Gluconeogenesis

Question 1

Brain requires ______ grams of glucose per day

Answer 1

120 grams

glycogen stores can supply about half that amount

Question 2

“New glucose synthesis”

Answer 2

gluconeogenesis

Question 3

gluconeogenesis is the net synthesis of glucose from non-carbohydrate sources, including:

Answer 3

• pyruvate
• lactate
• glycerol
• glucogenic amino acids (especially alanine and glutamine from muscle)
• TCA cycle intermediates (citrate, a-KG, succinyl-CoA, succinate, malate)
• propionyl-CoA

Question 4

Role of gluconeogenesis

Answer 4

to provide glucose for extrahepatic tissues (brain and nervous system, renal medulla, erythrocytes, testes, embryonic tissues) that require glucose as the main or only energy source

Question 5

Gluconeogenesis equation

Answer 5

2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H⁺ + 4 H₂O yields Glucose + 4 ADP + 2 GDP + 2 NAD⁺ + 6 Pi

Question 6

Gluconeogenesis occurs where in the body?

Answer 6

• liver
• renal cortex
• epithelial cells of small intestinal lining

Question 7

Compounds Whose Metabolism Cannot Yield Glucose

Answer 7

These include:

• (1) Acetyl-CoA, and
• (2) Even-chain fatty acids and strictly “ketogenic” amino acids lysine and leucine, which are metabolized exclusively to acetyl-CoA

Note that Acetyl-CoA loses its carbons as CO2 as it moves through the TCA cycle and therefore cannot be “recycled” back to oxaloacetate via the cycle and cannot be converted back to pyruvate because the reaction catalyzed by pyruvate dehydrogenase is irreversible in vivo

Question 8

How is the lactate that is produced during vigorous exercise dealt with?

Answer 8

It is converted back to glucose via gluconeogenesis in the liver as part of the Cori Cycle.

In this way, the body repays the “oxygen debt” incurred when glucose was broken down anaerobically because oxygen was limiting.

Question 9

The Three “Bypass” Steps of Gluconeogenesis where irreversible steps of glycolysis are being replaced

Answer 9

1. Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (replace pyruvate kinase of glycolysis)
2. Fructose 1,6-bisphosphatase (replaces PFK-1)
3. Glucose 6-phosphatase (replaces hexokinase)

Question 10

Bypass #3 (G6Pase) occurs only in certain tissues

Answer 10

Liver, kidney and epithelial cells of small intestine

Question 11

1st step occurs in _______ but rest occur in _______

Answer 11

mitochondrion; cytosol.

Question 12

Located in mitochondrion; a biotin-dependent enzyme

Answer 12

Pyruvate Carboxylase

Question 13

Pyruvate carboxylase is a heterotetramer, each subunit contains 4 domains:

Answer 13

• a biotin carboxylation domain (phase 1)
• a pyruvate carboxylase domain (phase 2)
• a domain that binds acetyl-CoA
• the domain to which biotin is attached via lysine residue

Question 14

Biotin group swings from _____ on one subunit to _____ on different subunit

Answer 14

site 1 ; site 2

Question 15

Two different isozymes exist for PEPCK, one in the ________ and one in the ________.

Answer 15

mitochondrion; cytosol

Question 16

Carries out decarboxylation and phosphorylation of oxaloacetate to give PEP

Answer 16

PEP Carboxykinase (PEPCK)

Question 17

Formation of unstable enol compound (PEP) is driven by _______ and is trapped by _______

Answer 17

decarboxylation; phosphorylation

Question 18

carboxylation-decarboxylation sequence is a way of _______ pyruvate

Answer 18

“activating”

Question 19

Bypass #2: Fructose 1,6-bisphosphatase replaces

Answer 19

PFK-1 of glycolysis

Question 20

Energy Charge regulation on pathways

Answer 20

Energy Charge

• Low E.C.
  
  • High [AMP]
  • More energy needed
  • Glycolysis stimulated
• High E.C.
  
  • Lots of ATP present
  • Biosynthetic reactions promoted
  • Gluconeogenesis stimulated, e.g., in order to store glucose as glycogen

Question 21

Fructose 2,6-bisphosphate regulation on pathways

Answer 21

Fructose 2,6-bisphosphate

• Acts mainly in liver
• Under starvation conditions
  
  • Glucagon released
  • F2,6BP levels decrease
  • Gluconeogenesis stimulated (glucose is synthesized)
• In fed state
• Insulin released F2,6BP levels increase
• Glycolysis stimulated (need to break down glucose from food for energy storage)

Question 22

Reciprocal Control of Glycolysis and Gluconeogenesis occurs at

Answer 22

• PFK-1/Fru-1,6-BPase
• PK/PC-PEPCK

The two pathways are responsive to enery charge

•Citrate and acetyl-CoA indicate cell is “flush” with biosynthetic precursors and energy

–gluconeogenesis activated

Question 23

Bypass #3: Glucose 6-phosphatase

Answer 23

• Five protein components
• Large negative DG; irreversible; enzyme is Mg2+-activated
• Operative only in certain tissues (liver, kidney, small intestine lining); in other tissues, pathway stops at glucose 6-phosphate
  
