Lecture 27

Question 1

The following all have what in common?

Hexose Monophosphate pathway
Pentose Shunt
Phosphogluconate pathway

Answer 1

All of these are synonyms for the “Pentose Phosphate Pathway”

Question 2

Describe what the following Enzymes do.

Glucose-6-Phosphate Dehydrogenase:

Ribulose-5-Phosphate Isomerase:

Ribulose-5-Phosphate Epimerase:

Transketolase:

Transaldolase:

Phosphoglucose Isomerase:

Gluconeogenic Enzymes:

Answer 2

Glucose-6-Phosphate Dehydrogenase: Forms Ribulose-5-Phosphate and NADPH from G6P

Ribulose-5-Phosphate Isomerase: Forms Ribose-5-Phosphate from Ribulose-5-Phosphate

Ribulose-5-Phosphate Epimerase: Forms Xylulose-5-Phosphate from Ribulose-5-Phosphate

Transketolase:

1. Takes Xylulose-5-P and Ribose-5-P and creates GAP and Sedoheptulose-7-P
2. Takes Xylulose-5-P and Erythrose-4-P and creates GAP and F-6-P

Transaldolase: Takes GAP and Sedoheptulose-7-P and creates Erythrose-4-P and F6P

Phosphoglucose Isomerase: Forms G6P from F6P

Gluconeogenic Enzymes: Forms F6P from GAP

Question 3

State the 3 enzymes that are involved in the Oxidative Phase of the pentose phosphate pathway. Also state the Reactants and products of this oxidative phase.

Answer 3

1. G6P Dehydrogenase
2. Lactonase
3. 6-Phosphogluconate Dehydrogenase

Reactant: G6P
Product: Ribulose-5-P

Question 4

_____decarboxylation of 6-phosphogluconate occurs to form Ribulose-5-P. NADP+ is ______ to form NADPH.

Answer 4

Oxidative (decarboxylation)

Reduced

Question 5

State the 2 main roles of NADPH in the cell

Answer 5

1. Synthesis of monomers

2. Reduction (of Oxidized glutathione and cytochrome p450 monooxygenases)

Question 6

Describe the structure of Glutathione and the AA’s that make up it’s structure

Answer 6

Glutathione is a Tripeptide

Composed of Glu (turned 90 degrees to the left), Cys, and Gly

(Glutamic Acid, Cysteine, and Glycine)

Question 7

State the 3 functions of glutathione’s role in Redox chemistry and include the mechanism/cofactors for these functions. Also give examples of these functions in action.

Answer 7

1. Interaction with proteins: via it’s Cysteines forming disulfide bonds
   ex. Disulfide bonds stabilize protein structures (in correctly and incorrectly folded proteins)
2. Inactivation of Peroxides: requires glutathione peroxidase
   ex. ROS species homeostasis is maintained this way (too much = damage ; too little = signaling mechanism cannot occur)
3. Regeneration of reduced glutathione: uses NADPH
   ex. Reduced glutathione is stored bc it is the more useful molecule (compared to oxidized glutathione)

Question 8

What is the most common genetic disease? describe it’s genotype/mutation.

Answer 8

G6P Dehydrogenase Deficiency (G6PDD)

G6PDD is an X-linked, autosomal recessive condition

Question 9

State the 2 phenotypes of G6PDD we should be familiar with (include their “Class”, “activity”, and what causes their acute hemolysis symptoms)

Answer 9

Mediterranean B- G6PDD: causes acute hemolysis due to “normal levels of oxidant stress”
Class II with <10% of G6PD activity

African A- G6PDD: causes acute hemolysis due to “high levels of oxidant stress”
Class III with 10-60% of G6PD activity

Question 10

In a pt with G6PDD, explain the impact that shortening the lifespan of G6PD will have on the lifespan of RBC’s. Also explain how “Bite cells” are formed.

Answer 10

Shortening the lifespan of G6PD will directly reduce the lifespan of RBC’s as well. This is bc Fe must be reduced by the NADHP molecules (created by G6PD activity) In order to be able to bind to oxygen.

