HMP pathway/Pentose phosphate pathway

Dr.Soumitra Chakravarty MD

Learning objectives Describe the important steps in the HMP pathway

and its importance

List the byproducts of HMP pathway and its uses

List the uses of NADPH in various biological reactions

Briefly describe the role of glutathione and NADPH in maintaining the integrity of RBC membranes

Describe G-6PD deficiency and its clinical presentation

Fate of glucose-6-PO4 molecule:

Glucose-6-P04

Pyruvate

Glycogen Ribose -5-

po4

Glycolysis

GlycogenesisHMP pathway

ATP

Energy Storage

Functions of HMP pathway:

Occurs in the cytosol:

Generation of NADPH – required for biosynthesis of fatty acids and steroids.

Generation of Ribose-5-phosphate for nucleotide synthesis

Interconversion of carbohydrates into glycolytic and gluconeogenic intermediates.

Primary tissues – for HMP shunt Liver, mammary glands and adipose tissue

– synthesis of fatty acids

Adrenal glands, testes and ovaries – synthesis of steroids.

RBCs – for maintaining Glutathione in reduced form.

HMP shunt has two pathways: Oxidative pathway – irreversible

- mainly occurs in tissues which require NADPH.

Non oxidative pathway – reversible

- Occurs in the tissues which require ribose for nucleotide synthesis.

Oxidative pathway

Rate limiting step

USMLE !!

VITAMIN B1

Transketolase Transfers a 2-C unit (C1 AND C2 ) of a ketose to aldehyde C of aldose

Transaldolase Transfers a 3-C unit (dihydroxyacetone) from Seduheptulose 7P to glyceraldehyde 3P

Ribulose 5P (C5)

Ribulose 5P (C5)

Ribose 5P (C5)

Ribulose 5P (C5)

Xylulose 5P (C5)

Xylulose 5P (C5)

Glyceraldehyde 3P (3C) Sedoheptulose 7P (C7)

Fructose 6P(6C)

½ Fructose 6P(6C)

Fructose 6P(6C)

½ Fructose 1,6 BP(6C)

Source of NADP:

Tryptophan

Niacin

Amino acid

Vitamin B3

SOURCE OF

NADH , NADPH

Most important source of NADPH :-HMP shunt

Other sources of NADPH 1. Cytosolic Isocitrate dehydrogenase:

2. Malic enzyme:

Malate + NADP

Pyruvate + NADPH +CO2

USMLE !!

Transketolase Wernicke-Korsakoff’s syndrome :- Genetic disease where a

mutation in gene for transketolase which decreases the affinity of enzyme transketolase for Thiamine pyrophosphate (TPP)

Treatment :- Give thiamine

Beri Beri Thiamine deficiency Thiamine dependent enzymes

Transketolase +

3 TENDER LOVING CARE FOR NANCY ENZYMES PDH ALPHA KETO GLUTARATE DH BRANCH CHAIN AA DH

ALCOHOL AND THIAMINE ALCOHOL INHIBITS THIAMINE ALCOHOL VERY LITTLE THIAMINE IN ALCOHOLIC

BEVERAGES POOR NUTRITION IN ALCOHOLICS

CHRONIC ALCOHOLICS WILL HAVE WERNICKE’S KORSAKOFF’S SYNDROME + SYMPTOMS OF THIAMINE DEFICIENCY

Uses of NADPH:

REDUCTIVE BIOSYNTHESIS e.g. Biosynthesis of steroids

Fatty acid synthesis

Drug metabolism – cytochrome p 450 system

Glutathione reductase

NADPH Oxidase respiratory burst phagocytosis.

Reduction of hydrogen peroxide

Synthesis of NO.

1.Cytochrome p-450 system

Group of Monooxygenase enzymes:

Converts hydrophobic substances into hydrophilic substances.

Types Mitochondrial

Microsomal

Cyt p -450 system: Mitochondrial system:

1. Conversion of cholesterol to steroid2. Bile acid synthesis3. Hydroxylation of vitamin D.

Microsomal system: liver1. Detoxification of drugs, carcinogens, petroleum

products, pesticides etc.

2.Phagocytosis by Neutrophils and macrophages: Oxygen dependent killing: Respiratory burst NADPH oxidase Myeloperoxidase system

Oxygen Independent killing: phagolysosome formation acidity

3.Respiratory burst: NADPH Oxidase

NADPH aids in synthesis of superoxide radical during respiratory burst.

Deficiency of enzyme NADPH oxidase – Chronic Granulomatous Disease.

Infection with Catalase positive bacteria

Oxygen dependent killing

Oxygen Superoxide Hydrogen peroxide

NADPH Oxidase

Superoxide dismutase

NADPH oxidase system

Hydrogen peroxide

Hypochlorous acid

Myeloperoxidase

Water

Catalase

Bacterial killing

Catalase is produced by catalase positive organisms

Chronic Granulomatous Disease Defect in the enzyme NADPH oxidase

Catalase Negative organisms produce hydrogen peroxide – used by myeloperoxidase of the host to kill it

Catalase positive organisms – have catalase which neutralise hydrogen peroxide – cannot be destroyed – granulomas – chronic recurrent infections and abscess formation

Staphylococcus, klebsiella, Serratia and Aspergillus fungus.

4.Glutathione reductase : NADPH acts as a co-enzyme along with

Glutathione reductase converts oxidized glutathione to reduced glutathione.

Function of reduced glutathione: acts along with Glutathione Peroxidse reduce hydrogen peroxide to water in RBCs. (Neutralization of free radicals)

Glutathione in RBC:

NADH

Super Oxide Dismutase(SOD)

Heinz Bodies Membrane Damage

Modified from Kaplan USMLE lecture notes

G6PD deficiency: X linked recessive: (-) G6PD Decreased production of NADPH Hemolysis of RBCs due to oxidative insult. Causes for increased oxidative stress:1. Infections2. Drugs –sulpha drugs, anti malarials, aspirin,

INH, nitrofurantoin3. Fava beans4. Certain chemicals

Protection against malaria – mainly in African and middle eastern population.

Heinz bodies: Elevation of LDH-2 iso-enzyme Hemoglobinuria Decreased haptoglobin Increased destruction in spleen - cross-

bonding and protein deposition in the red cell membranes.

Nitric Oxide: Relaxes vascular smooth muscles:

Prevent platelet aggregation

Acts as a neurotransmitter.

Role in macrophage function.

NADPH is co-factor in synthesis of NO NO synthase – 3

types.

Arginine Citrulline

NADPH is the co-enzyme.

A 19-year-old, African American male military recruit is about to be sent to Iraq on his assignment. In preparation for his tour of duty, he is given a prophylactic dose of primaquine to prevent malaria. Several days after he begins taking the drug, he develops fatigue and hemolytic anemia. Which of the following proteins is likely deficient?

Glucose -6- phosphatase Glucose 6-phosphate dehydrogenase Aldolase B Muscle phosphorylase Pyruvate kinase

Chronic granulomatous Disease is a condition where the macrophages cannot kill the bacteria with the help of oxygen mediated respiratory burst due to deficiency of which of the following enzymes?

Glucose-6-PO4 dehydrogenase NADPH Oxidase Glutathione peroxidase Peroxidase Catalase