PBL 5: Pentose Phosphate Pathway:

Pentose Phosphate Pathway

a metabolic pathway in the cytosol that runs parallel to upper glycolysis

Oxidative vs Non-Oxidative Series Functions:

Oxidative: creates 2 NADPH and a CO2 and finalises with ribulose-5-phosphate from glucose-6-phosphate

Non-Oxidative: isomerisation reactions of ribulose-5-phosphate into xylulose-5-phosphate and ribose-5-phosphate, and from here carbon-skeleton rearragements occur to produce fructose-6-phosphate and glyceraldehyde-3-phosphate

Oxidative Reactions of PPP and what is made

glucose-6-phosphate → 6-phosphogluconate

• glucose-6-phosphate dehydrogenase (NADPH made and H2O used)

6-phososphogluconate → ribulose-5-phosphate

• 6-phosphogluconate dehydrogenase (NADPH and CO2 made)

• C3 hydroxyl group is oxidised to orm beta-keta acid which is unstable and readily decarboxylates to form ribulose-5-phosphate

Non-Oxidative Reactions: Ribulose-5-phosphate

Ribulose-5-Phosphate into Xylulose-5-phosphate (ketose)

• phosphopentose-3-epimerase (inversion at carbon 3)

Ribulose-5-phosphate into Ribose-5-phosphate (aldose)

• phosphopentose isomerase

• Used as building block nucleic synthesis (pyramidine and purine)

`how does PPP link to Glycolysis?

The two five carbon sugars produced by isomerisation of ribulose-5-phosphate are needed and their carbon-skeletons are rearranged to create fructose-6-phosphate and glyceraldehyde-3-phosphate

• 2 xylulose and 1 ribose

Transketolase and Transaldolase

ketolase: transfers a two carbon unit and catalyses the first and third reaction in the reaarangement process

alodolase: transfers a three carbon unit and catalyses the second reaction

First Reaction of Carbon Rearrangement

• 2-carbon unit is transferred from xylulose-5-phosphate to ribose-5-phosphate to create sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate

  • transketolase

Second Reaction of Carbon Skeleton Rearrangement:

3-carbon unit of sedoheptulose-7-phosphate is transferred to glyceraldehyde-3-phosphate to make erythrose-4-phosphate and fructose-6-phosphate

• transaldolase

Third reaction of Carbon Skeleton Rearrangement

Another xylulose-5-phosphate is used to transfer 2-carbon unit to erythrose-4-phosphate to make another fructose-6-phosphate and glyceraldehyde-3-phosphate

• transketolase

Transketolase and Transaldolase: Reversible or Irreversible?

The reactions of transketolose and transaldolase are reversible which allows the pentose phosphate pathway to respond to the needs of the organism of NADPH

Net Reactions of Oxidative

6 Glucose-6-phosphate + 12 NADP+ + 6H2O → 6 Ribulose -5-phosphate + 12NADPH + 12H+ + 6CO2

Net Reactions of Non-Oxidative

3-Ribulose-5-Phsophate → 2 Fructose-6-phosphate + Glyceraldehyde-3-Phosphare

Net Reaction of Whole Pathway

3 glucose-6-phosphate + 6 NADP⁺ + 3 H₂O → 3 CO₂ + 6 NADPH + 6 H⁺ + 2 fructose-6-phosphate + 1 glyceraldehyde-3-phosphate

Oxidative Stress:

an imbalance between unstable molecules called free radicals and the bodies ability to counteract them with antioxidants

Free Radicals:

Unstable molecules that have at least one unpaired electron in its outer shell, making them highly chemically reactive as they try to capture electrons from other structures leading to damages in cells, proteins and DNA (oxidative stress)

Oxidative Damage:

The resulting structural degradation to cellular components—including lipid peroxidation of cell membranes, protein oxidation/denaturation, and DNA damage—caused by free radicals imbalance to antioxidants

Erythrocytes

Red blood cells that transfer oxygen from the lungs to body tissues

How does PPP help against oxidative stress:

the enzyme glucose-6-phosphate dehydrogenase produces NADPH which reduces glutathionine and protects red blood cells (erythrocytes) against ocidative stress

Function of Reduced Glutathionine:

the reduced form of glutathionine is able to donate electrons to neutralise reactive oxygen species thereby protecting erythrocytes from oxidative stress (especially hydrogen peroxide)

Why is the NADPH in PPP important for erythrocytes

PPP is the only pathway that generates NADPH in mature red blood cells

• maintaining a reducing atmosphere (High NADPH to NAD+ ratio) in such cells can prevent or undo oxidative damage to proteins, lipids and other sensitive molecules

• erythrocytes are directly exposed to oxygen and theredore the damaging free radicals

How does NADPH remove dangerous free radicals and and hydrogen peroxide (reactive oxygen species)

H2O2 is converted into H2O and O2 by oxidising glutathione

• this reaction is catalysed by glutothione peroxidase

Glutothione is then reverted to its reduced state through glutothione reductase with electrons from NADPH

What is the toxic product from fava beans?

Divicine- free radicals and reactive oxidative agents are generated through the actions of drugs like primaquionine and natural products like divicine

Glucoe-6-phosphate dehydrogenase deficiency:

leads to a decrease in NADPH being created

1. Oxidized glutathione (GSSG) cannot be reduced back to GSH.

1. Hydrogen peroxide and other ROS accumulate in the erythrocyte.

2. Unchecked oxidative damage causes lipid peroxidation of cell membranes and protein oxidation, causing red blood cells to lyse (break open) within 24 to 48 hours.

3. This hemolysis releases free hemoglobin into the blood, leading to jaundice and potentially acute kidney failure.

Erythrocyes in Favism

erythrocytes lyse due to favism because G6PD deficiency leaves them defenseless against severe oxidative stress triggered by compounds of fava beans

How does H2O2 accumulate in the cell?

accumulates in the cells as a consequence of aerobic respiration and exposure to toxic compounds

How does G6PD deficiency effect defense against super radicals

because NADPH is reliant on glucose-6-phosphate dehydrogenase, and if it is deficient it cannot make NADPH, which cannot reduce glutathione, and if glutathione is not reduced, it cannot reduce super radicals

Favism

severe hemolytic reaction (rapid breakdown of red blood cells) triggered by eating fava beans in people who have an inherited deficiency of the Glucose-6-phosphate dehydrogenase deficiency