Pentose Phosphate Pathway

Question 1

What is the PPP?

Answer 1

• A pathway which completely oxidizes glucose to CO2 BUT it neither produces nor consumes ATP

Question 2

The important and specialised metabolic needs that the PPP meets

Answer 2

• supplies ribose-5-phosphate for nucleotide and nucleic acid synthesis
• supplies reducing power in form of NADPH for fatty acid synthesis
• provides a route for surplus pentose sugars in diet to be brought into glucose metabolism
• recycles sugars according to the needs of the cell

Question 3

PPP enzymes in tissues

Answer 3

• tissues involved in fatty acid synthesis & steroid biosynthesis – liver, adipose tissue, adrenal cortex, testis & lactating mammary gland
• erythrocytes have large amounts of NADPH to maintain reduced glutathione

Question 4

PPP enzymes in skeletal muscle

Answer 4

• very low levels
• Skeletal muscle commits
  the vast majority of its glucose to glycolysis

Question 5

Percentage of PPP enzyme in glucose oxidation

Answer 5

• 80-90% of glucose oxidation is by Glycolysis 10-20% is by PPP

Question 6

1st phase of PPP

Answer 6

• irreversible oxidation
  of Glucose 6-Phosphate
• G-6-P converted to 6-phosphogluconate (via 6-phosphogluconolactone) by G-6-P dehydrogenase
• 6-phosphogluconate dehydrogenase reduces NADP+ and generates ribulose-5-phosphate (which was decarboxylated from b-keto acid

Question 7

Overall reaction of oxidative phase

Answer 7

• G-6-P + 2NADP+ + H2O –> Ribulose 5-phosphate + CO2 + 2NADPH + 2H+

Question 8

Control of NADPH regulation

Answer 8

• NADPH is a potent inhibitor of G 6-P Dehydrogenase (allosteric)
• High NADP+ stimulates G 6-P dehydrogenase activity & therefore NADPH production

Question 9

First step of non-oxidative phase of PPP

Answer 9

• R-5-P 3-epimerase converts 6 R-5-P into 2 xylulose-5-phosphate
• R-5-P isomerase converts R-5-P into 2 ribose-5-phosphate

Question 10

2nd step in non-oxidative phase (transketolase activity)

Answer 10

• transketolase: thiamine pyrophosphate co-factor
• converts 2 R-5-P into 2 glyceraldehyde-3-phosphate
• converts 2 X-5-P into 2 sedaheptulose-7-phosphate

Question 11

3rd step of non-oxidative phase (transaldolase activity)

Answer 11

• converts 2 S-7-P into 2 erythrose-4-phosphate

- converts 2 G-3-P into 2 fructose-6-phosphate

Question 12

Transketolase in 4th step of non-oxidative phase

Answer 12

• converts 2 E-4-P into 2 G-3-P

- 2 G-3-P reforms 1 glucose 6-P - put back into start of PPP

Question 13

Phosphoglucose isomerase in non-oxidative phase

Answer 13

• converts 2 F-6-P into 2 glucose-6-P

- which is put back into start of PPP

Question 14

Differential utilisation of PPP

Answer 14

• PPP substrates DO NOT have a clearly defined course
  1) If the need for NADPH and Ribose 5-P are balanced, mainly the oxidative part of the reaction is utilised
  2) If more NADPH than Ribose 5-P is required, the whole of the pathway is used
  3) If the cells primary need is for Ribose 5-P for nucleotide synthesis, the non-oxidative part of the pathway operating in reverse is used

Question 15

G6PD Deficiency

Answer 15

• X linked inherited disease (Xq28)
• > 400 x 106 people affected worldwide. Most common enzyme abnormality
• Most prevalent in the Middle East, tropical Africa & Asia & parts of the Mediterranean – remarkably similar distribution to malaria

Question 16

Haemolytic anaemia - G6PD deficiency

Answer 16

• class I: very severe (chronic haemolytic anaemia), residual activity <10%
• class II: severe (episodic/acute haemolytic anaemia, neonatal jaundice), residual activity <10%
• class III: moderate (episodic), residual activity 10-60%
• class IV: no clinical symptoms, residual activity >60%

Question 17

Effects of increased Oxidative stress on Erythrocytes

Answer 17

• Cleavage of fatty acid C-C bonds causes membrane damage –> lysis
• Oxygenation of sulphydryl gps in proteins including Hb –> Protein denaturation –> Heinz Bodies –> Cells removed by spleen

Question 18

Why is G6PD deficiency characterised by

breakdown of erythrocytes and not other cell types?

Answer 18

• All cells except erythrocytes have alternative sources of NADPH production, e.g., NADP+- dependent Malate dehydrogenase
• they have no nucleus or ribosomes to produce more enzyme & must survive on the enzymes they have at genesis/maturity
• Under severe oxidative stress they will consume all reduced glutathione. If G6DP is mutated then NADPH cannot be produced & thus glutathione will not be reduced

Question 19

Effects of G6PD deficiency

Answer 19

• Increased breakdown of erythrocytes
• Breakdown exceeds rate of erythropoiesis (i.e. production)
• Anaemia: Weakness/ fatigue, general malaise,
  poor concentration, dyspnea on exertion, palpitations, sweatiness, etc.
• Jaundice: Accumulation of breakdown products of Hb in blood. Urine dark brown in colour as breakdown products are excreted

Question 20

Precipitating factors

Answer 20

• oxidant drugs: Antibiotics, Antimalarials, Antipyretics
• infection: most common precipitating factor
• favism: Vicia faba also known as fava beans, Contain high levels of vicine, divicine, convicine & isouramil — all are oxidants