Biochemistry of carbohydrates/ pentose phosphate pathway · The pentose phosphate pathway is called sometimes as PPP, or hexose monophosphate shunt. It is not considered a metabolic

Biochemistry of carbohydrates/

pentose phosphate pathway الفريق الطبي األكاديمي

لكــية الطب البرشي

البلقاء التطبيقية / املركز

6102/6166أ حياها و من

Done By: - Khansaa’ Mahmoud

- Renad Aburumman

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Biochemistry of carbohydrates/ pentose

phosphate pathway

و من أحياها

6102/6166

**This topic starts from the minute (07:30) of the record, what was

previously said, is a revision of the previous lectures.

The pentose phosphate pathway is called sometimes as PPP, or hexose

monophosphate shunt.

It is not considered a metabolic pathway because it has no start nor end;

it is an intersection of a set of metabolic reactions, it is a multistep

reactions interacting with each other, so you are not going to memorise

this intersection pathway but you have to understand the segnifience and

physiologic importance of this pathway.

Here you see about 10 structures, all of them will participate in this

pathway. (You are not supposed to memorize their structures, except

those who are familiar to you, glucose for example).

Some of these sugars are hexoses, pentoses, ketoses and aldoses; they are

intermediates pf the PPP.

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The number in the figure are the indicators of enzymes, you are not

supposed to memorize them too.

PPP use glucose-6-phosphate that comes from -glycogen.

-the phosphorylation of glucose.

-Originate from fructose or

galactose.

This compound (glucose-6-phosphate) is oxidized with a catalyst enzyme,

called glucose-6-phosphate dehydrogenase. (This enzyme must be

memorized, because some chemical problems may occur when it is

deficient in cells, they are called glucose-6-phosphate deficiency

diseases).

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In this step, the glucose-6-phosphste is oxidized by the enzyme glucose-6-

phosphate dehydrogenase, and it uses -as coenzyme- NADP+. You usually

deal with NAD as a coenzyme, but this enzyme (G-6-P dehydrogenase)

uses NADP.

*As a summary: NADP coenzyme that is used by

G6Pdehydrogenase to oxidize glucose-6-phsphate to 6-

phosphogluconic acid(6PG).

**The important thing here is the production of NADPH, which is very

important molecule for the biosynthesis reactions, so the source of

NADPH is the PPP (the conversion of G-6-P to 6-phosphgluconic acid by

the enzyme G-6-P dehydrogenase ), NADPH is very important for the

fatty acids biosynthesis.

Fatty acids synthase and other enzymes use NADPH as a coenzyme.

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As a summary: Oxidation of glucose-6-phosphate to 6-phosphocluconic

acid then to and production of NADPH, how many NADPH in the oxidation

stage are produced? Two NADPH molecules because there are two

oxidation reactions(one from G6P6-PG/the other one from 6-PGRu-5-

P).

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The second feature what you have to understand and memorize is the

isomerization.

From the first stage riboluse-5 phosphate(Ru-5-P) is produced and this

could be isomerized to Xu5 phosphate or ribose 5 phosphate (R-5-P).

Ru-5-P(ketone) R-5-P(aldehyde) is an important step because it's

produce R-5-P &the importance of producing ribose 5 phosphate that it is

used in the synthesis of DNA and RNA(nucleotide metabolism) because

they contain a pentose sugar that is ribose 5 phosphate.

So where do we get the ribose sugar to synthesis of nucleotides from?

We get it from the pentose phosphate pathway, the isomerization of

ribulose-5-phosphate to ribose-5-phosphate.

What are the components of nucleotides?

*Nitrogenous base

*phosphate group

*Ribose sugar, what is it? Pentose sugar, what is it is source?

Isomerization of ribulose. Where does it come from? From the

dehydrogenation of glucose-6-phosphate.

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The third feature of the PPP is the carbon rearrangement (minute 21:00)

Here you see ribose 5 phosphate, Xu-5-phosphate, glyceraldehyde-3-

phosphate, S-7-P, fructose-6-phosphate, E-4-P. All of these are sugars you

are not supposed to memorize their names and structures, but you have

to understand them.)

