BiochemistrySixth Edition

Chapter 20:The Calvin Cycle and the Pentose Phosphate Pathway

Copyright © 2007 by W. H. Freeman and Company

Berg • Tymoczko • Stryer

Hexose Monophosphate

Pentose Phosphate Pathway Glycolysis, TCA, and oxidative phosphorylation are

primarily concerned with the generation of ATP. The PPP meets the need of all organisms for a source of

NADPH to use in reductive biosynthesis. The reducing power is NADPH. There is a fundamental distinction

NADH NADPH

The direction of HMP depends on the supply and demand for intermediates in the cycle…

Structure of nicotinamide derived e carriers• In NAD+, R = H• In NADP+, R = PO3

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Two Major Functions

1. NADPH

2. Ribose

Overall reaction:3G-6-P + 6NADP+ 3CO2 + 2G-6-P + Glyceraldehyde-3P + 6NADPH + 6H+

It occurs in the cytosol because NADP+ is used as a hydrogen acceptor.

There are two sequential reactions.

1. Oxidative

2. Nonoxidative

Oxidative and Nonoxidative Reactions

In oxidative, G-6-P undergoes dehydrogenation and decarboxylation to give a pentose ribulose-5-P.

In nonoxidative, ribulose 5-P is converted back to G-6-P by a series of reactions involving two enzymes1. Transketolase

2. Transaldolase

Dehydrogenation of G-6-P is the major biological control of the HMP.

G-6-PD is strongly inhibited by NADPH.

Oxidative Branch

Non-oxidative Branch

Ribose-5-P + Xylulose-5-P Transketolase, TPP Sedoheptulane-7-P + Glyceraldehyde-3-P

2C unit----- transferred by transketolase

This 2-C moiety is bound to TPP first then transferred.

Two products of transketolase then enter another reaction known as transaldolation.

3C unit---- transferred by transaldolase

Sedoheptulase-7-P + Glyceraldehyde-3-P Transaldolase F-G-P + Erythrope-4-P

Another transketolase reaction:‒ X-5-P + E-4-P Transketolase G-3-P + F-6-P‒ X-5-P serves as a donor of “active glycoaldehyde”.

Therefore, at the end of PPP:‒ NADPH ‒ Ribose-5-P ‒ Glyce 3-P and Fructose-6-P Gly

The differences between glycolytic pathway and PPP are:‒ NADPH‒ CO2

‒ ATP‒ Ribose-5-P for nucleotide synthesis

The PPP is much more active in adipose tissue than in muscle.

It is important in tissues such as adipose, liver, mammary gland, and adrenal cortex (NADPH depended synthesis of steroids).

Transketolase that is defective in TPP binding can cause a neuropsychiatric disorder.

• Wernicke-Korsakoff Syndrome

‒ Lack of TPP in susceptible people

‒ Paralysis of eye movements

‒ Abnormal gait

‒ Decreased mental function

‒ Severely impaired memory

‒ Transketolase from patients with the Wernicke-Korsakoff syndrome binds thiamine PP ten times less than does the enzyme from normal persons

The flow of Glc-6-P depends on the need for NADPH, ribose 5-P, and ATP

Mode 1• Much more ribose 5-P is needed than NADPH• It is seen in rapidly dividing cells• The stoichiometry of mode 1 is:

5 Glc 6-P + ATP 6 ribose 5-P + ADP + H+

Mode 2• The needs for NADPH = ribose 5-P are balanced.• Formation of 2 NADPH and 1 Ribose 5-P• The stoichiometry of mode 2 is:

Glc6-P + 2NADP+ + H2O ribose 5-P + 2NADPH + 2H+ CO2

Continue on modes

Mode 3• Much more NADPH than ribose 5-P is required. For example

adipose tissue requires a high level of NADPH for the synthesis of fatty acids.

– Glc6-P is completely oxidized to CO2

– 3 reactions are active• Oxidative phase forms 2 NADPH and 1 Ribose 5-P• Ribose 5-P F-6-P and Glyceraldehyde 3-P• Glc6-P is made from F-6-P and Glyceraldehyde 3-P

The sum of the mode 3 reaction is:• (The stoichiometry of mode 3) is:

Glc6-P + 12NADP+ + 7H2O 6CO2 + 12 NADPH + 12H++ Pi

Therefore Glc6-P can be completely oxidized to CO2 with the generation of NADPH

Continue on modes

Mode 4– Both NADPH and ATP are required. – Ribose 5-P pyruvate, F-6-P and glyceraldehyde 3-P– These enter glycolytic pathway

The stoichiometry of mode 4 is:

3 Glc6-P + 6 NAD+ + 5 Pi + 8 ADP 5 pyruvate + 3CO2 + 6 NADPH + 5 NADH + 8 ATP + 2 H2O + 8 H+

– Pyruvate can be oxidized more!

G-6-P dehydrogenase deficiency

G-6-P dehydrogenase deficiency causes a drug induced hemolytic anemia.

An antimalarial drug primaquine was introduced in 1926.

Some patients developed severe symptoms like:• Jaundice

• Hb decrease

• Massive destruction of red blood cells

• Death

In 1956, the basis of drug induced hemolytic anemia was elucidated

The primary defect is a deficiency in G-6-P dehydrogenase in red blood cells.

Role of NADPH in RBCs

GSSG GSH by glutathione reductase, which requires NADPH.

GSH keeps Cys residues in hemoglobin and other RBC proteins in the reduced state.

Normally, the ratio of the GSH/GSSG 500 in RBCs Electrons are transferred by NADPH to FAD first on the

reductase, then to a S-S bridge between 2 Cys residues in the enzyme subunit, and finally to GSSG.

GSH + ROOH GSSG + H2O + ROH Cells with low GSH are more susceptible to hemolysis

because ROOH eliminated by GSH preoxidase by using GSH as a reducing agent.

Glutathione reductase

Cells with low GSH

Cells with low GSH are more susceptible to hemolysis when fava beans are eaten.

In some regions where malaria is endemic (the middle east) fava beans are a staple food.

They are known to contain two beta glycosieds– Vicine – Convicine

They oxidize GSH! Individuals who eat fresh fava beans are protected to a

certain extent from malaria. A condition known as favism results when some Glc 6-P

deficient individuals develop a severe hemolytic anemia after ingestion of fava beans.

More about Glc 6-P dehydrogenase deficiency

In the absence of G-6-P dehydrogenase, Hb can no longer be maintained in the reduced form.

Hb molecules then cross-link with one another to form aggregates called Heinz bodies on cell membranes.

Membranes damaged by the Heinz bodies and ROS (reactive Oxygen Species) become deformed and the cell undergos LYSIS Hemolytic anemia!

The light micrograph shows RBC obtained from a person deficient in Glc 6-P dehydrogenase. The dark dots represent Hb aggregates. RBCs in such people lyse if there is oxidative stress (an increase in ROS)

Adeficiency of Glc 6-P dehydrogenase confers an evolutionary advantage in some circumstances

11% of African-Americans have this deficiency. This suggest that this deficiency may indeed be useful under certain

environmental conditions. In fact, deficiency of Glc 6-P dehydrogenase protects against

malaria! How??

– In order for the parasites (Plasmodium Falciparum) to survive, GSH is needed and products of PPP are also needed for optimal growth!!!

– Thus, Glc 6-P dehydrogenase deficiency is a mechanism of protection against malaria, which accounts for its high frequency in malaria-infested regions of the world.

WE SEE HERE ONCE AGAIN THE INTERPLAY OF HEREDITY AND ENVIRONMENT IN THE PRODUCTION OF DISEASE!