  • Lack enzyme; “want” to keep glucose as Glu-6-P, which cannot exit cell
• Phosphatase activity located on inner face of ER
• Glucose generated is transported out of cell via transporter GLUT2

Question 24

Glycogen storage disease due to defect in Glu6phosphatase

Answer 24

Von Gierke’s (a GSD)

Question 25

Produces ribose-5-phosphate, needed for synthesis of DNA and RNA, as well as the coenzymes ATP, NADH, FADH2 and coenzyme-A Especially important in rapidly dividing cells such as bone marrow, skin, and intestinal mucosa—also tumors

Answer 25

The pentose phosphate pathway

Question 26

Two phase of pentose phosphate pathway

Answer 26

oxidative and non-oxidative

Question 27

Location of pentose phosphate pathway

Answer 27

cytosol

Question 28

Glutathione exists in two interconvertible forms:

Answer 28

2 GSH (red.) = GSSG (ox.)

Question 29

GSH protects against

Answer 29

ROSs; it keeps RBCs in reduced state (2 GSH=GSSG)

Question 30

GSSG converted to GSH via Glutathione Reductase, which uses\_\_\_\_\_ as a cofactor

Answer 30

NADPH

Question 31

Gluconeogenesis

Answer 31

Question 32

Glucose CAN be made from

Answer 32

odd-chain fatty acids acids (from propionyl-CoA (succinyl-CoA)) and from the glycerol of triglycerides

Question 33

Glycerol conversion

Answer 33

Question 34

The first of the bypass steps occurs in __________ but the rest occurs in the \_\_\_\_\_\_\_\_\_

Answer 34

mitochondria; cytosol

Question 35

The pyruvate carboxylase reaction occurs in two phases:

Answer 35

1. carboxylation of biotin 2. tranfer of CO₂ group from carboxybiotin to pyruvate CO₂ (HCO₃^-) must be "activated" through phosphorylation first Absolute requirement for acetyl-CoA, a potent allosteric activator

Question 36

Oxaloacetate + GTP ---\> (yields)

Answer 36

PEP + CO₂ + GDP

Question 37

–Although DG⁰′ for pyruvate → PEP is small, DG in cell is large and negative because [PEP] is very low; therefore the 2-step process is \_\_\_\_\_\_\_\_\_\_

Answer 37

"irreversible"

Question 38

Stimulates transcription of the PEPCK gene (and others)

Answer 38

glucagon

Question 39

Glucagon also induces expression of other enzymes: And represses transcription of:

Answer 39

* Fru-1,6-BPase * Glu-6-PPase * various aminotransferases * It represses transcription of glycolytic enzymes PFK1, PK and glucokinase * Insulin has opposite effect

Question 40

The Malate-Aspartate Shuttle

Answer 40

Question 41

By means of malate shuttle, _________ produced in mitochondria is made available for a later step o of gluconeogenesis in cytosol where [] is low

Answer 41

NADH ## Footnote •A different route is followed when lactate is the precursor –Since NADH is produced in cytosol by LDH, shuttle unnecessary

Question 42

Fructose 1,6-bisphosphate + H₂O→

Answer 42

→ Fructose 6-phosphate + Pi Mg²⁺ -dependent; essentially irreversible

Question 43

Bypass #2 is tightly and reciprocally controlled \_\_\_\_\_\_\_and \_\_\_\_\_\_\_

Answer 43

allosterically and hormonally ## Footnote –When one pathway is “on,” the other is “off” –Prevents uncontrolled “futile cycling,” which wastes energy (6 – 2 = 4 NTPs) –But futile cycling can play role in fine-tuning regulation of flux through the two pathways

Question 44

Answer 44

Question 45

The Pentose Phosphate Pathway meets the needs of all organisms for NADPH, which is used for reductive biosynthesis of a number of important biomolecules like:

Answer 45

* Fatty acids * Cholesterol annd the steroid hormones * nucleotides * Neurotransmitters –Protection against reactive oxygen species (ROSs) arising from oxidative stress, which is especially important in erythrocytes and lens and cornea of the eye

Question 46

Where does the Pentose Phosphate Pathway Occur?

Answer 46

Question 47

Oxidative Generative of NADPH * Steps of oxidative phase are \_\_\_\_\_\_\_\_\_\_ * Controlled by the \_\_\_\_\_\_\_\_\_\_\_\_ * Increased reductive biosynthesis causes \_\_\_\_\_\_\_\_\_\_ * When demand for NADPH slows, \_\_\_\_\_\_\_\_\_\_\_\_

Answer 47

* irreversible * NADP+ -to- NADPH ratio * NADP+ to rise, leading to allosteric activation of Glu6PDHase & greater flux through PPP * [NADP+] drops and PPP slows

Question 48

causes cellular damage; especially harmful to RBC's

Answer 48

ROSs

Question 49

GSH acts as “sulfhydryl buffer” in RBCs:

Answer 49

* Maintains Cys residues of Hb in reduced state * Keeps iron in reduced, Fe2+, state * Detoxifies peroxides and hydroxyl free radicals •Otherwise: Hb tetramers-Heinz bodies-hemolysis-hemolytic anemia