hexose monophosphate shunt pathway: Steps, Diagram, Uses

What Is the Hexose Monophosphate Shunt Pathway?

Another significant pathway in cellular metabolism is the hexose monophosphate shunt pathway, also known as the pentose phosphate pathway (PPP). It is an important process for the formation of nicotinamide adenine dinucleotide phosphate, a key means of reducing power for biosynthetic reactions and cellular redox balance, and ribose-5-phosphate for nucleotide and nucleic acid synthesis.

The PPP is a cytoplasmic process subdivided into two major phases: an oxidative phase that produces the reduced co-factor NADPH, and a non-oxidative phase that yields ribose-5-phosphate and other sugars. In addition to having several important anabolic applications, this pathway substantially enhances the capacity of the cell for redox homeostasis and, therefore, in more general terms, metabolic plasticity and cell health.

Importance In Cellular Metabolism

This PPP, besides providing cells both with the reducing power and essential metabolic intermediates, also provides NADPH in anabolic processes like the biosynthesis of fatty acids, cholesterol, and detoxification of reactive oxygen. The requirement of ribose-5-phosphate in nucleotide biosynthesis is extremely vital for cell growth and repair. Hence, the pathway supports cell functions and growth, especially in tissues exhibiting high biosynthetic activity or oxidant stress.

Location & Significance

The Hexose Monophosphate Shunt pathway takes place in the cytoplasm of cells.

Cellular Location

Unlike glycolysis, which also occurs in the cytoplasm, the PPP can function independently, even though it is in association with glycolytic and several other metabolic pathways.

Highly Active In

This pathway is very active in tissues with wide biosynthetic requirements and high oxidative stress, like the liver, adipose tissue, adrenal glands, and red blood cells. These tissues use the PPP to fulfil their requirements for NADPH and ribose-5-phosphate.

Phases of the Pentose Phosphate Pathway

The different phases are:

Oxidative Phase (Irreversible)

Enzymes Involved:

The important enzymes in the oxidative phase of this pathway are composed of glucose-6-phosphate dehydrogenase (G6PD), 6-phosphogluconactonase, and 6-phosphogluconate dehydrogenase.

Key Outputs:

This reaction will convert glucose-6-phosphate into ribulose-5-phosphate and will aid in the generation of NADPH. The generation of NADPH is for reductive biosynthesis and, at the same time, removes oxidative stress.

Non-Oxidative Phase (Reversible)

Enzymes Involved:

The enzyme predominant in the non-oxidative pathway is transketolase.

Key Outputs:

• Ribose-5-phosphate

• Glyceraldehyde-3-phosphate

• Fructose-6-phosphate

Functions of the HMP Shunt

The functions of the HMP shunt includes:

NADPH Production

NADPH is used for:

• Fatty acid synthesis

• Cholesterol/steroid synthesis

• Glutathione regeneration (RBC protection)

• Detoxification reactions (cytochrome P450)

Ribose-5-Phosphate Production

Ribose-5-phosphate is needed for:

• DNA & RNA synthesis

• ATP, NAD⁺, FAD, CoA formation

Redox Homeostasis

• NADPH maintains reduced glutathione (GSH)

• Neutralizes H₂O₂ → H₂O, preventing oxidative injury

Clinical Significance

The clinical significance is described below-

G6PD Deficiency

Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency is a common genetic disorder that impacts the oxidative phase of the PPP. The lack of formation of NADPH can lead to a reduction in red blood cells' ability to withstand oxidative stress with the administration of certain drugs or during infections, thus resulting in hemolytic anemia.

Hemolytic Anemia

Decreased generation of NADPH by the cells compromised the integrity of red cells in G6PD-deficient patients, leading to their lysis and consequent anaemia.

PPP vs Glycolysis vs Entner–Doudoroff Pathway

The difference between the three pathways is given in the table below:

Hexose Monoshunt Pathway NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Phases of HMP Shunt

• Functions of HMP Shunt

Practice Questions for NEET

Q1. Who discovered the hexose monophosphate shunt pathway?

1. Peter Mitchell

2. Warburg and Dickens

3. Calvin

4. Emerson

Correct answer: 2) Warburg and Dickens

Explanation:

The hexose monophosphate shunt, alternatively known as the pentose phosphate pathway (PPP), was discovered in 1938 by Otto Warburg and Franck Dickens. This is an auxiliary pathway parallel to glycolysis and mainly contributes to producing NADPH, as well as ribose-5-phosphate, which is essential in many biosynthetic processes. The discovery was made based on Warburg's oxidation of glucose-6-phosphate experiment, and it greatly contributed to insights into cellular respiration in plants.

Hence, the correct answer is option 2) Warburg and Dickens.

Q2. The hexose monophosphate shunt pathway takes place when there is

1. Less concentration of NADP

2. More concentration of NADP

3. Less concentration of FAD

4. More concentration of FADH

Correct answer: 2) More concentration of NADP

Explanation:

Hexose monophosphate shunt occurs when the concentration of NADP is higher. The pentose phosphate pathway, sometimes referred to as the hexose monophosphate shunt, is a special pathway that produces substances that the body needs for a variety of purposes. Nicotinamide adenine dinucleotide phosphate (NADPH) and ribose-5-phosphate are produced via the HMP shunt, an alternate pathway to glycolysis.

Hence, the correct answer is option 2) More concentration of NADP.

Q3. The enzyme which converts 6-phosphogluconolactone to 6-phosphogluconate is

1. Ribulose-5-phosphate isomerase

2. Transaldolase

3. Glucose-6-phosphate dehydrogenase

4. Gluconolactonase

Correct answer: 4) Gluconolactonase

Explanation:

Gluconolactonase is the enzyme that changes 6-phosphogluconolactone into 6-phosphogluconate.

The pentose phosphate pathway includes this reaction, in which gluconolactonase hydrolyzes the lactone ring of 6-phosphogluconolactone to produce 6-phosphogluconate. The pentose phosphate pathway, which produces nucleotides and NADPH and is essential to cellular metabolism, depends on this step to continue.

Hence, the correct answer is option 4)Gluconolactonase.

Also Read:

• MCQs on Hexose Monophosphate Shunt Pathway

• Aerobic and Anaerobic Respiration

• Lactic Acid Fermentation

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