These sugars - you see- some of them are composed of 7 carbon atoms

some are composed of six carbon atoms, Some have three carbon atoms

some with four carbon atoms.

so in this stage there is a rearrangement between the sugar in which they

give different types of sugars, by enzymes that will arrange or do some

reactions to produce different types of sugars.

The importance of this for the E-4-P(compose of 4 C atoms) sugar it is

essential precursor synthesis of aromatic amino acids like phenylalanine,

tryptophan and tyrosine,In order to synthesis these important amino acids

in our bodies E-4-P is required.

From where do we get this sugar?

From the rearrangement of carbons.

(To see that clearly in details check slides 7 + 9)

This is one imp. thing about carbon rearrangement , another important

one fructose-6-phosphate and glyceraldehyde-3-phosphate, where did

you see those molecules (sugars) took places as intermediates? In

glycolysis.

So supposedly, you have shortage in these intermediates pentose

phosphate pathway provides these intermediates for the glycolysis to be

continued, and this another physiological importance of PPP in carbon

rearrangement is providing intermediates for the glycolysis and

gluconeogenesis.

As a summary:

The importance of the pathway is:

1. production of NADPH, in the oxidation stage, important for biosynthetic

reactions such as fatty acids synthesis.

2. Provides ribose-5-phosphate, for nucleotide synthesis, in the

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isomerization stage.

3. Carbon rearrangement. *to provide intermediates for glycolysis and

gluconeogenesis and *precursors for aromatic amino acid synthesis

tryptophan, tyrosine and phenylalanine. The sugar that is the precursor is

E-4-P.

Why this pathway is called pentose not Hexose, as long as the sugars are

Hexoses not pentoses? Because the pentoses are the most important

sugars among the others, to produce nucleotides for nucleic acids. This

pathway also has other names as mentioned before.

So this pathway is multifunctional not directional like conventional

pathways such as *glycolysis (from glucose to pyruvate) and

gluconeogenesis (from pyruvate to glucose), or *oxidation of fatty acids

(from fatty acid to acetyl co a). So it is a multifunctional pathway

producing different molecules according to * the need of the cell and

*the availability of substrates.

(32:54) quick summary:

The importance of oxidation stage is production of NADPH; it produces

two molecules of NADPH through two reactions. The first is the

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conversion of glucose-6-phosphate to 6-phosphoglugonic acid, and the

second one is the conversion of 6-phosphoglugonic acid to riboluse-5-

phosphate. The importance of producing NADPH is that it contributes

biosynthesis fatty acids.

The importance is isomerization is producing ribose-5-phosphate, the

importance of ribose-5- phosphate is the synthesis of nucleotides which

contributes nucleic acids synthesis.

The importance of carbon rearrangement is producing different sugars,

(fructose-6-phosphate and glyceraldehyde-3-phosphate which are used in

glycolysis and gluconeogenesis) and (E-4-P which is used an automatic

amino acids synthesis such as tyrosine, phenylalanine and tryptophan).

(Slides 8 + 9)

R-5-P and Xu-5-P both have five carbon atoms and 5+ 5=10, which equals

3 + 7, which are S-7-P and GLAYAL-3-phsphate.

Two Sugars, which one is five carbon atoms ,converted to Sugars one of

them 7 and 3 carbon atoms.

This is the carbon rearrangement.

How does that happen? By the enzyme transketonase (memorize its

name), the portion of CH2OOH from the Xu-5-P is taken to produce

heptolose. It took two carbon atoms adding them to the 5 atoms of R-5-P

to give S-7-P, the left three carbon atoms are converted to

glyceraldehyde-3- phosphate, and which is important in glycolysis. Then

those products would be rearranged to give 4 and 6 carbon atoms sugars

which are fructose-6-phosphate (used in glycolysis) and E-4-P (used as a

precursor for automatic amino acid synthesis).

Heptolose + triose will be converted to hexose + pentose. The enzyme

used is called Transaldolase because it transfers an aldehyde group.

(Memorize the enzymes name).

Thank you

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Biochemistry of carbohydrates/ pentose phosphate pathway · The pentose phosphate pathway is called sometimes as PPP, or hexose monophosphate shunt. It is not considered a metabolic