The Pentose Phosphate Shunt

(AKA: Pentose Phosphate Pathway, PPP)

Uses Glucose 6P to produce 3, 4, 5, 6 and 7 carbon sugars.

In the process of doing this it reduces NAD+ to NADPH which is needed for reductive biosynthesis.

NADPH is needed for synthesis of fatty acids

Fatty acids are a highly reduced energy store

Palmitate is the primary fatty acid, made to 16Cs by the serial addition of 2Cs from Acetyl-S-CoA

Acetyl-S-CoA

Just attaching Acetylstogether would yieldthis, which is NOT found

This is palmitate the primary product of the following reaction

Fatty acid synthesis and NADPH continued

8Acetyl-CoA +7ATP +7H+ + H20 + 14 NADPH a8 CoA + 7Pi +7ADP +Palmitate + 14 NADP+

So Palmitate and all other reduced fatty acids require the oxidation of NADPH to NADP+.

Oxidation is the opposite of reduction. If something is reduced then something else must be oxidized.

NADP+ is reduced to NADPH in the pentose phosphate pathway

In NADP+(oxidized)

In NADPH (reduced)

This phosphate is not present in NAD+ or NADH

NADPHNH2

derived from niacin (vitamin B3)

Dietary Deficiency of Niacin - PellegraDisease characterized by the “three Ds” - dematitis, diarrhea, and dementia.

“Pellegra” - Italian for “rough skin”.

Mental hospitals in the southern states filled up every late winter/early spring with patients with pellegra. In 1930 there were about 200,000 cases of pellegra in the US.

A man with an early stage of pellagra known as “pellagra gloves”.

Pellegra or “corn disease” was originally thought to be an infectious disease or result from eating contaminated corn.

80% of niacin in corn is bound to a substance making it unavailable.

Native Americans did not get pellegra because they treated their corn with alkali (lime or wood ashes) to make hominy before grinding it.

Dr. Joseph Goldberger

- realized pellagra was due to a dietary deficiency and was not an infectious disease.

Figure 22.22 The pentose phosphate pathway.

The Oxidative Steps of the Pentose Phosphate Pathway

1. Glucose-6-P Dehydrogenase Irreversible 1st step - highly regulated!

2. Gluconolactonase The uncatalyzed reaction happens too

3. 6-Phosphogluconate DehydrogenaseAn oxidative decarboxylation (in that order)

The Oxidative Steps of the Pentose Phosphate Pathway

Figure 22.23 The glucose-6-phosphate dehydrogenase reaction is the committed step in the pentose phosphate pathway.

Figure 22.24 The Gluconolactonase Reaction

Figure 22.24 The gluconolactonase reaction. The uncatalyzed reaction also occurs.

The Oxidative Steps of the Pentose Phosphate Pathway

Figure 22.25 The 6-phosphogluconate dehydrogenase reaction. The initial NADP+-dependent dehydrogenation yields a β-keto acid, 3-keto-6-phosphogluconate, which is very susceptible to decarboxyation (the second step). The resulting product, D-ribulose-5-P, is the substrate for the nonoxidative reactions of the pentose phosphate pathway.

Figure 22.22 The pentose phosphate pathway.

The Nonoxidative Steps of the Pentose Phosphate Pathway

Five steps, but only 4 types of reaction...Phosphopentose isomerase

converts ketose to aldose

Phosphopentose epimeraseepimerizes at C-3

Transketolase ( a TPP-dependent reaction)transfer of two-carbon units

Transaldolase (uses a Schiff base mechanism)transfers a three-carbon unit

Figure 22.22 The pentose phosphate pathway.

The Nonoxidative Steps of the Pentose Phosphate Pathway

Figure 22.26 The phosphopentose isomerase reaction converts a ketose to an aldose. The reaction involves an enediol intermediate.

Figure 22.22 The pentose phosphate pathway.

The Nonoxidative Steps of the Pentose Phosphate Pathway

Figure 22.27 The phosphopentose epimerase reaction interconverts ribulose-5-P and xylulose-5-phosphate. The mechanism involves an enediol intermediate and occurs with inversion at C-3.

Figure 22.22 The pentose phosphate pathway.

The Nonoxidative Steps of the Pentose Phosphate Pathway

Figure 22.28 The transketolase reaction of step 6 in the pentose phosphate pathway. This is a two-carbon transfer reaction that requires thiamine pyrophosphate as a coenzyme. TPP chemistry was discussed in Chapter 19 (see the pyruvate dehydrogenase reaction).

Figure 22.22 The pentose phosphate pathway.

The Nonoxidative Steps of the Pentose Phosphate Pathway

Figure 22.31 The transaldolase reaction. In this reaction, a 3-carbon unit is transferred, first to an active site lysine, and then to the acceptor molecule.

Figure 22.22 The pentose phosphate pathway.

The Nonoxidative Steps of the Pentose Phosphate Pathway

Figure 22.29 The transketolase reaction of step 8 in the pentose phosphate pathway. This is another two-carbon transfer, and it also requires TPP as a coenzyme.

