HMP pathway 15 marks long answer mbbs

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Hexose Monophosphate (HMP) Pathway / Pentose Phosphate Pathway

Introduction

The hexose monophosphate (HMP) pathway - also called the pentose phosphate pathway or HMP shunt - is an alternative route for the oxidation of glucose 6-phosphate that operates in the cytosol of cells. Unlike glycolysis, it does not produce ATP and does not involve pyruvate. Its two cardinal products are:

NADPH - the main cellular reductant for anabolic and detoxification reactions
Ribose 5-phosphate - for nucleotide and nucleic acid biosynthesis

The pathway is active in all tissues that synthesize nucleotides and nucleic acids, and is especially prominent in the liver, lactating mammary glands, adipose tissue, adrenal cortex, gonads, and red blood cells.

Location

All reactions occur in the cytosol. No mitochondrial involvement.

Two Phases of the Pathway

The HMP pathway consists of two distinct phases:

Phase	Reactions	Reversibility
Oxidative	Glucose 6-P → Ribulose 5-P + 2 NADPH + CO2	Irreversible
Non-oxidative	Interconversion of pentose sugars → fructose 6-P + glyceraldehyde 3-P	Reversible

Phase 1: Irreversible Oxidative Reactions

Reactions of the pentose phosphate pathway showing oxidative (irreversible) and non-oxidative (reversible) phases

Step 1 - Glucose 6-phosphate dehydrogenase (G6PD) reaction:

Glucose 6-phosphate + NADP+ → 6-Phosphogluconolactone + NADPH + H+
Enzyme: G6PD (glucose 6-phosphate dehydrogenase)
This is the committed, rate-limiting, and regulated step
G6PD is competitively inhibited by NADPH (product inhibition)
When NADPH/NADP+ ratio falls, G6PD activity increases - the pathway is "demand-driven"
Insulin upregulates G6PD gene expression

Step 2 - Hydrolysis of the lactone:

6-Phosphogluconolactone + H2O → 6-Phosphogluconate
Enzyme: 6-Phosphogluconolactone hydrolase (lactonase)
Spontaneous but accelerated by this enzyme

Step 3 - Oxidative decarboxylation:

6-Phosphogluconate + NADP+ → Ribulose 5-phosphate + CO2 + NADPH + H+
Enzyme: 6-Phosphogluconate dehydrogenase
Carbon 1 of glucose is released as CO2
A second molecule of NADPH is produced
Product is a 5-carbon ketose - ribulose 5-phosphate

Net result of oxidative phase (per molecule of glucose 6-phosphate):

Glucose 6-P + 2 NADP+ → Ribulose 5-P + 2 NADPH + 2H+ + CO2

Phase 2: Reversible Non-Oxidative Reactions

Ribulose 5-phosphate can be channeled in two directions depending on cellular needs:

A. Isomerization to Ribose 5-phosphate (if nucleotide synthesis is needed)

Ribulose 5-P → Ribose 5-P
Enzyme: Ribose 5-phosphate isomerase
Ribose 5-P is used for synthesis of ATP, NAD+, FAD, CoA, and nucleic acids

B. Epimerization to Xylulose 5-phosphate

Ribulose 5-P → Xylulose 5-P
Enzyme: Phosphopentose epimerase

If NADPH demand > ribose demand - the ribulose 5-phosphates are converted back to glycolytic intermediates by the following interconversions:

Transketolase reaction (first):

Xylulose 5-P (5C) + Ribose 5-P (5C) → Sedoheptulose 7-P (7C) + Glyceraldehyde 3-P (3C)
Transfers a 2-carbon unit from ketose to aldose
Coenzyme: Thiamine pyrophosphate (TPP) - clinically important
This reaction is used to test thiamine status (transketolase activity assay)

Transaldolase reaction:

Sedoheptulose 7-P (7C) + Glyceraldehyde 3-P (3C) → Erythrose 4-P (4C) + Fructose 6-P (6C)
Transfers a 3-carbon unit from ketose to aldose
Uses an active lysine residue (Schiff base mechanism) - no cofactor needed

Transketolase reaction (second):

Xylulose 5-P (5C) + Erythrose 4-P (4C) → Fructose 6-P (6C) + Glyceraldehyde 3-P (3C)
Again requires TPP

The net result: 3 ribulose 5-P (15C) → 2 fructose 6-P (12C) + 1 glyceraldehyde 3-P (3C)

These products re-enter glycolysis.

