7. Pentose Phosphate Pathway (Hexose Monophosphate Shunt)/HMP Shunt

What is the pentose phosphate pathway?

• The pentose phosphate pathway is an alternate route to glycolysis & TCA cycle for glucose oxidation

• 10% of glucose molecules/day are entering this pathway. The liver & RBC metabolize about 30% of glucose by this pathway.

Where does PPP take place?

Cytosol

Is the PPP more anabolic or catabolic?

Anabolic

What are the 2 main functions of PPP?

1. Generation of NADPH: mainly used for reductive synthesis of fatty acids, steroids, amino acids via glutamate dehydrogenase; and production of reduced glutathione in erythrocytes and other cells

2. Production of ribose for nucleotides and nucleic acid synthesis

Describe the 2 phases of HMP Shunt pathway.

1. Oxidative phase: where NADH is produced

2. Non-oxidative phase: synthesis of 5-carbon sugars: hexoses —> pentoses (ribose-5-p)

Describe the reactions in the HMP shunt pathway.

• Glucose-6-phosphate is converted to 6-phosphogluconate using the enzyme glucose-6-phosphate dehydrogenase (G6PD)

• NADP+ is reduced to NADPH

• 6-phosphogluconate is converted into ribulose-5-phosphate

• NADP+ is reduced to NADPH

• CO2 is produced (decarboxylation)

• Reactions are oxidative so are irreversible

How is the HMP shunt pathway regulated?

• NADPH is a competitive inhibitor of glucose-6-phosphate dehydrogenase (G6PD)

• NADPH ratio is high under most conditions so G6PD is inhibited

• NADPH ratio is low when there is demand for NADPH (G6PD is active)

• NADP+ stimulates G6PD to produce more NADPH

What are the differences between NADH and NADPH?

• NADH is used for reducing reactions in catabolic processes (e.g. TCA cycle)

• NADH enters ETC and ATP is generated

• NADPH is only generated by HMP pathway

• NADPH is used for reductive biosynthetic reactions

• NADPH does not enter ETC and does not generate ATP

What is the overall equation for the HMP pathway?

Glucose-6-phosphate + 2NADP+ + H2O → ribose-5-phosphate + CO2 + 2NADPH + 2H

Why is there no ATP in the equation?

• ATP is neither used nor produced by HMP shunt pathway because the cells do not use HMP shunt pathway for energy production

What is the physiological significance of the HMP shunt pathway?

It takes place in the following organs:

• Liver

• RBC

• Adipose tissue

• Lens of eyes

• Testes and Ovaries

• Adrenal cortex

• Mammary glands

  • Oxidative phase seen in all above organs

  • Non-oxidative phase is present in all tissues of the body, so synthesis of ribose is possible in all tissues of the body.

What is the importance of NADPH?

• Synthesis of FAs, steroids, and certain amino acids

• Protect cells from free radical damage through antioxidant reactions involving NADPH

• Detoxification of drugs and toxins by hydroxylation reaction requires NADPH (by liver using p450 enzyme along w/NADPH)

• Preserves integrity of RBC membrane

• Keeps Hb iron in reduced form (Fe2+) in RBCs

Describe how NADPH protects cells from free radical damage.

• O2 is converted into superoxide anion which means it has an electron taken out so now it is a radical

• 2 things can now happen

• The superoxide anion can be converted into a deadly chemical called peroxy nitrite; this can only happen when this is a nitric oxide radical

• The enzyme superoxide dismutase converts superoxide anion into less dangerous molecule called hydrogen peroxide, H2O2

• H2O2 is converted into dangerous molecule called hydroxyl radical, OH˙

• Glutathione peroxidase converts H2O2 into H2O and oxygen

• Glutathione has thiol group in reduced form (SH)

• It is a naturally occurring antioxidant

• 2 Glutathiones can react with H2O2 can converts it into water and oxygen using glutathione peroxidase

• When the glutathione in reduced form unloads its hydride onto H2O2, they get converted into oxidized form; they can no longer give hydride ions to H2O2, so cannot get rid of free radicals

• NADPH get oxidized (NADPH → NADP+) to turn oxidized Glutathiones into reduced glutathiones

• Enzyme used is glutathione reductase

What are the dangerous effects of peroxy nitrite?

Peroxy nitrite can oxidize proteins which can destroy them, it can nitrate proteins which alter their activity, it can inactivate enzymes that control channels within the body, and they can cause lipid peroxidation which have dangerous effects on cell membranes

What are the dangerous effects of OH radicals (hydroxy radicals) in the body?

Can cause DNA damage, cause lipid peroxidation, and leads to cytotoxic effect

Erythrocytes need _____ to maintain their ___________ levels.

NADPH
glutathione (GSH)

Describe how NADPH helps maintain RBC membrane integrity.

• Oxidized glutathione (GSSG) leads to rigid and fragile RBC membrane

• Glutathione reductase uses NADPH to regenerate reduced glutathione

Glutathione reductase

GSSG + NADPH —> 2GSH + NADP

• NADPH, GSH, and glutathione reductase all contribute to RBC membrane integrity

How does NADPH prevent met-hemoglobinemia?

• Met-hemoglobin: Iron in the ferric state Fe 3+ and not Fe 2+

• This is not good because the hemoglobin will be physiologically inactive

• NADPH is used to supply reducing equivalents to Met hemoglobin reductase, the enzyme that returns the hemoglobin to the Fe+2 state.

What is the role of NADPH in the lens of the eye?

• Max. conc. of NADPH is seen in lens of eye

• Helps with preserving transparency of lens

What is hemolytic anemia caused by?

1. Oxidant drugs (primaquine (antimalarial drug), sulfa drug (antibiotic))

2. Ingesting fava bean (divicine) produce hemolytic jaundice.

3. Severe infection result in generation of free radicals —>destroy RBC —>anemia.

What is the mechanism of hemolytic anemia?

1. Lack of G6DP leads to loss of NADPH production

2. This leads to loss of glutathione production (antioxidant system)

3. This leads to increased oxidative stress leads to membrane damage

4. Red blood cell lysis occurs

Describe G6P deficiency.

• Common deficiency

• Transmitted as an X-linked recessive trait

• Deficiency occurs in all cells but severe in RBC

• Major cause of hemolysis, leads to hemolytic anemia

• Reducing the activity of G-6-PD impairs the production of NADPH in RBC, this result in accumulation of MetHb & peroxides in RBC which lead to hemolysis.

How does G6DP deficiency linked with resistance to malaria?

• Parasite that cause malaria are dependent on availability of GSH for their optimum growth in RBC, which will not be available in deficiency of G6PD

• Common disease in some countries

• It is regarded as adaptability of people living in malaria infected regions.

Describe Wernik-korsakof syndrome.

• Genetic disorder associated with HMP shunt

• Cause: defect in transketolase enzyme (found in non-oxidative phase), which require TPP

• Disorder is seen in alcoholics and those with thiamine deficiency

• Symptoms: mental retardation, loss of memory, partial paralysis

What are the outcomes of the Pentose Phosphate Pathway?

1. Generation of reducing equivalents in the form of NADPH, used in reductive biosynthesis reactions within cells (e.g. FA synthesis

2. Production of ribose-5-phosphate used in synthesis of nucleotides and nucleic acids