Utilization of Glucose-6-P Depends on the Cell’s Need for ATP, NADPH, and Rib-5-P

Glucose can be a substrate either for glycolysis or for the pentose phosphate pathway

The choice depends on the relative needs of the cell for biosynthesis and for energy from metabolism

ATP can be made if G-6-P is sent to glycolysis

Or, if NADPH or ribose-5-P are needed for biosynthesis, G-6-P can be directed to the pentose phosphate pathway

Depending on these relative needs, the reactions of glycolysis and the pentose phosphate pathway can be combined in four principal ways

Four Ways to Combine the Reactions of Glycolysis and Pentose Phosphate

1) Both Ribose-5-P and NADPH are needed by the cell

In this case, the first four reactions of the pentose phosphate pathway predominate

NADPH is produced and ribose-5-P is the principal product of carbon metabolism

2) More Ribose-5-P than NADPH is needed by the cell

Synthesis of ribose-5-P can be accomplished without making NADPH, by bypassing the oxidative reactions of the pentose phosphate pathway

Four Ways to Combine the Reactions of Glycolysis and Pentose Phosphate

Case 1: Both ribose-5-P and NADPH are needed

Figure 22.33 When biosynthetic demands dictate, the first four reactions of the pentose phosphate pathway predominate and the principal products are ribose-5-P and NADPH.

Four Ways to Combine the Reactions of Glycolysis and Pentose Phosphate

3) More NADPH than ribose-5-P is needed by the cell

This can be accomplished if ribose-5-P produced in the pentose phosphate pathway is recycled to produce glycolytic intermediates

4) Both NADPH and ATP are needed by the cell, but ribose-5-P is not

This can be done by recycling ribose-5-P, as in case 3 above, if fructose-6-P and glyceraldehyde-3-P made in this way proceed through glycolysis to produce ATP and pyruvate, and pyruvate continues through the TCA cycle to make more ATP

Xylulose-5-Phosphate is a Metabolic Regulator

In addition to its role in the pentose phosphate pathway, xylulose-5-P is also a signaling molecule

When blood glucose rises, glycolysis and the pentose phosphate pathways are activated in the liver

The latter pathway makes xylulose-5-P, which stimulates protein phosphatase 2A (PP2A)

PP2A dephosphorylates PFK-2/F-2,6-Bpase and also carbohydrate-responsive element-binding protein (ChREBP)

Glycolysis and lipid biosynthesis are both activated as a result

Xylulose-5-Phosphate is a Metabolic Regulator

Activation of PP2A triggers dephosphorylation of PFK-2/F2,6-BPase, which raises F-2,6-BP levels, activating glycolysis and

inhibiting gluconeogenesis.

Figure 22.34 Dephosphorylation of ChREBP elevates expression of genes for lipogenesis.

Aldose Reductase and Diabetic Cataract Formation

The complications of diabetes include a high propensity for cataract formation in later life

Hyperglycemia is the cause, but why?

Evidence points to the polyol pathway, in which glucose and other simple sugars are reduced in NADPH-dependent reactions

Glucose is reduced by aldose reductase to sorbitol, which accumulates in lens fiber cells, increasing pressure and eventually rupturing the cells

Aldose reductase inhibitors such as tolrestat and epalrestat suppress cataract formation

Aldose Reductase and Diabetic Cataract Formation

Glucose is reduced by aldose reductase to sorbitol, which accumulates in lens fiber cells, increasing pressure and eventually rupturing the cells

Aldose reductase inhibitors such as tolrestat and epalrestat suppress cataract formation

Glucose 6-Phosphate Dehydrogenase Deficiency

- affects more than 400 million people in the world.

- most common enzyme abnormality (enzymopathy) in people.

- the disease results in acute hemolytic anemia.

- red blood cells are particularly affected because they lack

mitochondria.

- NADPH is important for maintaining adequate levels of reduced

glutathione (GSH) in the cell.

- The GSH is critical for destroying hydrogen peroxide and

maintaining the cysteine residues in hemoglobin and other rbc

proteins in the reduced state as well as the iron in hemoglobin

in the ferrous state.

More than 400 variants of G6PD deficiency .

Most of the mutations are single-base changes that result in an

amino acid substitution.

The G6PD gene is on the X chromosome so males are primarily

affected.

Most female carriers (heterozygotes) are asymptomatic.

G-6-PD deficiency affects all races. The highest prevalence is

among persons of African, Asian, or Mediterranean descent.

Severity varies significantly between racial groups because of

different variants of the enzyme.

Glucose 6-Phosphate Dehydrogenase Deficiency (cont’d)

About 11% of African-American males are

affected by the so-called A-type.

In Africa, female carriers of the mutation were

found to have an increased resistance to malaria.

People originating in the Mediterranean region

may have another, more serious variant, called the

Mediterranean type.

People are usually asymptomatic but certain

drugs can trigger a severe episode of hemolytic

anemia within hours of exposure.

Glucose 6-Phosphate Dehydrogenase Deficiency (cont’d)

Drugs that can bring on an acute reaction include:

• antimalarial agents• sulfonamides (antibiotic)• aspirin• nonsteroidal anti-inflammatory drugs • nitrofurantoin• quinidine• quinine• others

Bacterial and viral infections can also trigger episodes.