Overall Balanced Equation

When 3 molecules of glucose 6-phosphate enter the combined oxidative + non-oxidative pathway:

3 Glucose 6-P + 6 NADP+ → 6 NADPH + 6H+ + 3 CO2 + 2 Fructose 6-P + 1 Glyceraldehyde 3-P

Regulation

Regulator	Effect on G6PD
NADPH (high ratio)	Inhibits G6PD - shuts down pathway
NADP+ (low NADPH/NADP+)	Activates G6PD - increases flux
Insulin	Upregulates G6PD gene expression

The pathway is entirely driven by the cell's need for NADPH, not for ATP.

Functions / Significance of Products

1. NADPH

NADPH is the major reducing coenzyme produced by this pathway. Its key roles:

Function	Tissue
Fatty acid biosynthesis (FAS enzyme)	Liver, adipose, lactating breast
Cholesterol/steroid synthesis	Liver, adrenal cortex, gonads
Glutathione reductase - reduces GSH (antioxidant defense)	All cells, especially RBCs
NADPH oxidase - respiratory burst (O2- production for killing bacteria)	Neutrophils, macrophages
Cytochrome P450 monooxygenase system - drug detoxification	Liver
Nitric oxide synthase (NOS) - NO synthesis	Endothelium, macrophages
Vitamin D activation	Liver, kidney
Reduction of H2O2 via glutathione peroxidase	RBCs

Note: NADPH/NADP+ ratio in cytosol is ~0.1 (high NADPH), favoring reductive biosynthesis - opposite of the NAD+/NADH ratio (~1000) which favors oxidation.

2. Ribose 5-phosphate

Used for synthesis of:

Purines and pyrimidine nucleotides (RNA, DNA)
ATP, NAD+, FAD, CoA, PRPP

G6PD Deficiency - Clinical Importance

Inheritance: X-linked recessive - predominantly affects males

Pathophysiology: G6PD is the only source of NADPH in RBCs (which lack mitochondria). Deficient G6PD → low NADPH → glutathione remains oxidized (G-S-S-G) → H2O2 accumulates → oxidative damage to hemoglobin (Heinz bodies) and RBC membrane → hemolytic anemia

Triggers of hemolytic crisis:

Oxidant drugs (primaquine, dapsone, rasburicase, nitrofurantoin, some sulfonamides)
Fava beans (favism) - contain oxidant isouramil
Severe infections (generate free radicals)

Features:

Hemolytic anemia + jaundice episodically
Heinz bodies on peripheral smear (denatured hemoglobin)
Bite cells (splenic removal of Heinz body inclusions)
Neonatal jaundice (prolonged)
Malaria-protective advantage in heterozygous females (geographic distribution in malaria belt)

Important: Degree of anemia depends on amount of residual enzyme activity. Older RBCs have less G6PD than young RBCs - they are more susceptible.

Tissues where HMP Pathway is Most Active

Tissue	Reason
Liver	Fatty acid and cholesterol synthesis (need NADPH)
Lactating mammary gland	Fatty acid synthesis
Adipose tissue	Fatty acid synthesis
Adrenal cortex, gonads	Steroid hormone synthesis
Red blood cells	NADPH to maintain reduced glutathione
Neutrophils/macrophages	NADPH oxidase - respiratory burst
Corneal epithelium, lens	Protection from oxidative damage

Comparison with Glycolysis

Feature	Glycolysis	HMP Pathway
ATP production	Yes (net 2 ATP)	None
NADH production	Yes	None
NADPH production	None	Yes (2 per G6P)
CO2 release	No	Yes (C1 of glucose)
Location	Cytosol	Cytosol
Reversibility	Mostly irreversible	Phase 2 reversible
Key enzyme	PFK-1	G6PD
Product	Pyruvate	Ribose 5-P, NADPH

Thiamine (B1) Deficiency and the HMP Pathway

Transketolase requires thiamine pyrophosphate (TPP) as a coenzyme. Thiamine deficiency (beriberi, Wernicke's encephalopathy) impairs the non-oxidative phase. Erythrocyte transketolase activity is used as a sensitive assay for thiamine status.

Summary

The HMP pathway is a metabolic shunt that:

Oxidizes glucose 6-P to generate 2 NADPH and ribose 5-phosphate (oxidative, irreversible phase)
Interconverts pentose sugars back to glycolytic intermediates via transketolase and transaldolase (non-oxidative, reversible phase)
Is regulated entirely by the NADPH/NADP+ ratio via G6PD
Does not generate ATP or CO2 in large quantities (only 1 CO2 per glucose)
Is clinically relevant through G6PD deficiency causing drug- or infection-induced hemolytic anemia

Sources: Lippincott Illustrated Reviews: Biochemistry, 8th ed., Chapter 13; Basic Medical Biochemistry: A Clinical Approach, 6th ed., Chapter 27

HMP pathway 15 marks

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