Glycolysis and hemolysis

Question 1

describe the GLUTs

Answer 1

• GLUTs are transporters that transport glucose via facilitated diffusion
  
  • GLUT 2: liver
  • GLUT 1 and 3: neurons and brain
  • GLUT 1: RBC
  • GLUT 4: in adipose tissue and muscle is responsive to insulin
    
    • the # of GLUT4 increase in the presence of insulin

Question 2

contrast glucokinase and hexokinase

Answer 2

• both convert glucose into glucose 6-phosphate
• glucokinase: in liver has a high Km (low affinity) for glucose (more active when the blood glucose is elevated)
  
  • also present in B-cells of the pancreas
  • high Vmax
• hexokinase: has a low Km for glucose (high affinity), found in other tissues (RBC)
  
  • low Vmax

Question 3

describe glycolysis

Answer 3

• one of the principal pathways for generating ATP
  
  • brain and RBCs
• glycolysis can generate ATP in the presence and absence of O2 and in the presence and absence of mt
• RBCs have no mt–glycolysis is the only source of ATP in these cells
• glycolytic enzymes are present in the cytosol

Question 4

describe the 3 phases of glycolysis

Answer 4

Question 6

describe the energy investment phase

Answer 6

• 2 ATP molecules used
• PFK-1 is the second irreversible reaction of glycolysis and also the most important regulated step (allosteric regulated enzyme)

Question 7

describe cleavage of 6C sugar into 3C intermediates

Answer 7

• aldolase A is present in muscle and aldolase B is present in liver

Question 8

describe the energy generation phase

Answer 8

• all rxns are reversible
• 1,3-BPG is a high energy intermediate that results in the formation of ATP without involving the mt ETC
  
  • this rxn forms ATP without mitochondria

Question 9

describe the second part of the energy generation phase (2-phosphoglycerate to pyruvate)

Answer 9

• phosphoenolpyruvate is a high energy compound that results in the formation of ATP without the involvement of the ETC in mt (substrate level phosphorylation)
  
  • SLP important during hypoxia and in RBC
• pyruvate kinase is an irreversible rxn

Question 10

describe the anaerobic fate of pyruvate

Answer 10

• pyruvate formed in glycolysis is metabolized anaerobically in the absence of mt or poorly vascularized tissues
• the NADH formed by the glyceraldehyde 3-P dehydrogenase rxn is reoxidized to form NAD+; this allows glycolysis to proceed even in the absence of mt
• LDH is a cytosolic enzyme
• the direction of the rxn depends on the NADH/NAD+ ratio
  
  • higher the NADH levels, the rxn is driven towards lactate formation

Question 11

describe the Cori cycle

Answer 11

Question 12

name the 3 irreversible rxns (regulatory steps) in glycolysis

Answer 12

• glucokinase/hexokinase
• phosphofructokinase-1
• pyruvate kinase

Question 13

name the 2 SLP rxns in glycolysis

Answer 13

• phosphoglycerate kinase (reversible)
• pyruvate kinase (irreversible)

Question 14

name inhibitors of glycolysis

Answer 14

• pentavalent arsenate poisoning results in inhibition of glyceraldehyde 3-P dehydrogenase enzyme
• in clinical labs, blood is collected in fluoride containing tubes for the estimation of blood glucose
  
  • if fluoride is not added, there would be reduction in the glucose due to its utilization by RBC and WBC, which leads to erroneously low blood glucose
  • fluoride inhibits enolase

Question 15

describe formation and significance of 2,3-BPG

Answer 15

• 1,3-BPG –> 2,3-BPG by the enzyme mutase
• 2,3-BPG binds to the B-chains of Hb and helps to unload O2 at the tissues
  
  • the formation of 2,3-BPG is increased when a person moves to a high altitude to facilitate unloading of O2

Question 16

describe hemolytic anemia caused by pyruvate kinase

Answer 16

• pyruvate kinase deficiency leads to a decrease in ATP
• ATP is required in RBCs to maintain electrolyte concentration of the RBCs (Na/K/ATPase)
  
  • defects in glycolysis would result in hemolysis and hemolytic anemia due to decreased ATP generation
• pyruvate kinase deficiency leads to higher 2,3-BPG levels

Question 17

describe how pyruvate kinase/hexokinase deficiency can lead to RBC lysis

Answer 17

Question 18

describe when lactic acidosis is observed

Answer 18

• an increased conversion of pyruvate to lactate (increased NADH/NAD+ ratio)
• inherited deficiency of pyruvate dehydrogenase (Leigh disease)
• decreased blood supply due to cardiac failure resulting in anaerobic metabolism to peripheral tissues
  
  • there is a defect in the Cori cycle and lactate formed is not efficiently transported to liver)