Glycolysis and Gluconeogenesis

Question 1

What does phosphorylation of glucose accomplish?

Answer 1

Traps glucose in the cell and raises energy of glucose to prepare for following rxns

Question 2

Why isn’t glucose phosphorylation the “committed step” in glycolysis?

Answer 2

Possible to take different paths other than glycolysis (glucose-6-phosphate = metabolic cross roads)

Question 3

What are potential fates of Glc-6-P? Do you think Glc-6-P would be directed to the pentose phosphate pathway if the ADP/ATP ratio was high? Why or why not?

Answer 3

Potential fates:
Glucose 1-phosphate - to glycogen (energy storage)
Glucose 6-phosphate to fructose 6-phosphate - to glycolysis
6-phosphoglucono-ð-lactone to ribose 5-phosphate - to nucleotide biosynthesis

ANSWER QUESTION

Question 4

Would hexokinase I or glucokinase (hexokinase IV) be more active at lower glucose concentrations?

Answer 4

Low Km for glucose (high affinity), active at low levels of glucose

Glucokinase - larger Km (lower affinity) and needs higher conc to be active

Question 5

Why is Glucokinase predominantly in liver cells?

Answer 5

Liver cells = where glucose storage occurs (glucokinase can deal w/ sudden incr in conc glucose)

Question 6

What reactions require ATP in glycolysis? Why is this considered a good “investment”?

Answer 6

Phosphorylation rxns in prep phase use 2 ATP

WHY GOOD INVESTMENT

Question 7

Why is the PFK1 step the considered commitment step? Why are the highly exergonic reactions the most regulated steps?

Answer 7

Commitment step b/c can only proceed in one direction

WHY EXERGONIC REGULATED

Question 8

Why aren’t the exergonic reactions identical for both glycolysis and gluconeogenesis?

Question 9

How does ATP levels affect Phosphofructokinase 1 (PFK1)?

Answer 9

High levels inhibit PFK1 - no longer need ATP, stops glycolysis from continuing

Low levels activate PFK1

Question 10

How do AMP levels affect Fructose-1,6-bisphosphatase (F1,6BPase)?

Answer 10

FBPase needed for gluconeogenesis

High levels of AMP inhibit FBPase b/c AMP signals need for ATP (AMP activates PFK1)

Low levels of AMP activate FBPase

Question 11

How does Fructose-2,6-BP affect PFK1?

Answer 11

Allosteric activator of PFK1

Incr F2,6BP, incr PFK1, incr glycolysis

Question 12

Where does Fructose-2,6-Bisphosphate (F2,6BP) come from?

Answer 12

Generated from PFK2 enzyme

Question 13

How does F2,6BP affect the direction of carbohydrate metabolism?

Question 14

What is F2,6BP’s impact on homeostatic regulation of glycolysis?

Question 15

Why does F2,6BP predominate in only gluconeogenic tissues?

Question 16

What happens to ATP regulation of PFK1 in the presence of F2,6BP?

Answer 16

ATP no longer able to efficiently inhibit, ATP conc negligible for regulation of PFK-1

Question 17

What are the 2 activities of the enzyme Phosphofructokinase 2 (PFK2)?

Answer 17

Bifunctional - kinase domain and phosphatase domain (which is active depends on hormone regulation)

Active kinase (PFK-2) - incr in F-2,6-BP, incr glycolysis

Active phosphatase (FBP-2) - decr in F-2,6-BP, incr gluconeogenesis

Question 18

How does glucagon and insulin signaling affect PFK2’s activities? What proteins are involved in this regulation?

Answer 18

Liver wants to inhibit PFK-2 when blood sugar is low (needs gluconeogenesis); PFK-2 normally generates F-2,6-BP which inhibits FBPase-1 (used in gluconeogenesis)

PROTEINS

Question 19

What is the product of PFK2’s phosphatase activity? Where does that product go?

Question 20

Substrate level phosphorylation

Answer 20

Formation of ATP by transferring a phosphate group directly from substrate directly to ADP

Question 21

What reactions in glycolysis perform substrate level phosphorylation?

Answer 21

Phosphoglycerate kinase and pyruvate kinase

Question 22

How is pyruvate kinase regulated? What regulation is in all cells? Why is there a special regulation mechanism in the liver?

