Lecture 20 - Glycolysis

Question 1

What cell types exclusively use glucose for their energy needs?

L20 S7

Answer 1

• RBCs

- brain (under non-starvation conditions)

Question 2

What are the main glucose transporters, what is their glucose affinity (Km), and where are they located?

L20 S8

Answer 2

GLUT1:

• expressed ubiquitously expressed
• highly expressed in RBCs and brain
• high affinity (Km 1mM)

GLUT2:

• liver
• low affinity (Km 10mM)

GLUT3:

• neurons
• high affinity (Km 1mM)

GLUT4:

• skeletal and cardiac muscle
• adipose tissue
• high affinity (Km 5mM)
• insulin dependent

Question 3

What is the general function and chemical equation of each stage of glycolysis?

L20 S11

Answer 3

Stage 1:

• traps glucose by phosphorylating it
• converts glucose into 3 carbon sugar
• glucose + 2 ATP -> 2 glyceraldehyde 3-phosphate

Stage 2:

• generation of ATP
• 1,3-BPG -> pyruvate + 2 ATP (x2)

Question 4

What is the function of hexokinase?

L20 S14

Answer 4

Phosphorylation of glucose

Uses ATP

Traps glucose

Irreversible

Question 5

What is the function of phosphoglucose isomerase?

L20 S14

Answer 5

Isomerizes glucose 6-phosphate to fructose 6-phosphate

Question 6

What is the function of phosphofructokinase?

L20 S14

Answer 6

Phosphorylates fructose 6-phosphate to fructose 1,6-bisphosphate

Rate limiting step of glycolysis

Irreversible

Question 7

What is the function of aldolase?

L20 S15

Answer 7

Breaks fructose 1,6-bisphosphate down into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate

Takes 6 carbon sugar and breaks it down into two 3 carbon sugars.

Question 8

What is the function of triose phosphate isomerase?

L20 S15

Answer 8

Converts DHAP, which can’t be used in glycolysis, into glyceraldehyde 3-phosphate, which can.

Question 9

What is the function of glyceraldehyde 3-phosphate dehydrogenase (GAPDH)?

L20 S17

Answer 9

Oxidative phosphorylation of GAP to 1,3-BPG

Reduces NAD+ to NADH

Question 10

What is the function of phosphoglycerate kinase?

L20 S18

Answer 10

Uses 1,3-BPG to phosphorylate ADP to produce ATP and 3-phosphoglycerate

Question 11

What is the function of phosphoglycerate mutase?

L20 S18

Answer 11

Converts 3-phosphoglycerate into 2-phosphoglycerate by moving the phosphate

Question 12

What is the function of enolase?

L20 S19

Answer 12

Dehydrogenates 2-phosphoglycerate to form phosphoenolpyruvate (forms double bond)

Question 13

What is the function of pyruvate kinase?

L20 S19

Answer 13

Uses phosphate from phosphoenolpyruvate to phosphorylate ADP producing ATP and pyruvate

Irreversible

Question 14

What are the possible fates of pyruvate?

L20 S21

Answer 14

Can be converted into :

• ethanol (yeast)
• lactate (low O2/regenerates NAD+)
• acetyl CoA

Question 15

In what steps do other sugars enter the glycolysis pathway and what, if anything, is significant about this?

L20 S23

Answer 15

Galactose:
-enters pathway as glucose 6-phosphate

Fructose:

• enters as DHAP or glyceraldehyde 3-phosphate
• bypasses rate limiting step

Question 16

What are the regulatory mechanisms of glycolysis?

L20 S29-30

Answer 16

High [glucose 6-phosphate] inhibits hexokinase via substrate level inhibition

High [ATP] inhibits phosphofructokinase and pyruvate kinase (both of which produce ATP) via substrate level inhibition

Low [ATP] promotes phosphofructokinase

Fructose 1,6-bisphosphate promotes pyruvate kinase

Question 17

What is glucokinase and what is its significance?

L20 S14;30

Answer 17

Glucokinase replaces hexokinase in the liver which is the only place it is found.

It facilitates the same reaction of phosphorlating glucose into glucose 6-phosphate, however it is not inhibited by glucose 6-phosphate like hexokinase is.

Promotes trapping and storage of glucose.