Glycolysis

Question 1

What is the normal basal level of blood glucose? How long after a meal does blood glucose peak? How long until it returns to the basal level?

Answer 1

• 5 mmol = 90 mg/dL

- peaks at about 45 minutes after a meal, and returns to the basal level at about 90 minutes

Question 2

Types of glucose transporters.

Answer 2

• GLUT-1 and GLUT-3: found in most tissues; Km is 1 mM, so glucose enters these cells at a constant, slow, steady rate
• GLUT-2: found in the liver and pancreatic B cells (and kidneys); Km is 15 mM, so when glucose levels are high, more glucose enters these cells
• GLUT-4: found in fat and muscle tissues; Km is 5 mM, so glucose enters these cells more rapidly than via GLUT-1 and GLUT-3; GLUT-4 is insulin dependent

Question 3

Which enzyme converts glucose into ____________? What does this reaction require?

Answer 3

• hexokinase (glucokinase in the liver and pancreatic B cells) converts glucose into glucose-6-phosphate
• requires ATP

Question 4

How are hexokinase and glucokinase regulated? Why is this difference important?

Answer 4

• hexokinase is inhibited by its product, glucose-6-phosphate
• glucokinase is hormonally regulated (activated by insulin)
• since glucokinase isn’t inhibited by G6P, excess glucose is able to continue to enter the liver (and pancreatic B cells) to maintain BGLs (most cells will stop glucose uptake once hexokinase is inhibited)

Question 5

What is the purpose of converting glucose into glucose-6-phosphate?

Answer 5

• the phosphorylation of glucose prevents it from flowing back out of the cell via GLUT transporters; it keeps the glucose in the cell
• it also maintains the glucose gradient favoring entry into a cell since G6P technically isn’t the same as glucose

Question 6

Which enzyme converts fructose-6-phosphate into ____________? How is it regulated? What does this reaction require?

Answer 6

• phosphofructokinase-1 (PFK-1) converts F6P into fructose-1,6-bisphosphate
• requires ATP
• inhibited by (excess) ATP and citrate
• activated by AMP and fructose-2,6-bisphosphate

Question 7

Fructose-1,6-bisphosphate is acted on by which enzyme to form __________ + ___________? What do these products do?

Answer 7

• aldolase A converts F-1,6-BP into glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone-phosphate (DHAP)
• G3P is used in the next step of glycolysis
• DHAP is used for fatty acid synthesis, but is also isomerized into G3P to partake in glycolysis

Question 8

Glyceraldehyde-3-P is acted on by a dehydrogenase to form what product? What is generated in the process? What does this reaction require?

Answer 8

• the oxidation of G3P converts it into 1,3-bisphosphoglycerate (1,3-BPG) and generates NADH in the process (so NAD is required)
• (this is the only oxidation step involved in glycolysis)

Question 9

What is important about 1,3-bisphosphoglycerate?

Answer 9

• in RBCs, 1,3-BPG can be converted into 2,3-BPG, which is involved in the Bohr shift of the Hb-O2 saturation curve

Question 10

The conversion of 1,3-bisphosphoglycerate into ___________ generates ______, making it what type of reaction?

Answer 10

• 1,3-BPG into 3-phosphoglycerate generates ATP
• it is the 1st of 3 substrate-level phosphorylation reactions (these reactions generate ATP using a substrate rather than oxidation)

Question 11

What enzyme converts phosphoenolpyruvate into __________? What does the reaction generate?

Answer 11

• pyruvate kinase converts PEP into pyruvate

- the reaction generates ATP, making it the 2nd of 3 substrate-level phosphorylations

Question 12

Where does glycolysis take place?

Answer 12

• in the cytoplasm
• (glucose to pyruvate takes place in the cytoplasm; pyruvate then enters the mitochondria to be converted to acteyl-CoA to enter the TCA cycle and F.A.S.)

Question 13

Explain the pathology associated with a pyruvate kinase deficiency.

Answer 13

• PK deficiency manifests as hemolytic anemia and elevated 2,3-BPG levels
• cells use glycolysis to generate pyruvate which will enter the TCA cycle to generate ATP; in RBCs, this ATP is needed for proper cation transport, so when this fails, hemolytic anemia results
• (this deficiency doesn’t hurt other cells as much because other cell types have alternate ways of generating ATP)

Question 14

Why are RBCs solely dependent on glucose for fuel?

Answer 14

• RBCs lack nuclei, mitochondria, ERs, Golgi complexes, and peroxisomes, and so RBCs only have 2 metabolic pathways (glycolysis and the HMP shunt)

Question 15

What is the rate determining enzyme for glycolysis?

Answer 15

• PFK-1

- (again, activated by AMP, inhibited by ATP and citrate; activated by fructose-2,6-bisphosphate in the liver)

Question 16

Since phosphofructokinase-1 is turned off by ATP, glycolysis will end once enough ATP is generated. This will stop the formation of pyruvate, which is needed for fatty acid synthesis in addition to the ATP generation. How does the body get around this issue?

Answer 16

• in the LIVER, an extra enzyme exists (PFK-2) that converts fructose-6-phosphate into F-2,6-BP
• F-2,6-BP overrides any inhibition of PFK-1, keeping glycolysis active, and generating pyruvate for fatty acid synthesis even after enough ATP is generated

Question 17

How is phosphofructokinase-2 regulated?

Answer 17

• PFK-2 is activated by insulin and inhibited by glucagon
• (glucagon actually converts PFK-2 into fructose bisphosphatase-2, which converts F-2,6-BP back into fructose-6-phosphate to promote gluconeogenesis)
• glucagon results in PKA activity, which phosphorylates the enzyme into FBPase-2; insulin results in decreased PKA activity, which dephosphorylates the enzyme into PFK-2

Question 18

Explain the effectiveness of the liver soaking up any extra glucose.

Answer 18

• all excess glucose in the blood enters the liver via GLUT-2 (because of its high Km), glucose is kept in the liver via glucokinase (which is NOT inhibited by its product, but is stimulated by insulin), glycolysis is continued here for fatty acid synthesis despite high ATP (because with insulin, PFK-2 creates F-2,6-BP, which keeps PFK-1 active)

Question 19

The oxidation step of glycolysis (glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate) requires NAD, which is very limited in the cell - how are repeated cycles of glycolysis able to occur?

Answer 19

• the resulting NADH is recycled back into NAD!
• in the presence of O2, NADH uses its electron to generate ATP via the ETC, returning NADH to NAD
• without O2, lactate dehydrogenase (LDH) transfers the electron from NADH to pyruvate to create lactate and NAD

Question 20

How much energy is generated from one molecule of glucose undergoing glycolysis?

Answer 20

• a net of 2 molecules of ATP are generated, along with 2 NADH (1 NADH = 3 ATP via ETC) = 8 ATP
• 2 ATP are needed to fuel glycolysis, but each glyceraldehyde-3-P generates 2 ATP, and each molecule of glucose forms two molecules of G3P
• (without oxygen, the NADH are unable to generate ATP, so only 2 molecules of ATP are generated from glycolysis in anaerobic conditions)

Question 21

Which steps of glycolysis require ATP? Which steps generate ATP?

Answer 21

• glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose-1,6-bisphosphate require ATP
• 1,3-bisphosphoglycerate to 3-phosphoglycerate, and phosphoenolpyruvate to pyruvate generate ATP (x2)
• (glyceraldehyde-3-P to 1,3-bisphosphoglycerate generates NADH) (x2)