor’ • Othertissues–hexokinase(lowKm)
• Glucokinase mutations: Hyperglycemia and MODY-2
– -cells of pancreas cannot respond to high blood glucose levels!!!
What are the fates of glucose 6 phosphate in the liver?
Glycolysis
Glycogenesis
Ribulose 5-P NADPH (PPP)
Give an overview of the pentose phosphate pathway
CYTOSOL
• Glucose 6-phosphate dehydrogenase (key enzyme)
– Does NOT form ATP (energy) • Forms NADPH
– Fatty acid biosynthesis, cholesterol biosynthesis and steroid hormone synthesis in liver, adipocytes and endocrine glands
- Malic enzyme also forms NADPH in these tissues
- Forms Ribose phosphate – purine and pyrimidine synthesis
• Transketolase requires TPP (vitamin B1)
– Thiamine deficiency: Low transketolase activity
What are the functions of glycolysis ?
Generate ATP (energy) in presence or absence of O2 (aerobic and anaerobic) and in presence or absence of mitochondria
• Generates ATP (energy)
• Glycolytic enzymes are present in cytosol
– In brain (fed and fasted state), liver (fed state)
– Tissues that lack mitochondria or hypoxic (Only pathway for energy generation)
– Actively contracting skeletal muscle (Glucose obtained from glycogen or plasma glucose)
Give an overview of the glycolysis phases
Stage 1: energy investment phase( glucose to fructose 1,6-bisphosphate)
Stage 2: cleavage of 6C sugar into 3C intermediates (fructose 1,6 biphosphate splits to glyceraldehyde 3 phosphate and dihydroxyacetoje phosphate
Stage 3 energy generation phase
Each glyceraldehyde 3 phosphate and DHA phosphate is used to form pyruvate
2 ATP formed and NADH formed
How does Acetyl CoA enters the mitochondria?
Via shuttle systems
Describe the ATP formed substrate level phosphorylation
• Directly form ATP from ADP
• DO NOT require Electron Transportchainor
mitochondria or Oxygen
- ONLY reactions that form ATP in cells that lack mitochondria and in hypoxic conditions
- High energy compounds – 1,3-Bisphosphoglycerate – Phosphoenolpyruvate
What are the fates of Pyruvate ?
Aerobic- forms Acetyl CoA (Pyruvate dehydrogenase complex-requires TPP and is a mitochondrial enzyme)—> enters TCA cycle
Anaerobic- Anaerobjc-lactate (lactate dehydrogenase and is a cytosolic enzyme)—> goes to liver via Cori cycle
What situations call for anaerobic use of Pyruvate?
Absence of mitochondria (red blood cells, lens, cornea, leukocytes)
Poorly vascularized tissues or hypoxia (contracting skeletal muscle, cardiac muscle hypoxia)
Pyruvate dehydrogenase deficiency or thiamine deficiency
Explain the anaerobic fate of Pyruvate
• NADHformedbyglyceraldehyde3-phosphate dehydrogenase is reoxidized to form NAD+
– Allows glycolysis to proceed in absence of mitochondria
• LDH is cytosolic enzyme
• Direction of reaction: NADH/NAD+ ratio
– Higher NADH, increases lactate
- Identify states of high NADH/NAD+ ratio
- Contrast between muscle (fast-twitch) and liver
Describe the energetics of aerobic glycolysis
- Overall reaction: Glucose→2 Pyruvate
- 2 NADH+ H+ (glyceraldehyde 3-phosphate dehydrogenase) = 2x3 = 6ATP
- (1 NADH=2.5 ATP and 1 FADH2=1.5 ATP; Approximate 1 NADH = 3 ATP and 1 FADH2 = 2 ATP)
- NADH in cytosol by glycolysis transported into mitochondria by shuttle systems (malate-aspartate or glycerol-phosphate shuttle) – Refer Shuttles and OxPhos
- 4 ATP (substrate level phosphorylation) (2ATP from a glyceraldehyde 3-phosphate)
- -2ATP used
- Total = 8 ATP
Describe the energetics of glycolysis under anaerobic conditions
- OverallreactionGlucose→2 Lactate
- 2 NADH+ H+ (glyceraldehyde 3- phosphate dehydrogenase) used in lactate dehydrogenase
- 4ATP(substratelevel phosphorylation) (2ATP from a glyceraldehyde 3-phosphate)
- -2ATPused
- Total = 2 ATP
Describe lactate formation during anaerobic exercise in muscle
• Lactate threshold and endurance training:
Weight loss advice
– 60-70% of maximum heart rate: Fat oxidation by
muscle
– 80-85% of MHR: Anaerobic oxidation forms
lactate
• Anaerobic (strenuous) activity, blood lactate
levels increase
– Lactate in muscle → pH drops and cramps
What are the major comparisons of aerobic exercise?
• Oxygen present
• Glycogenolysis is active and may last
hours
• Glycolysis is active
• Kreb’s cycle and electron transport
chain are both active, producing ample ATP; Oxygen is the final electron acceptor.
- Fatty acid oxidation
- ATP production is maximal
What are the features of anaerobic/strenuous/ischemic exercise?
• Oxygen absent
• Glycogenolysis is active, but of
short duration
- Glycolysis is active
- Pyruvate is reduced to lactate
- ATP production is rapid but minimal
What are the defects in muscle glycolysis?
