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HMP SHUNT [Hexose Monophosphate Pathway] / HMP SHUNT [Hexose Monophosphate Pathway] / PPP [Pentose Phosphate Pathway] PPP [Pentose Phosphate Pathway]

• Principle pathway for oxidation of Glucose-Glycolysis & TCA

• HMP-alternative pathway for oxidation of Glucose-not for Energy

• Occours in cytosol

• Provides: NADPH [reductive synthesis] & Pentoses [for NA synth]

• Most common at the site of synthesis: Liver [phospholipid, FA synth, Cholesterol], adipose tissue [FA synth], lactating mammary gland [FA synth], adrenal cortex [Cholesterol, steroid hormone synth], testes & other endocrine glands concerned with steroid synthesis and RBC

• NADPH produced: required for – Reductive biosynthesis of FA, TG, cholesterol, steroids[NADH – reduction in Catabolic pathways (NADH enters ETC→ ATP);NADPH – reduction in Synthetic pathways]

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HMPHMP

Anabolic pathway that utilizes the 6 carbons of glucose to generate 5 carbon sugars and reducing equivalents

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Oxidative reactionsOxidative reactions

1. Dehydrogenation of G-6-P by G6PD: • Irreversible oxidation to 6-PGL- sp. coenzyme NADP+

• 1st NADPH is produced• HMP-primarily regulated at G6PD• NADPH-comp inhibitor of G6PD

2. Hydrolysis of 6-PGL to 6-P-Gluconate by 6-PGL hydrolase or lactonase - irreversible

3. Oxidative decarboxylation of 6-P-Gluconateby 6-PGluconate dehydrogenase

• Irreversible • 2nd NADPH is produced

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Non-oxidative reactionsNon-oxidative reactions

• Interconversion of 3-, 4-, 5- and 7-Carbon Sugars

• Ribulose 5-P to be converted to ribose 5-P [for Nucleic acid synthesis]

• Or to intermediates of Glycolysis : F-6-P and Glycerladehyde-3-P

• Non-oxidative part - controlled by availability of intermediates

• Only coenzyme required: TPP for Transketolase reaction

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Products of HMP ShuntProducts of HMP Shunt

G6P

NADPH Ribose 5-P

Biosynthesis (fatty acids, steroid

hormones)

Nucleic acid synthesis (DNA, RNA)

6G-6-P→ 5G-6-P regenerated, 6 CO2 eliminated,12 NADPH generated, 1 Glucose is completely oxidized

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Uses of NADPH Uses of NADPH

1.1. Reductive biosynthesis:Reductive biosynthesis:NADPH is a high energy molecule and electron donor. It is required as a source of electrons for biosynthesis of FA, cholesterol, sterols, hormones, and bile salts.

2.2. Reduction of HReduction of H22OO22: : ROS- damage DNA, proteins, unsaturated lipids– reperfusion injuries, cancer, inflammatory diseases, aging. Several protective mechanisms:

a. Enzymes that catalyze Antioxidant reactions:Glutathione peroxidase or GOD [ using reduced Glutathione or GSH which is active as antioxidant, present in most cells which can detoxify HH22OO22 ]Glutathione reductase or GR [regenerates GSH from oxidized glutathione formed in above reaction] using NADPH NADPH as a source of electrons

b. Antioxidant chemicals: Vit E, A, C, uric acid, bilirubin, ceruloplasmin etc

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3.3. CYP 450 or Cyt P-450: major pathway for hydroxylation of CYP 450 or Cyt P-450: major pathway for hydroxylation of xenobioticsxenobioticsSupply of NADPHNADPH is critical for liver microsomal cyt P-450

4.4. Phagocytosis of microorganisms esp bacteria, foreign Phagocytosis of microorganisms esp bacteria, foreign particles etc by Neutrophils & Macrophages:particles etc by Neutrophils & Macrophages:imp defense mechanism

a. Oxygen dependent system- MPO [myeloperoxidase system]-most potent, NADPH OXIDASE [needs NADPH] in WBC cell memb, converts OO22 into Superoxide FR into Superoxide FR [Respiratory burst]→Superoxide is converted by SOD [superoxid dismutase] into HH22OO22 →lysosomal MPO converts it to hypochlorous acid HOCl·→ kills bacteria

NADPH OXIDASE deficiency- Chronic granulomatosis

b. Oxygen independent system- pH changes in phagolysosomes and lysosomal enzymes- destroy pathogens

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G6PD and NADPH in the RBCG6PD and NADPH in the RBC

HMP

2 ADP

2 ATP

2 Lactate

G6P

Glucose

Glucose

G6P NADP

NADPH6-Phospho-gluconate

Glucose 6-phosphatedehydrogenase

GS-SG

2 GSH

2 H O

H O

Glutathionereductase

2 2

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Glycolyticpathway

Glutathioneperoxidase

G6PD deficiency preventsthe production of NADPHin the erythrocyte. Lack ofNADPH results in hemolysis.