Answer 22

Allosteric regulation - ATP, acetyl-CoA, long-chain fatty acids (neg regulators)

WHY SPECIAL REG?

Question 23

What are the potential fates of pyruvate? What happens to pyruvate in anaerobic conditions?

Answer 23

Energy needs - Used in TCA/citric acid cycle

Low oxygen - pyruvate used for anaerobic fermentation

Question 24

How does lactate dehydrogenase increase the rate of glycolysis?

Answer 24

Lactate dehydrogenase catalyzes conversion of lactate to pyruvate

Allows reoxidization of NADH to NAD+ which allows glycolysis/TCA to continue after slowing down from NADH buildup

Prolonged muscle activity causes ATP demand to exceed oxygen supply - muscle cells become anerobic, pyruvate and NADH can’t go to TCA cycle, builds up and slows glycolysis

Question 25

What’s the point of the Cori cycle?

Answer 25

Couples glucose fermentation in exercising muscle w/ gluconeogenesis in liver

Prevents lactic acidosis (excessive accumulation of lactate) in muscle under anaerobic conditions

Question 26

How is pyruvate converted to PEP in gluconeogenesis? Why is pyruvate to PEP done in two steps?

Answer 26

CO2 group added to pyruvate w/ ATP, makes oxaloacetate
Oxaloacetate decarboxylated and phosphorylated, makes PEP

WHY 2 STEPS

Question 27

Why is biotin important and what does it do?

Question 28

Why is oxaloacetate a useful intermediate for gluconeogenesis—where else can it come from?

Question 29

Glycogen

Answer 29

What is it made of? Why does it exist and in which tissues? What enzyme makes glycogen? How is it regulated? What are its substrates and products? Why would energy be required for glycogen synthesis,a nd how does energy status of the cell affect it? How is its activity regulated by insulin and glucagon?

Question 30

How does glucagon affect glycogen synthase?

Answer 30

Glucagon promotes GS phosphorylation (inactivation)

Activates cAMP-dependent protein kinase A (PKA) and other kinases to inactivate GS and PP1

Question 31

If the phosphorylated form of a glycolytic enzyme is less active, which hormone would direct this? How?

Question 32

Which hormone would increase the activity of Glycogen synthase? How?

Answer 32

Insulin wants glycogen synthesis

Wants to prevent GS phosphorylation/inactivation by glycogen synthase kinase 3 (GSK3)

Activates phosphoprotein phosphatase 1 (PP1) to reverse any phosphorylation of GSK3

Question 33

Glc-6-P that is produced by either gluconeogenesis or glycogen catabolism is converted to Glucose by a phosphatase that is inside of the ER. Why does it make sense for a liver cell to have this in the ER rather than in the cytoplasm?

Question 34

What does fermentation in animals produce? Why does low oxygen drive this?

Question 35

Why is the NADH/NAD+ ratio important?

Question 36

What is aerobic glycolysis?

Answer 36

Where/when is it observed? Why don’t these cells use O2 when it is around to produce ATP? What else are they producing?

Question 37

What is the end product of glucose metabolism if the TCA cycle is active?

Answer 37

3 NADH, 1 ATP, 1 FADH

Question 38

Why would rapidly dividing cells prefer to keep the TCA cycle minimally active?

Answer 38

Rapidly dividing cells rely primarily on glycolysis for energy production (Warburg effect), TCA cycle moves on from glycolysis - cells want to stay in glycolysis

Question 39

How do insulin and glucagon affect PFK1 and F-1,6-Bisphosphatease activities?

Question 40

What is Protein Kinase A? How is it activated and how is its activity regulated by insulin vs glucagon?

Answer 40

Protein kinase A - key regulatory enzyme regulated by cyclic-AMP

Insulin activates PKA

Glucagon activates cAMP-dependent PKA and other kinases to inactivate glycogen synthase and PP1

Question 41

How is Pyruvate kinase affected by insulin and glucagon, and why is this regulation mostly/only in the liver?

Answer 41

Glucagon inhibits pyruvate kinase, signals liver to activate gluconeogenesis

WHY LIVER