Pentose Phosphate Pathway Flashcards
Where is the pentose phosphate pathway active?
Liver, lactating mammary gland, adipose tissue (NADPH for fatty acid biosynthesis)
Adrenal cortex, testis, ovaries (NADPH for synthesis of steroid hormones)
Erythrocytes (NADPH for reduction of glutathione )
White blood cells and macrophages (NADPH for phagocytosis )
What is the oxidative pathway of the pentose phosphate pathway?
- Irreversible reactions
- forms NADPH and pentose phosphate
- Enzymes: dehydrogenase (G6PD)
Describe the non- oxidative pathway of the pentose phosphate pathway
- reversible reactions
- Takes place under conditions where pentoses are not required by cells
Involves interconversion of sugars resulting in formation of glycolyticontermedkates
Enzymes: transketolase (TPP) and transaldolase
What is the regulated enzyme (and disorder associated of) the oxidative pathway?
Glucose-6 phosphate dehydrogenase (G6PD)
What are the reactions of the oxidative phase if the pathway?
Two reactions: catalyzed by G6PD 1 and 6-phosphogluconate dehydrogenase
Both reactions form NADPH+ H+ (reducing equivalents)
Oxidative decarboxylation by 6-phosphogluconate dehydrogenase enzyme forms Ribulose 5-phosphate (pentose phosphate)
Regulatory enzyme if pathway: G6PD
Where and when does the non-oxidative phase of the pentose pathway occur?
- Occurs in cells/ tissues that do not require pentose phosphate
- Convert excess Ribulose 5-phosphate into glycolytic intermediates (excess pentose s are metabolized by major pathways)- to recycle carbon atoms
To understand the non-oxidative phase, we Imagine that 3 molecules of glucose 6-phosphate are converted to 3 molecules of Ribulose 5-phosphate by oxidative reactions
Which pentose does epimerase interconvert?
Ribulose 5-P(formed from G6P)
And xylulose 5-P
What pentoses does does isomerase interconvert in the oxidative phase?
Ribulose 5-P (from G6P)
And ribose 5-P
What pentoses does does isomerase interconvert in the oxidative phase?
Ribulose 5-P (from G6P)
And ribose 5-P
What is the significance of thiamine (vitamin B1) to transketolase?
Transketolase uses TPP (thiamine pyrophosphate) as coenzyme
Transketolase activity in RBCs used an index of nutritional thiamine (vitamin B1) status
Laboratory test to identify nutritional thiamine deficiency
Patients with beriberi (thiamine deficiency) have low erythrocyte transketolase activity
What cells require both NADPH and pentoses?
- Only oxidative phase operates, to form NADPH and peptide phosphate
- Ribise 5-phosphate required for nucleotide biosynthesis is formed from Ribulose 5-P by isomerization
What cells only require NADPH?
RBCs
Oxidative phase operates to produce NADPH- Used in various reactions
Pentoses formed in oxidative phase are converted to glycolytic intermediates (recycling of unwanted/excess pentoses)
What cells require only pentoses ?
- Non-oxidative reversible reactions (transketolase, transaldolase, epimerase abd isomerase) convert glycolytic intermediates into pentose phosphates
- Only the non-oxidative phase operates to form pentose phosphates
What are the uses of NADPH?
- Reductive biosynthesis freactions in fatty acid synthesis and synthesis of steroid hormones
- RBCs: detoxification of hydrogen peroxide and reactive oxygen species
- Cytochrome P450 system (drug metabolizing enzymes)
- Phagocytosis in WBCs (forms reactive oxygen species)
- Synthesis of nitric oxide(NO)
Explain phagocytosis by WBCs
- On phagocytosis, neutrophils and macrophages increase their oxygen consumption and increase generation of reactive oxygen species- ‘respiratory or oxidative burst’
- Superoxide free radicals (O2-) formed using NADPH oxidase (requires NADPH)
- Superoxide converted to hydrogen peroxide by superoxide dismutase (SOD)
- H2O2 forms hydroxyl radicals (toxic to bacteria)
- H2O2 forms HOCl (hydrochloride acid) which kills bacteria and fungi
- Infections result in increased production of oxidants
What is NADPH activity responsible for?