+ H +

Certain DrugsInfectionsFava Beans

Oxidant Stress

Erythrocyte

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G-6-PD deficiency• Inherited disease-most common disease producing enzyme

abnormality in humans- X-linked• Hemolytic anemia due to inability to detoxify oxidizing agent• 400 different types of mutations [point mutations]• Shortened life span due to complications• Increased resistance to falciparum malaria in female carriers of

mutation• ↓activity of G6-PD→↓NADPH [HMP] →↓detoxification of FR &

peroxides• RBC- most vulnerable as HMP is only means for NADPH

production. (other tissues NADP-dependent malate dehydrogenase also]

• Precipitating factors- Oxidant drugs[ A-antibioticsA-antibiotics (sulfamethoxazole), A-antimalarial (primaquin),A-antipyritics (sulfamethoxazole), A-antimalarial (primaquin),A-antipyritics (acetanilid)(acetanilid), ingest Fava beans [favism], severe infection-free radical generation in macrophages diffuse to RBC-hemolysis

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Regulatory Mechanisms

Enzymes G6PD and 6PGD catalyze irreversible steps of the HMP shunt.

[NADPH] inhibits these enzymes via feedback inhibition

[ATP]: a putative inhibitor of these steps

[G-6P] increases flux through the HMP shunt (G-6P is a substrate)

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TPP is coenzyme for Transketolase

Transketolase reaction is measured in RBC as index of Thiamine status.

Wernicke-Korsakoff syndrome: seen in alcoholics and persons with Thiamine deficiency is due to genetic defect in the enzyme Transketolase

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Uronic acid Pathway of GlucoseUronic acid Pathway of GlucoseImportance in humans:• Provides UDP-glucuronic acid for conjugation [conjugation of

bilirubin, steroids etc] and synthesis of glycosaminoglycans.

• In lower animals (not in primates- deficiency of enzyme L-gulonolactone oxidase), this pathway leads to synthesis of Vit C.

• Essential Pentosuria: one of Garrod’s tetrad [alkaptonuria, albinism, pentosuria, cystinuria- inborn error of metabolism]: *1 in 2500 births due to deficiency of xylitol dehydrogenase → L-xylulose excreted in urine gives + benedict’s test-not harmful. *Diffentiated from DM by + Bials test [orcinol in HCL-Bial’s reagent] by pentose sugars.

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URONIC ACID PATHWAYURONIC ACID PATHWAY

G-6-PG-6-P

PhosphoglucomutasePhosphoglucomutase

G-1-PG-1-P

+ UTP [UDPG Phosphorylase]UTP [UDPG Phosphorylase]

UDP- GlucoseUDP- Glucose

enters Uronic acid pathway

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Aldose reductaseAldose reductase- Glucose to Sorbitol [glucitol]:

Lens, retina, Schwann cell of peripheral nerves, kidney, placenta, RBC, cells of ovaries and seminal vesicles.

Sorbitol dehydrogenaseSorbitol dehydrogenase- Sorbitol to FructoseGlucoe to Sorbitol to Fructose: in seminal vesicles for sperm cell [fructose is preferred carbohydrate energy source]

Hyperglycemia: Uncontrolled DM-large amt Glucose enters Hyperglycemia: Uncontrolled DM-large amt Glucose enters Lens, Retina,Nerve, Kidney – with action of aldose reductaseLens, Retina,Nerve, Kidney – with action of aldose reductase

→↑→↑Sorbitol, cannot pass through cell memb, so trapped inside Sorbitol, cannot pass through cell memb, so trapped inside cell. cell.

Sorbitol dehydrogenase is absent in Lens, retina, kidney and Sorbitol dehydrogenase is absent in Lens, retina, kidney and nerve cell →↑sorbitol accumulates →Osmotic effects →cell nerve cell →↑sorbitol accumulates →Osmotic effects →cell swelling and water retention:swelling and water retention:

cause of cataract formation, peripheral neuropathy, vascular cause of cataract formation, peripheral neuropathy, vascular problems leading to nephropathy and retinopathyproblems leading to nephropathy and retinopathy