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pentose phosphate pathway

Pentose Phosphate Pathway

The pentose phosphate (PP) pathway serves a number of functions. As the name implies, pentose biosynthesis is through this pathway. Ribose, ribulose, xylulose, are typical 5-carbon sugars made in the pathway. Because pathway reactions lead to fructose-6-PO4 and glyceraldehyde-3-PO4 in a round about way, early workers considered its only function was to backup glycolysis. The PP pathway, however, is a major producer of NADPH for the synthesis of fatty acids, which you will study later. Look for two unique enzymes “transketolase and transaldolase” in the PP pathway. It is certainly not a “ho-hum” pathway and to consider PP a backup to glycolysis is demeaning.

Gateway to Ribose and NADPH

slide2

COO-

H-C-OH

CH2OPO3=

O

HO-C-H

NADP+ NADPH

CH2OPO3=

OH

H-C-OH

OH

O

HO

=O

H-C-OH

OH

OH

HO

CH2 OPO3=

OH

Glucose-6-PO4

6-phosphoglucono--lactone

6-phosphogluconate

The hexose phase of the pathway starts with glucose-6-PO4 (click 1). Focus on the carbonyl group (C-1) for the next step (click 1). The first reaction is an oxidation of C-1catalyzed by the enzyme glucose-6-PO4 dehydrogenase (click 1). The product 6-phosphoglucono--lactone is an internal ester, not a hemiacetal. Note that C-1 is oxidized to a –COOH group (click 1) You would expect NAD+ to be the oxidizing coenzyme, but the cell has other surprises in mind. NADP+ is used instead (click 1). The product NADPH is a very important coenzyme for biosynthetic reactions. An internal ester of a sugar is called a “lactone”. Remember this, because you want to remember the name of the first intermediate and the next reaction in the pathway. In the next reaction the lactone ring is opened by the enzyme lactonase (click 1). The final hexose product is a sugar acid, 6-phosphogluconate. These are the 3 reactions of the “hexose” phase of the pathway . Note the major event was to oxidize the sugar with the phosphate group staying on the molecule. You did not see this in glycolysis. Click 1 to go on.

Glucose-6-PO4

dehydrogenase

Lactonase

Hexose Phase

slide3

COO-

H

CH2OH

H-C-OH

H-C-OH

H-C-OH

NADP+ NADPH

C=O

CO2

HO-C-H

+

HO-C

HO-C-H

H-C-OH

H-C-OH

H-C-OH

H-C-OH

H-C-OH

H-C-OH

H-C-OH

H-C-OH

CH2 OPO3=

CH2 OPO3=

CH2 OPO3=

CH2 OPO3=

6-phosphogluconate

Ribulose-5-PO4

The second phase is catalyzed by one enzyme, 6-phosphogluconate dehydrogenase. The sugars ribulose-5-PO4 (Ru5P) is the first 5-C sugar to appear (click 1). To make this sugar, a carbon as CO2 must be removed (click 1). The CO2 comes from a –COOH group. This is why it was necessary to oxidize C-1 of glucose-6-PO4. Recall, that when a –COOH group is removed the electron pair stays with the source molecule (click 1). The electron pair attracts a proton to form a –CH2OH on the new C-1 (click 1). In the final reaction a hydride ion (red) is abstracted from C-2 leaving a carbonium ion on C-2. That reaction requires a NADP+ and in the process a second NADPH is formed in the pathway. The final step is to rearrange electrons on C-2 to give the product ribulose-5-PO4 . Note that there are two intermediate forms that remain bound to the enzyme (click 1). Click 1 to go on.

6-phosphogluconate dehydrogenase

H

[Enzyme-bound intermediates]

slide4

CH2OH

C=O

CH2OH

CHO

H-C-OH

C=O

H-C-OH

H-C-OH

HO-C-H

H-C-OH

CH2OP

H-C-OH

H-C-OH

CH2OP

CH2OP

Having made the ribulose-5-PO4 (click 1), it’s a simple matter to make the other 5-C sugars in the pathway. This is accomplished by two enzymes, an isomerase and a epimerase. The epimerase changes the stereochemistry of C-3 (click 1). The isomerase interchanges the keto-aldo groups (click 1). The result is two new pentoses, ribose-5-PO4 and xylulose-5-PO4 (click 1). These are to two sugars that will take part in the next series of reactions. Click 1 to go on.

Ribulose-5-PO4

Ribulose-5-PO4

Epimerase

Ribulose-5-PO4

Isomerase

Ribose-5-PO4

Xylulose-5-PO4

slide5

CH2OH

C=O

HO-C-H

CH2OH

CH2OH

CHO

C=O

H-C-OH

+

C=O

H-C-OH

H-C-OH

HO-C-H

HO-C-H

H-C-OH

CHO

CH2OP

H-C-OH

H-C-OH

H-C-OH

CHO

CH2OH

CH2OP

CH2OP

H-C-OH

H-C-OH

C=O

CH2OP

CH2OP

HO-C-H

H-C-OH

CH2OP

Glyceraldehyde-3-PO4

The last phase of this most interesting pathway involves two special enzymes, a transketolase and a transaldolase. The transketolase starts the pathway by condensing ribose-5-PO4 with xylulose-5-PO4 (click 1). In the reaction the keto group of the xylulose-5-PO4 (first 2 carbons) is transferred to the receiving ribose-5-PO4(click1) This results in the formation of a 7 carbon intermediate, sedoheptulose-7-PO4 (S7P) leaving behind 3 carbons that rearrange to form glyceraldehyde-3-PO4 (GAP) (click 1). The GAP reacts with S7P in a reaction catalyzed by transaldolase to form a 6-carbon fructose-6-PO4, leaving behind erythrose-4-PO4 (E4P) (click 1). E4P condenses with a second xylulose-5-PO4 to give a second molecule of F6P and GAP (click 1). That last reaction is catalyzed by a transketolase.

Transketolase

Transaldolase

Fructose-

6-PO4

Trans-

ketolase

Xylulose-5-PO4

Ribose-5-PO4

Sedoheptulose-7-PO4

Erythrose-4-PO4

Xylulose-5-PO4

slide6

What you have learned. Click 1 for answers.

1. How many ATPs are produced by the oxidation of 1 glucose molecule in the pentose phosphate pathway?

None. The pathway is not designed to produce ATP

2. How many ATPs are required to produce the final products of the pathway?

None. The pathway does not require ATP

3. How many NADPHs are produced by oxidation of 3 glucose molecules in the pentose phosphate pathway?

Six. Two for each glucose oxidized.

4. Is the pentose phosphate pathway considered anaerobic or aerobic? Anabolic or catabolic? Explain.

Anaerobic and Anabolic. The pathway does not require oxygen. Therefore, it is an anaerobic pathway. Since it results in the synthesis of pentoses and NADPH, it must be considered primarily anabolic.