2012-11-27 3&4: Gluconeogenesis; Pentose Pi Pathway

Question 1

Quantitatively, what is the most important gluconeogenesis precursor?

Answer 1

lactate

Question 2

Where does gluconeogenesis occur?

Answer 2

most: liver
some: kidney

Question 3

Gluconeogenesis is in many ways just the reverse of glycolysis, but it bypasses some of the enzymes. Which are those?

Answer 3

The non-reversible steps

1. pyruvate kinase
2. PFK
3. hexokinase

Question 4

Which takes the place of pyruvate kinase in gluconeo?

Where does this occur?

What else (beside enzyme(s)) is required?

What regulates it?

Answer 4

Two steps actually req’d:

1. pyruvate—pyruvate carboxylase—>oxaloacetate [mitochondria]
2. oxaloacetate—PEP carboxykinase—>PEP [cytosoll]

requires 1 ATP and 1 GTP (respectively); pyruvate carboxylase also requires biotin (B7)

regulated by Acetyl-CoA which signals low glucose

Question 5

How do you get oxaloacetate across the mito membrane?

Answer 5

in mito: oxaloacetate—malate DH—>malate

in cyto: malate—MDH—>oxaloacetate(—>etc.—>glucose)

Question 6

What bypasses the PFK-1 rxn in gluconeo?

Answer 6

F-1,6-BP—F-1,6-BPase—>F-6-P

Question 7

What regulates F-1,6-BPase?

Answer 7

Activate: citrate and ATP

Inhibit: F-2,6-P and ADP (coordinate reg so you don’t do both gluconeo and glycolysis at same time)

Question 8

When would you do both gluconeo and glycolysis at same time?

Answer 8

to generate heat

Question 9

What conditions would you expect would favor gluconeogenesis and slow down glycolysis?

Answer 9

Lactate levels: low in a.m.; really high s/p exercise

Question 10

What conditions would favor gluconeogenesis and slow down glycolysis?

Answer 10

High ATP would do both

high lactate and high mito [NADH,H+] favor gluconeogenesis

Question 11

What enzyme is used to bypass hexokinase?

Answer 11

glucose-6-phosphatase

Question 12

What is the Cori Cycle?

Answer 12

Muscle: Glucose—>Lactate
——lactate to liver via blood——
Liver: Lactate—>glucose
——glucose to muscle via blood—

Question 13

How much ATP does Cori cycle require?

Answer 13

6

Question 14

Does gluconeogenesis from glycerol require more or less ATP than using lactate or a.a.?

Answer 14

Less b/c glycerol jumps right into G3P with one ATP.

Skips the two other ATP-requiring steps

Question 15

What steps in gluconeo require ATP?

Answer 15

1. pyruvate—pyruvate carboxylase—>oxaloacetate
2. oxaloacetate—PEP carboxykinase—>PEP
3. 3-PGA—>1,3-BPG

Question 16

How can you make pyruvate?

Answer 16

from 1) glycolysis and 2) from lactate (via LDH)

Question 17

What are the possible fates of pyruvate?

i.e. What is the first metabolite it would become going down these various pathways? Via which enzymes?

Answer 17

1) lactate via LDH [in anaerobic resp]
2) acetyl-CoA via PDH [in CAC]
3) oxaloacetate via pyruvate carboxylase [in gluconeogenesis]

Question 18

What is PDH?

Answer 18

pyruvate DH is a oligomeric protein made up of 3 separate enzymes:
E1 (pyruvate decarboxylase), E2 (dihydrolipoyl transacetylase) and E3 (dihydrolipoyl DH)

It requires five co-factors!

Located in mito

Question 19

Overall what is the rxn PDH catalyzes?

Answer 19

pyruvate + CoA + NAD+ —PDH—>acetyl-CoA + NADH,H+ + CO2

Question 20

How is PDH regulated?

Answer 20

inhibited by a protein kinase

—kinase is stimulated by ATP, acetyl-CoA, NADH (makes sense b/c shows you have already done enough glycolysis)

—kinase inhibited by pyruvate, ADP

activated by a protein phosphatase
—in turn activated by Ca2+ (muscle contraction link), also insulin?

Question 21

What is lactose made of?

Answer 21

galactose + glucose

Question 22

What is sucrose made of?

Answer 22

fructose + glucose

Question 23

What is maltose made of?

Answer 23

2 glucoses

Question 24

What causes galactosemia? Sx?

Answer 24

loss of f(x) in Galactose-1-P uridyl transferase causes galactol to build up

sx = causes galactol build-up in lenses—>cataracts

Question 25

What causes fructose intolerance?

Answer 25

loss of f(x) in liver aldolase; F1P accumulates causing liver damage

Question 26

What is G6PDH deficiency?

Answer 26

most common enzymopathy

• affects RBCs ability to make NADPH b/c they can only do so via G6PDH
• only symptomatic during periods of ox. stress (e.g. drugs, foods, infection)
• confers protection against malaria

Question 27

In which tissues does the pentose phosphate pathway MORE active?

Answer 27

adipose, liver, lact mamm, adrenal cortex, thyroid, testes, RBCs

Question 28

In which tissues does the pentose phosphate pathway LESS active?

Answer 28

cardiac and skel muscle

Question 29

Where are disaccharides broken down to hexoses?

Answer 29

the surface of endothelial cells in the intestines

Question 30

What does the pentose phosphate pathway make?

Answer 30

pentoses, NADPH and ATP

Question 31

What step in the pentose phosphate pathway makes NADPH?

Answer 31

The first two steps:

1. G-6-P—G-6-Pi DH—>P-gluconate
2. P-gluconate—P-Gluconate DH—>Ribulose-5-P

—The stoich here is 6 for all involved, so 12 total NADPHs are produced!

Question 32

What step in the pentose phosphate pathway makes ribose?

Answer 32

The third step:

3. Ribulose-5-P—pentose isomerase–>Ribose-5-P—>—>ribose

Question 33

What are the major end products of the PP pathway? What is formed along the way?

Answer 33

1. synth of ribose-5-Pi (forms NADPH, too)
2. conversion back to G-6-Pi (NADPH is formed, too)
3. Metabolism to lactate via F6P to G3P (NADPH and ATP formed, too)

Question 34

If glucose in converted to ribose-5-Pi and all of the! ribose-5-Pi is converted to deoxyribose-5-Pi for! synthesis of DNA…

(a) there will be no NADPH formed
(b) there will be no ATP formed
(c) there will be no ATP nor NADPH formed
(d) both ATP and NADPH will be formed.

Answer 34

B