Bite cells: are formed when Heinz bodies are removed from an RBC

Question 11

True or False:

G6PD is the only enzyme that can make enough NADPH to sustain the human body. explain.

Answer 11

False? (mostly)

1. Isocitrate Dehydrogenase
2. Malic Enzyme (M1)
   can both make NADPH, but not enough to support the needs of the human body.
   RBC’s are especially dependent bc they cannot synthesize enough NADPH without active G6PD (bc they don’t have enough machinery to do so)

Question 12

What is the most severe effect of G6PDD and briefly explain the physiology behind this issue

Answer 12

Jaundice

G6PDD means that the liver cannot conjugate bilirubin, which leads to the excessive amounts of unconjugated bilirubin that causes Jaundice
(kernicterus = jaundice severe enough to enter the brain)

Question 13

Describe the 3 structural forms of G6PD that can be found in the human body and include their activity level

Answer 13

1. Monomers (single molecule) which are inactive
2. Dimers (Homodimer) which are active
3. Tetramers (dimer of dimers) which are active

Question 14

______is both a substrate and a coenzyme. Explain this.

Answer 14

NADP+

NADP+ is a substrate when it is acted on by G6PD to form NADPH

Structural NADP+ molecules function to hold 2 monomers of G6PD together to form G6PD dimers (active)

Question 15

Briefly explain what Ataxia Telangiectasia has to do with G6PD. (there is an Hsp to be mentioned here as well)

Answer 15

AT(Ataxia Telangiectasia) is caused by an autosomal recessive mutation to the ATM gene

The ATM gene controls the rate of cell division and “sounds the alarm” when it detects double stranded DNA damage

Upon detecting Double stranded DNA damage, ATM activates Hsp27

Hsp27 promotes G6PD dimerization (activating it)

(basically, AT decreases the activity level of G6PD by interfering with the pathway that activates it)

Question 16

State the 3 methods by which G6PD is regulated. Divide the following actions into the categories of Activators or Inhibitors of G6PD activity.

Transcription factors for antioxidant genes
Cell cycle and synthesis activators
Apoptosis-signaling proteins 
Insulin
Phosphorylation
Dimerization

Answer 16

1. Transcriptional/Translational control
2. Location in the cell
3. Post-translational controls

Activators:
Dimerization
Transcription factors for antioxidant genes
Cell cycle and synthesis activators
Insulin

Inhibitors:
Phosphorylation
Apoptosis-signaling proteins

Question 17

Describe the relationship between G6PDD and diabetes. What does the effect of high external glucose (common in diabetics) have on the Pancreatic Beta-islet cells?

Answer 17

G6PDD pt’s are likely to develop diabetes bc Insulin conducts it’s blood glucose lowering effects by activating G6PD.

(Pancreatic Beta-islet cells produce insulin and naturally express low levels of G6PD)

High levels of external glucose result in decreased G6PD

Question 18

The nonoxidative phase of the pentose phosphate pathway shuffles carbons. For the 2nd and 3rd “shuffles” state what stabilizes their transition state and the enzyme that catalyzes that “shuffle”

Answer 18

Shuffle 2: Transketolase enzyme
Stabilized by TPP (Thiamine pyrophosphate) coenzyme

Shuffle 3: Transaldolase enzyme
Stabilized by Lys sidechain

Question 19

During the last “shuffle” of the nonoxidative phase of the pentose phosphate pathway, state the pathway that is used to regenerate G6P

Answer 19

The gluconeogenesis pathway

Question 20

For modes 1-4 of the pentose phosphate pathway, state the “need” that the cell will most likely be addressing. Include the primary products of each mode.

Answer 20

Mode 1: PP = Ribose-5-Phosphate
addressing the need for more nucleotides

Mode2: PP = Ribose-5-phosphate & NADPH
addressing the need for “a little bit of everything”

Mode 3: PP= NADPH
Addressing the need for NADPH

Mode 4: PP= Fructose-6-Phosphate + GAP (pyruvate)
Addressing the need for “energy”