CHAPTER 5. PENTOSE PHOSPHATE PATHWAY docx

PENTOSE PHOSPHATE PATHWAY INTERNATIONAL UNIVERSITY SCHOOL OF BIOTECHNOLOGY BIOCHEMISTRY... glucose glucose pentose 6-phosphate phosphate pathwayglycogen...  Provide pentose phosphate f

CHAPTER 5 PENTOSE PHOSPHATE

PATHWAY

INTERNATIONAL UNIVERSITY SCHOOL OF BIOTECHNOLOGY

BIOCHEMISTRY

Learning objectives

To know the enzyme joining the PPP

To understand the pathways to form

the ribose 5-phosphate

To understand the regulation of PPP

 Non-oxidative reactions leading to the

formation of intermediates of glycolysis

glucose glucose pentose 6-phosphate phosphate pathway

glycogen

 Provide pentose phosphate for

nucleic acid synthesis

Phosphopentose pathway

Produces NADPH and ribose-5-phosphate

NADH and NADPH although chemically similar they are not metabolically exchangeable.

Ratios of [NAD+]/[NADH] ~ 1000 favors metabolite oxidation, whereas ratios of [NADP+]/[NADPH] ~ 0.01 favors reductive biosynthesis.

Many anabolic pathways require the reducing power of NADPH for

synthesis including Fatty acid synthesis and the synthesis of cholesterol.

3G-6-P + 6NADP + + 3H 2 O 6NADPH + 6H+ 3CO 2 + 2F6P + GAP

2 Isomerization and epimerization reactions:

3Ribulose-5-PO4 Ribose -5-PO4 + 2Xylulose-5-PO4

3 A series of C-C bond cleavage and formations:

Ribose-5-PO4 + 2Xyluose-5-PO4 2F-6-P + GAP

Glucose-6 phosphate dehydrogenase

Phosphogluconate dehydrogenase

Ribulose-5-PO4 isomerase

Two enzymes control the rearrangement of carbon skeletons which result in the production of

Glyceraldehyde-3-phosphate and

Transketolase requires TPP

The transition of carbon skeletons in the Phosphopentose pathway

The pentose pathway control

The need for NADPH is

controlled by glucose

dehydrogenase, however,

when ribose -5-phosphate is

needed (DNA and RNA

synthesis) it can be made

from the reverse of the

transaldolase and

transketolase reactions from

Fructose-6-PO4 and GAP

One fate of G6P is the pentose pathway.

The pentose pathway is a shunt.

 The pathway begins with the glycolytic

intermediate glucose 6-P.

 It reconnects with glycolysis because two of the end products of the pentose pathway are glyceraldehyde 3-P and fructose 6-P; two

intermediates further down in the glycolytic pathway.

 It is for this reason that the pentose pathway is often referred to as a shunt.

It’s a shunt

What does the pentose phosphate pathway achieve?

 The pathway yields reducing

potential in the form of NADPH to be used in anabolic reactions requiring electrons.

 The pathway yields ribose

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NADPH is a phosphorylated

form of NADH

In general, with some exceptions, NADH is used to drive the

phosphorylation of ADP to ATP NADPH

is used where reducing potential is required for synthetic

The pentose pathway can be

divided into two phases

Non-oxidative interconversion of sugars

reaction is the rate

limiting step and is

essentially irreversible

There is a medical story

for this enzyme

Cells have a greater

5 carbon atoms

Regulatory enzyme

Regulatory enzyme

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Transketolase requires the coenzyme

thiamine pyrophosphate (TPP), the

transaldolase does not.

Remember which reactions require TPP

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Regulation of the Pentose

Pathway

regulatory enzyme.

inhibitor of the enzyme.

so the enzyme is inhibited.

NADPH, the ratio decreases and

portion of the pentose pathway are readily reversible.

and reactants can shift depending on the metabolic needs of a particular

cell or tissue.

Rapidly dividing cells require more ribose 5-

phosphate than NADPH

The need for NADPH and ribose 5-phosphate is

balanced

More NADPH is needed than ribose 5-phosphate;

Fatty acid synthesis in adipose cells

The cell needs both NADPH and ATP

Thank you for your kindly listening