Lecture 26

Question 1

Regulation of PPP:

Answer 1

• Flux of pathway depends on the need of the cells.

• Examples:
• When NADPH is being used NADP+ drives the oxidative phase
• When ribose is not needed carbons are diverted to glycolysis.

Note that pentoses can be made even without running the oxidative phase of PPP.

(Ribose-5-phosphate is a precursor for Nucleotides needed to make DNA and RNA.)

Question 2

Importance of PPP for detoxification

Answer 2

The cellular weapon against damaging reactive oxygen species (e.g. peroxides) is glutathione:

NADPH generated in PPP is used to regenerate (reduce) glutathione.

People with mutations that decrease Glc-6-P-DH activity are hypersensitive to oxidative stress.

Question 3

Glucose-6-Phosphate Dehydrogenase Deficiency

Answer 3

• 400 million people worldwide have G6PDH deficiency
• They are more resistant than normal people to malaria.
• But they also are more sensitive to oxidizing agents because they can’t make enough NADPH to keep Glutathione reduced.
• In erythrocytes reduced glutathione helps keep Fe in the Fe2+ oxidation state.
• Fe3+ hemoglobin does not bind O2 and it causes erythrocytes to change shape.
• Erythrocytes get broken down - causing anemia.

Question 4

Gluconeogenesis why

Answer 4

To provide glucose for brain (120g/day) and erythrocytes (red blood cells, 160 g/day).

To convert lactate back into glucose during exercise.

Question 5

Gluconeogenesis where

Answer 5

In liver , NOT in muscle, fat or brain

Mostly in cytosol

Question 6

Gluconeogenesis when

Answer 6

starvation - (precursor- amino acids)

exercuse (precursor- lactate)

Question 7

Anabolic pathway, where does the energy come from?

Answer 7

From fatty acid oxidation - Here we are burning fat!

Fatty acids are NOT a precursor for glucose because we can NOT convert acetyl CoA into pyruvate. Plants can do it!

Question 8

Gluconeogenesis summary

Answer 8

2pyruvate +4ATP +2GTP +2NADH +2H+ +6H2O —->

glucose +4ADP +2GDP +6Pi +2NAD+

Question 9

ΔG numbers in glycolysis*
hexokinase
phosphofructokinase
pyruvate kinase

Answer 9

h=-27.2
pfk= -25.9
pk= -13.9

*ΔG, NOT ΔG0

Question 10

From pyruvate to PEP: Two enzymes and a shuttle

Answer 10

Pyruvate Carboxylase uses Biotin (Vitamin B7)
Required activator:Acetyl-CoA

pyruvate +pyruvate carboxylase –> oxaloacetate

oxaloacetate –(PEPCK)–> phosphoenol-pyruvate (PEP)

Required activator: Acetyl-CoA

Question 11

how is oxaloacetate shuttled from mitochondrial matrix into cytoplasm

Answer 11

Oxaloacetate is shuttled as malate from the mitochondrial matrix into cytoplasm.

This also shuttles reducing power to the cytoplasm.

Question 12

two ATP equivalents

Answer 12

what is used to reverse the pyruvate kinase step?

Question 13

Regulation of glycolysis and gluconeogenesis in muscle and liver

Answer 13

It is important to avoid “Futile Cycles”
that just hydrolyze ATP without accomplishing
anything.

1. Phosphofructokinase 1
2. Phosphofructokinase 2
3. Pyruvate Kinase

Question 14

Fructose bis phosphatase-1 converts

Answer 14

F1,6BP into F6P

Question 15

Activation of PFK-1 by F-2,6BP

Answer 15

F-2,6BP is an mportant allosteric effector in liver that shifts the inactive state to the active state for PFK-1.

F-2,6BP is made in liver cells only when blood sugar is high.

Note that Vmax increases (a little) and Km decreases for the F-6-P substrate.

Question 16

regulation steps

Answer 16

1. Phosphofructokinase-1 and Fructose -1,6 bisphosphatase
2. Phosphofructokinase-2 and Fructose-2,6 bisphosphatase
3. 1. Allosteric regulation of PFK-2 / FBPase-2
4. 2. Hormonal regulation of PFK-2 / FBPase
5. Pyruvate kinase
6. 1 allosteric regulation
7. 2 hormonal regulation of pyruvate kinase (only in liver)

Question 17

2. Phosphofructokinase-2 and Fructose-2,6 bisphosphatase

Answer 17

PFK-2 and FBPase-2 are unique in that both catalytic domains reside on one polypeptide chain (bifunctional enzyme).

Question 18

2.1. Allosteric regulation of PFK-2 / FBPase-2

Answer 18

Consider PFK-2 a sensor for Fructose-6-phosphate

In summary, this mechanism is a feed forward activation of PFK-1 by its substrate, fructose-6-phosphate.

What other enzyme in glycolysis is regulated by feed forward activation?

Question 19

2.2. Hormonal regulation of PFK-2 / FBPase

Answer 19

(only in liver – muscle PFK-2 has no phosphorylation site)

Why are both activities, PFK-2 and FBPase-2, located on one protein?

Phosphorylation can now function like a switch:
ON = phosphatase OFF = kinase

Question 20

summary

Answer 20

low blood glucose
increased glucagon secretion
increased [cAMP]
increased enzyme phosporylation
activation of FBPase-2 and inactivation of PKF-2
decreased [F2,6P]
inhibition of PFK and activation of FBPase
increased gluconeogenesis

Question 21

what is a precursor for glucose in gluconeogenesis

Answer 21

alanine

Question 22

what is made in the liver and released into blood

Answer 22

Glucose made in the liver by gluconeogenesis is released into the blood by Glucose-6-Phosphatase

Question 23

why can only the liver make glucose

Answer 23

because only the liver has Glucose-6-Phosphatase

Question 24

what released glucose made in the liver

Answer 24

Glucose-6-Phosphatase releases glucose made in the liver either from Glycogen Breakdown or from Gluconeogenesis.

Question 25

what has a high km and why

Answer 25

Hexokinase in the liver has a very high Km so it won’t rephosphorylate the glucose.

Question 26

The Cori-Cycle

Answer 26

• When MUSCLES are very active they cannot oxidize pyruvate as fast as they make it by glycolysis.
• They release LACTATE.
• The LIVER “recycles” the LACTATE to GLUCOSE to support more glycolysis in the muscle.

Question 27

what supplies carbons for gluconeogenesis

Answer 27

pyruvate and amino acids

Question 28

what provides energy for gluconeogenesis

Answer 28

fatty acids

Question 29

carbons from acetyl-CoA ____ be used to increase carbohydrates

Answer 29

cannot

Question 30

Key Concepts for Gluconeogenesis

Answer 30

• Occurs primarily in liver (and kidney)
• Provides glucose under starvation conditions.
• Carbons come from lactate and amino acids, ATP comes from fat oxidation.

• Exergonic reactions from glycolysis require specific types of reactions to be reversed.
– Pyruvate carboxylase, PEPCK, F1,6 Bisphosphatase, Glucose-6-Phosphatase
– Regulation PFK2, PK regulation

• CoriCycle