- Muscle cramps (due to low ATP) during high-intensity anaerobic exercise; hemolysis if associated RBC glycolytic defect
- Myoglobin uria and high serum CK-MM levels following high intensity exercise
- NO increase in blood lactate levels with high intensity exercise (Ischemic forearm test)
- Aerobic exercise tolerated (Oxidation of fatty acids not affected)
- PFK-1 deficiency (Tarui disease; Glycogen storage disease Type VII)
What is the cori cycle?
A fate of lactate
Glucose (liver) and lactate (Skeletal muscle; RBC)
Lactate from skeletal muscle and RBC, transported to liver for
gluconeogenesis
Whaat is the effect of arsenic poisoning?
- Glyceraldehyde 3-phosphate dehydrogenase
* PDH complex and -ketoglutarate dehydrogenase complex (mitochondrial enzymes)
What are the other fates of pyruvate?
Forms alanine(important substrate for gluconeogenesis; link to amino acid metabolism)
Forms acetyl coa (for aerobic oxidation, by Pyruvate dehydrigenase complex)
Forms lactate(via lactate dehydrogenase)
Forms oxaaloacetate(via pyruvate carboxylase)
What is lactic acidosis?
• High anion gap metabolic acidosis
– pH=low; HCO3–decreased; PCO2 decreased (compensation)
– High anion gap; Elevated lactate levels (unmeasured anion)
Whhat are the conditions of lactic acidosis?
• Conditions of Lactic acidosis
– Increased NADH/NAD+ ratio: Binge alcohol consumption
– Pyruvate dehydrogenase deficiency (Leigh disease)
– Thiamine deficiency (pyruvate dehydrogenase complex)
– Gluconeogenesis defect – von Gierke disease – Defect in Cori cycle
– Decreased blood supply (circulatory shock): hypoxia and anaerobic metabolism
– Pulmonary embolism (poor oxygenation)
What is the significance of glycolysis ?
- Aerobic Tissues (brain, skeletal muscle) Glucose→ pyruvate→ acetyl CoA → TCA cycle
- Adipose tissue: Dihydroxyacetone phosphate (forms glycerol 3-P) used for triacylglycerol formation (Link to lipid metabolism)
• Tissues of eye: cornea, lens (no mitochondria)
• Tumor cells
• Wound and healing injured tissue
• Red blood cells, anaerobic glycolysis
– Only source of energy (ATP)
– Forms 2,3 bisphosphoglycerate (2,3 BPG)
Whwt is clmmon in gkycolysus and gluconeogenesis?
- Reversible reactions of gluconeogenesis are same as glycolysis (but in reverse direction)
- Four irreversible reactions in gluconeogenesis to bypass the three irreversible reactions of glycolysis
How is glycolysis regulated in liver?
- Liver glycolysis and gluconeogenesis are reciprocally regulated • Glycolysis active when high blood glucose levels/ insulin
- Gluconeogenesis: Low blood glucose levels/ glucagon
- Irreversible/ regulated reactions:
- Glukokinase
- Phosphofructokinase-1(PFK-1)
- Pyruvare kinase
What is the significance of glucokinase?
• Glucokinase (liver) high Km; high Vmax
– More active as blood glucose level elevated (after a meal)
• No product inhibition
- ‘Glucose sensor’ in liver and beta-cells of pancreas
- Enzyme sequestered in nucleus when plasma glucose level is low
- GlucokinaseMutations:HyperglycemiaandMODY-2
What is the significance of PFK-1?
- Allosteric enzyme and committed enzyme
- Muscle: ATP(inhibits) & AMP(stimulates)
- Liver:Fructose2,6-bisphosphate(stimulates) and citrate(inhibits)
Explain PFK-2/FBPase-2?
• PFK-2 forms Fructose 2,6-bisphosphate
– Allosteric regulator of glycolysis and gluconeogenesis
• Fructose 1,6-bisphosphate: intermediate of glycolysis and gluconeogenesis
- Fructose 2,6-bisphosphate degraded by Fructose bisphosphatase-2 (FBPase-2)
- PFK-2/ FBPase-2 :‘Bifunctional enzyme’
- Phosphorylation by glucagon
- Dephosphorylation by insulin
Explain the regulation of bifunctuonal enzyme
Insulin (high blood glucose level) – Dephosphorylation
– Active PFK-2
– Increases [Fructose 2,6-bisphosphate] – Allosterically activates PFK-1 (glycolysis)
Glucagon (low blood glucose level) – Phosphorylation
– FBPase-2 active
– Decreases [Fructose 2,6-bisphosphate]
– Activates Fructose 1,6-bisphosphatase (gluconeogenesis)
Explain pyruvate kinase regulation
• Feed forward activator: Fructose1,6
bisphosphate
• Phosphorylation(glucagon) decreases activity
• Low blood glucose: Liver glycolysis less active, and gluconeogenesis active.
• Alanine (substrate for gluconeogenesis) inhibits pyruvate kinase
How is glycolysis regulated?
• High carbohydrate diet and insulin: induce glycolytic enzymes (Increased synthesis increases concentration of enzymes)
– Low levels of insulin (diabetes mellitus) represses glycolytic enzymes
• Glucagon and fasting repress glycolytic enzymes