Responsible for respiratory burst (increased oxygen consumption) following phagocytosis
Explain the results of NADPH oxidase deficiency
Chronic granulomatous disease (CGD):
Severe persistent bacterial and fungal infections
-Neutrophils show lack of increased oxygen consumption on phagocytosis (defective respiratory burst/ negative respiratory burst test)
What are the results from Myeloperoxidase deficiency ?
Increased risk of fungal infections
-Normal respiratory burst (positive respiratory burst) but decreased HOCl formation
-G6PD deficiency usually does NOT affect WBCs
Explain detoxification of H2O2 (Reactive oxygen species)
- Oxidant drugs (sulfa drugs/primaquine) or infection or Fava bean increase production of free radicals (reactive oxygen species)
- In RBCs, H2O2 detoxified to H2O by glutathione peroxidase
- Hydrogen donated by reduced glutathione (GSH)
- Selenium containing enzyme
- Oxidized glutathione (GS-SG) converted back to reduced glutathione (GSH) by glutathione reductase, using NADPH formed by pentose phosphate pathway
- GSH functions as antioxidant to detoxify H2O2 (ROS) in RBCs
- Defect in pentose pathway results in H2O2 accumulation (reactive oxygen species) that damage RBC membrane resulting in hemolysis
Explain the case report of G6PD deficiency
A 16 year old boy is diagnosed with pneumonia. He is treated with sulfamethoxazole (sulfa drugs/ sulfonamides)
On the third day of treatment, he is slightly jaundiced (increased serum bilirubin) and he feels fatigued.
His hemoglobin has fallen by 3.5 g/dL
Urine is reddish brown and is positive fir hemoglobin
Evidence of acute hemolysis
What is the most common enzyme abnormality?
G6PD deficiency
What is G6PD deficiency?
X-linked recessive disorder resulting in hemolysis
Highest prevekance in Middke East, Africa, Asia and Mediterranean
Reduced activity of G6PD (pentose phosphate pathway)—> impairs NADPH formation
Decreases cellular reduced glutathione pool (remember: GSH is major antioxidant in RBCs)
Also, mature RBCs are unable to synthesize new enzyme (do not have nucleus and cellular organelles)
Older RBCs more susceptible to hemolysis
What are the effects of G6PD deficiency in RBC ?
Impaired detoxification of free radicals and peroxide’s
-H2O2 accumulates and damages RBC membrane (unsaturated fatty acids in membrane phospholipids), resulting in hemolysis
- GSH maintains thiol (-SH) groups in proteins
- G6PD deficiency results in formation of denatured insoluble proteins (Heinz bodies)
- G6PD deficiency most severe in RBCs
- (PPP is ONLY pathway generating (NADPH in RBCs)
Summarize the mechanism of hemolysis in G6PD deficiency
Oxidative stress: infections/fava beans/ antimalarials/sulfa drugs—> GSH converted to GS-SG fir detoxifying oxidants (GSH used up for detoxification of oxidants)
Thus leads to increased requirement of NADPH to convert oxidized glutathione to reduced form (GSH)-(remember NADPH production in G6PD deficiency is low)
On one hand this leads to oxidants to accumulate, causing damage of fatty acids containing double bonds (cell membrane phospholipids), leading to RBC lysis
On the other hand this leads to denaturation of proteins due to GSH depletion, this causes formation of Heinz bodies
Explain the precipitating factors of G6PD deficiency
Old (senile) RBCs are vulnerable to G6PD deficiency, as mature RBC have no nucleus or ribosomes, and cannot synthesize new enzyme.
Precipitating factors (oxidants) Oxidant drugs -antibiotics (sulfa drugs) -antimalarials (promaquine ) -antipyreptics (aspirin)
Fava (broad ) beans- ‘favism’ (Mediterranean)
Infections
What are the G6PD variants?
About 400 pathogenic variants of G6PD known
G6PD A-: moderate hemolysis
- Young RBCs with G6PD A- have sufficient levels of the enzyme
- lower enzyme stability
- Hemolysis in presence of precipitating factors
-G6PD Mediterranean variant: severe hemolysis
