CPR 53 - Glycolysis and Hemolysis Due to Enzyme Deficiency

Question 1

Which GLUT transporter is responsible for getting glucose into a RBC?

Answer 1

GLUT 1

Question 2

What are the two enzymes that can convert glucose to G6P? Where are they located? What is the key difference between them?

Answer 2

Glucokinase and Hexokinase

Glucokinase is in the liver and β-cells of the pancreas

Hexokinase is everywhere else

Glucokinase has a high Km for glucose while hexokinase has a low Km.

Question 3

What are the two possible fates of G6P in the RBC?

Answer 3

Glycolysis to become pyruvate

The Pentose Phosphate Pathway

Question 4

What are the different ways that RBCs can generate ATP?

Answer 4

Only through glycolysis (there is no mitochondria)

Question 5

What is Stage 1 of glycolysis often referred to as? What are the steps to this stage?

Answer 5

Energy Investment Phase

1. Glucokinase/Hexokinase use ATP to phosphorylate glucose into G6P
2. Phosphoglucose isomerase isomerizes G6P to F6P
3. PFK-1 uses ATP to phosphorylate F6P into F16BP

Question 6

What is stage 2 of glycolysis usually referred to as? List the steps to this stage.

Answer 6

Cleavage of one 6C Sugar to two 3C sugars

1. Aldolase A or B cleaves F16BP into G3P and DHAP
2. Triosphosphate isomerase isomerizes DHAP into G3P.

We now have two molecules of G3P so the rest of glycolysis is happening twice for each glucose molecule that enters

Question 7

What is stage 3 of glycolysis usually referred to as? List the steps to this stage.

Answer 7

Energy Generation Phase

1. G3P dehydrogenase uses NAD+ to dehydrogenate G3P into 1,3-BPG and making NADH in the process
2. Phosphoglycerate kinase uses ADP to dephosphorylate 1,3-BPG into 3-phosphoglycerate, making ATP in the process
3. Phosphoglycerate mutase converts 3PG into 2PG
4. Enolase converts 2PG into phosphoenolpyruvate
5. Pyruvate kinase uses ADP to dephosphorylate PEP into pyruvate, making ATP in the process

Question 8

What are the irreversible reactions of glycolysis?

Answer 8

1. The Glucokinase/Hexokinase reaction that converts glucose to G6P
2. The PFK-1 reaction that phosphorylates F6P to F16BP
3. The pyruvate kinase reaction that dephosphorylates PEP to pyruvate

Question 9

What are the substrate level phosphorylation reactions of glycolysis?

Answer 9

1. Phosphoglycerate kinase converting 1,3-BPG to 3-phosphoglycerate
2. Pyruvate kinase converting PEP to pyruvate

Question 10

What happens to pyruvate under aerobic conditions? When can this not happen?

Answer 10

When there is an abundance of O2 AND mitochondria pyruvate dehydrogenase will dehydrogenate pyruvate to Acetyl CoA which will then enter the TCA cycle.

Question 11

What happens to pyruvate under anaerobic conditions? Why is this reaction important to glycolysis?

Answer 11

Lactate dehydrogenase uses NADH to hydrogenate pyruvate to lactate, forming NAD+ in the process. Under anaerobic condictions, NADH builds up and NAD+ depletes which slows glycolysis way down. This LDH reaction is what allows glycolysis to continue under anaerobic conditions by converting NADH to NAD+.

Question 12

Why does lactate accumulation occur most frequently in skeletal muscle tissue?

Answer 12

In actively contracting skeletal muscle, NADH formed as a result of glycolysis and the TCA cycle exceeds the oxidative capacity of the ETC resulting in a high NADH/NAD+ ratio. This favors the conversion of pyruvate to lactate.

Question 13

Why doesn’t cardiac muscle often accumulate lactic acid?

Answer 13

It has an abundance of mitochondria and good vascularization. Both of these keep the NADH/NAD+ ratio low. If cardiac muscle becomes hypoxic (ie - MI) then lactate formation will occur.

Question 14

What happens to the lactate that accumulates in the peripheral tissues?

Answer 14

It is delivered to the liver where LDH-5 converts it into pyruvate and then undergoes gluconeogenesis.

Question 15

What is the total energy outcome of aerobic and anaerobic glycolysis per molecule of glucose?

Answer 15

Aerobic glycolysis yields 2-ATP, 2-NADH, and 2-pyruvates which will eventually be converted into 2-acetyl CoA

Anaerobic glycolysis yields 2 ATP

Question 16

What are the inhibitors of glycolysis we need to know for this exam?

Answer 16

Arsenate - inhibits the G3PD enzyme

Fluoride - inhibits the enolase enzyme

Question 17

Describe the 2,3-BPG triangle.

Answer 17

In glycolysis, 1,3-BPG is phosphorylated to 3-phosphoglycerate by phosphoglycerate kinase. Both of these molecules can be converted to 2,3-BPG.

Mutase converts 1,3-BPG to 2,3-BPG

2,3-BPG phosphatase can then convert 2,3-BPG to 3-phosphoglycerate which can then re-enter glycolysis.

Both of these reactions are reversible

Question 18

Describe how an enzymatic defect in the glycolytic pathway can lead to hemolytic anemia. What are the most common enzymatic deficiencies that cause this?

Answer 18

A defect in the glycolytic pathway leads to a reduction in ATP production. This causes the activity of the NA+/K+ ATPase to be compromised. This leads to an osmotic imbalance and RBC lysis.

Pyruvate Kinase and Hexokinase deficiency are the most common enzymatic deficiencies that cause hemolytic anemia

Question 19

How can you distinguish between a pyruvate kinase deficiency and a hexokinase deficiency?

Answer 19

A pyruvate kinase deficiency will also present with increased levels of 2,3-BPG.

Question 20

When is lactic acidosis most commonly observed?

Answer 20

1. There is an increased conversion of pyruvate to lactate (increased NADH/NAD+ ratio)
2. There is an inherited deficiency of pyruvate dehydrogenase
3. There is a defect in the Cori cycle and lactate formed is not efficiently transported to the liver.

Question 21

What is the key regulatory step in glycolysis and how is it regulated?

Answer 21

The conversion of F6P to F16BP by PFK-1.

PFK-1 is an allosterically regulated enzyme. AMP, which is present in an energy deficient state, activates PFK-1 while ATP inhibits it.

Question 22

Draw out the entire glycolytic pathway. Be sure to include all reactants, products, and enzymes involved. Indicate which steps are reversible/irreversible and in which steps substrate level phosphorylation occurs. Also indicate which steps are blocked by arsenate and fluoride.

Answer 22

Refer to image.

Arsenate blocks G3PD and Fluoride blocks Enolase