Pentose Phosphate Pathway: Source of NADPH for Reductive Biosynthesis (Biochemistry)

Introduction to Pentose Phosphate Pathway
The pentose phosphate pathway (Hexose monophosphate pathway) is the metabolic pathway that occurs in the cytosol and generates NADPH (nicotinamide adenine dinucleotide phosphate) which is utilized in various biosynthetic pathways. This pathway can be broadly classified into two phases i.e oxidative phase and non-oxidative phase. 

In the oxidative stage of the pentose phosphate pathway, the glucose-6-phosphate is converted to five carbon ribulose-5 phosphate with generation of NADPH. 

During the non-oxidative stage, the interconversion of pentose sugars and hexose sugars is catalyzed by isomerases and aldolases. The fate of these reversible steps depends on the anabolic and energy status of the cells. 

The two primary functions of the pentose phosphate pathway are;
a) Provide NADPH (a coenzyme) for biosynthetic pathways
b) Provide pentose sugars for nucleotides (DNA and RNA) synthesis

Two phases of Pentose phosphate pathway:

The oxidative and non-oxidative phases of the pentose phosphate pathway are interconnected, and the specific steps in the non-oxidative phase can vary depending on the metabolic needs of the cell.

i) Oxidative phase of Pentose Phosphate Pathway:

The oxidative stage of the pentose phosphate pathway is a stepwise oxidation-reduction reaction followed by decarboxylation.

-The first step of the pentose phosphate pathway is the conversion of glucose-6-phosphate to 6-phosphogluconolactone and generation of NADPH from NADP+. This oxidation-reduction coupled reaction is catalyzed by the enzyme glucose-6-phosphate dehydrogenase (G6PD). G6PD is a rate-limiting enzyme of the pentose phosphate pathway.

-The second step is the hydrolysis of 6-phosphogluconolactone to 6-phosphogluconate catalyzed by enzyme gluconolactone hydrolase.

-The third enzyme 6-phosphogluconate dehydrogenase catalyzed an oxidation-reduction reaction of the pentose phosphate pathway to yield 3-keto-6-phosphogluconate which is non-enzymatically decarboxylated to form ribulose-5-phosphate. During this conversion, the second NADPH is generated and CO2 is released.

 

Altogether, the conversion of glucose-6-phosphate to ribulose-6-phosphate yields two NADPH which is used for various biosynthetic and physiological processes of the cells.

Figure 1: Oxidative Stage of the Pentose Phosphate Pathway.

Enzymes for the pathway: 1) Glucose-6-phosphate dehydrogenase; 2) Gluconolactone hydrolase; 3) 6-phosphogluconate dehydrogenase; 4) nonenzymatic decarboxylation

ii) Non-oxidative phase of Pentose Phosphate pathway:

Once the ribulose-5-phosphate is formed, the series of reversible steps can result in the formation of six sugars. Transketolase and trans aldolases are the enzymes that catalyze a series of reversible reactions to convert pentose sugar into fructose-6-phosphate which can again re-enter glycolysis or pentose phosphate pathway depending on the cellular demand of ATP and NADPH.

          The non-oxidative phase involves the interconversion of various sugar phosphates without the                involvement of NADPH or NADP+.

-First, the ribulose-5-phosphate is converted into xylulose 5-phosphate and ribose-5-phosphate catalyzed by enzyme 3-epimerase and keto-isomerase respectively.

-Second, the xylulose-5-phosphate and ribose-5-phosphate are converted into glyceraldehyde-3-phosphate and sedoheptulose by the enzyme Transketolase. Transketolase is a thiamine-contained enzyme that transfers 2 carbons from keto-sugars to Aldo-sugar (or vice-versa).

-Third, the glyceraldehyde-3-phosphate and sedoheptulose are converted to fructose-6-phosphate and erythrose-4-phosphate catalyzed by the enzyme Transaldolase. Transaldolase transfers 3 carbons from keto-sugar to Aldo-sugar.

-Lastly, another 2 carbons from xylulose-5-phosphate (keto-sugar) to erythrose-4-phosphate to yield fructose-6-phosphate and glyceraldehyde.

Overall, three pentose sugars are utilized to form 2 fructose-6-phosphate and 1 glyceraldehyde-3-phosphate. 

Figure 2: Non-oxidative Stage of the Pentose Phosphate Pathway

Cellular demand for ATP, NADPH, and pentose sugar modulate pentose phosphate pathway
-Scenario 1: When ribose-5-phosphate and NADPH are the prime need of the cells (especially dividing cells), most of the glucose-6-phosphate is converted into pentose sugar and NADPH. The oxidative reaction is most active during this state of cells.

-Scenario 2: When NADPH is required for other biosynthetic pathways, the pentose sugars are converted into fructose-6-phosphate which can be re-converted into glucose-6-P by glycolytic enzyme phosphogluco isomerase. The glucose-6-phosphate thus formed re-enters into the pentose phosphate pathway. In this mode, one glucose-6-phosphate can generate 12 NADPH.

-Scenario 3: When NADPH and ATP are required (for example erythrocytes) 3 glucose-6-phosphate is converted into a pentose phosphate pathway and generates 6 NADPH. The 3 pentose sugar formed during the oxidative stage is converted into 2 fructose-6-phosphate and 1 glyceraldehyde-3-phosphate. These can enter glycolysis to form additional ATPs and NADH.