Chapter 20: Pentose Phosphate Pathway

The Pentose Phosphate Pathway generates.. and synthesizes..?

Generates NADPH and synthesizes pentoses

What si the pentose phosphate pathway?

Pathway generating a crucial source of NADPH for use in reductive biosynthesis and for protection against oxidative stress

Where does pentose phosphate pathway occur?

In the cytoplasm of all organisms

The pentose phosphate pathway consists of how many phases?

2

The first phase in the pentose phosphate pathway is?

The oxidative generation of NADPH

When does the first phase of pentose phosphate pathway occur?

When glucose-6-phosphate is oxidized to ribulose 5-phosphate

What is the second phase of pentose phosphate pathway?

The nonoxidative interconversion of 3, 4, 5,6 and 6 6-carbon sugars

The interconversions of the second phase of pentose phosphate pathway rely on?

The same reactions leading to regeneration of ribulose 1,5-biphosphate in the Calvin-Benson cycle

Oxidative phase of the pentose phosphate pathway yields?

2 molecules of NADPH

What isomerizes ribulose 5-phosphate into ribose 5-phosphate?

Phosphopentose

The pentose phosphate pathway and glycolysis are…. linked

Reversibly

Transketolase and transaldolase catalyze?

Three successive reactions

The net results of transketolase and transaldolase is the conversioin od

3 pentoses into 2 hexoses and one triose

Transketolase catalyzes the formation of.. from?

glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate from 2 pentoses

Transaldolase catalyzes the formation of… from?

A six-carbon and a 3 carbon sugar from a 4 carbon and a 5 carbon sugar

Xyulose phosphate can be formed from

Ribose 5-phosphate by phosphoepentose isomerase and phosphopentose epimerase.

The metabolism of glucose 6-phosphate by the pentose phosphate pathway is coordinated with?

Glycolysis

Glucose 6-phosphate is metabolized by?

Both the glycolytic pathway and the pentose phosphate pathway

The fate of glucose-6-phosphate is controlled by?

The cytoplasmic concentration of NADP+

The rate of the oxidative phase of the pentose phosphate pathway is controlled by?

The level of NADP+

The first reaction of the pentose phosphate pathway is?

The dehydrogenation of glucose 6-phosphate by glucose 6-phosphate dehydrogenase

The first reaction fo the pentose phosphate pathway is?

A rate limiting step of the pathway, and the reaction is also essentially irreversible

What limits dehydrogenation fo glucose 6-phosphate and why?

Low levels of NADP+ because it is needed as the electron acceptor

Low levels of NADP+ limit dehydrogenation of glucoe 6-phosphate ensures?

NADPH is not generated unless needed for reductive biosynthesis or protection against oxidative stress

The pentose phosphate pathway can operate in how many modes?

4

What is the 1st mode?

Much more ribose 5-phosphate than NADPH is required

What is an example of mode 1?

When rapidly dividing cells need ribulose 5-phosphate for the synthesis of nucleotide precursors of DNA

In mode 1, glucose 6-phosphate is converted into

Fructose 6-phosphate and glyceraldehyde 3-phosphate by the glycolytic pathway

In mode 1, transaldolase and transketolase convert 2 molecules of F-6P and one molecule of GAP into?

3 molecules of ribose 5-phosphate

What is mode 2?

The needs of ribulose 5-phosphate and NADPH are balanced

In mode 2, glucose 6-phosphate is processed to

One molecule of ribulose 5-phosphate while generating 2 molecules of NADPH

In mode 2, ribulose 5-phosphate is converted into

Ribose 5-phosphate

In Mode 3,

Much more NADPH than ribose 5-phosphate is required

What is an example of mode 3?

When adipose tissues requires a high level of NADPH for the synthesis of fatty acids

In mode 3, glucose 6-phosphate is completely oxidized to

CO2

Glucose 6-phosphate is completely oxidized to CO2 through 3 groups of reactions?

-The oxidative phase of the pentose phosphate pathway forms two molecules of NADPH and one molecule of ribulose 5-phosphate.
– Ribulose 5-phosphate is converted into F-6P and GAP by
transketolase and transaldolase.
– Glucose 6-phosphate is resynthesized from fructose

What is mode 4?

Both NADPH and ATP are required

In mode 4, ribulose 5-phosphate formed from glucose 6-phosphate can be converted into

Pyruvate

F-6P and GAP derived from ribose 5-phosphate enter the.. instead of?

The glycolytic pathway instead of reverting to glucose 6-phosphate

In mode 4, pyruvate formed can be oxidized to?

Generated more ATP or used as a building block in a variety of biosynthesis

The pentose phosphate pathway is required for?

Rapid cell growth

Rapidly dividing cells, such as cancer cells require?

• Ribose 5-phosphate for nucleic acid synthesis

• NADPH for fatty acid membrane lipid synthesis

Rapidly dividing cells switch to?

Aerobic glycolysis to meet ATP needs

Rapidly dividng cells divert what?

Glucose 6-phosphate and glycolytic intermediates to the nonoxidative phase of the pentose phosphate pathway to generate NADPH and ribose 5-phosphate

Glucose 6-phosphate dehydrogenase plays a key role in?

Protection again reactive oxygen species

What is reduced gluthione?

A tripeptide with a free sulfhydryl group that compats oxidative stress by reducing ROS to harmless forms

What is oxidized glutathione must be?

Must be reduced to regenerate GSH

Reducing power for oxidized glutathione is supplied by?

The NADPH generated by glucose 6-phosphate dehydrogenase int eh pentose phosphate pathway

Reduced glutathione maintains?

The structure of hemoglobin

Reduced from of glutathione serves as.. that does what?

Serves as a sulfydryl buffer that keeps the residues of hemoglobin in the reduced sulfydryl form

Without adequate levels of reduced glutathione, hemoglobin molecules..

Cross link with one another to form aggregates called heinz bodies on cell membranes

Membranes damaged by heinz bodies and reactive oxygen species become

Deformed.. these cells are more likely to undergo lysis

A deficiency of glucose 6-phosphate dehydrogenase confers?

An evolutionary advantage in some circumstances

Glucose 6-phosphate dehydrogenase deficiency?

Characterized by a 10 fold reduction in enzymatic activity in red blood cells

Glucose 6-phosphate dehydrogenase deficiency is more common in?

Americans of African heritage.. protects agains thte deadliest form of malaria, which is. prevalent in Africa’s subtropical regions

Parasites causing malaria require?

NADPH for growth and infection induces oxidative stress in infected human cells

Because the pentose phosphate pathway is compromised,

The cells and parasites die from oxidative damage