Metabolic Pathways to Break Glucose into Pyruvate

The following points highlight the three main metabolic pathways to break glucose into pyruvate. The pathways are: 1. Glycolysis 2. Pentose Phosphate Pathway or Hexose Monophosphate Pathway 3. Entner-Doudoroff Pathway.

Metabolic Pathway # 1. Glycolysis:

Glycolysis (Gk. glykys = sweet, lysis = splitting), also called glycolytic pathway or Embden-Meyerhof-Parnas (EMP) pathway, is the sequence of reactions that metabolises one molecule of glucose to two molecules of pyruvate with the concomitant net production of two molecules of ATP.

Glycolysis is almost an universal central pathway of glucose catabolism, and the complete pathway of glycolysis was elucidated by 1940, largely through the pioneering contributions of G. Embden, O. Meyerhof, J. Parnas, C. Neuberg, O. Warburg, G. Cori, and C. Cori. However, glycolysis occurs in all major groups of microorganisms and functions in the presence or absence of oxygen. It is located in the cytoplasmic matrix of the cells of an organism.

The whole process of glycolysis (i.e., the breakdown of the 6-carbon glucose molecule into two molecules of the 3-carbon pyruvate) occurs in ten steps (Fig. 24.1). The first five-steps constitute the preparatory phase while the rest live-steps represent the payoff phase (oxidation phase).

In preparatory phase there is phosphorylation of glucose and its conversion to glyceraldehyde 3-phosphate at the expense of two molecules of ATP. Oxidative conversion of glyceraldehyde 3-phosphate to pyruvate and the coupled formation of ATP and NADH is the feature of payoff phase.

The step-wise concise account of glycolysis is the following:

1. Glucose (hexose sugar) is activated for subsequent reactions by its phosphorylation to yield glucose 6-phosphate, with ATP as the phosphoryl donor. This reaction, which is irreversible under intracellular conditions, is catalyzed by enzyme hexokinase, which requires Mg²⁺ for its activity.

2. Enzyme phosphohexose isomerase (phosphoglucose isomerase) catalyzes the reversible isomerization of glucose 6-phosphate (an aldose) to fructose 6- phosphate (a ketose). Phosphohexose isomerase requires Mg²⁺ and is specific for glucose 6-phosphate and fructose 6-phosphate.

3. Enzyme phosphofructokinase catalyses the transfer of a phosphoryl group from ATP to fructose 6-phosphate to yield fructose 1, 6-bisphosphate. This reaction is essentially irreversible under cellular conditions. Phosphofructokinase also requires Mg²⁺ for its activity.

4. The enzyme fructose 1, 6-bisphosphate aldolase, often called simply aldolase catalyses the cleavage of fructose 1,6-bisphosphate to yield two different triose sugar phosphates, glyceraldehyde 3-phosphate (an aldose) and dihydroxyacetone phosphate (a ketose).

5. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-convertible. Only glyceraldehyde 3-phosphate is directly degraded in the subsequent steps and, therefore, dihydorxyacetone phosphate is rapidly and reversibly converted to glyceraldehyde 3-phosphate by the enzyme triose phosphate isomerase. This reaction completes the preparatory phase of glycolysis.

6. This step is the first step of payoff phase of glycolysis, Glyceraldehyde 3-phosphate oxidises to 1, 3- bisphosphoglycerate with the involvement of enzyme glyceraldehyde 3- phosphate dehydrogenase. During this reaction NAD⁺ is reduced yielding NADH (oxidative phosphorylation).

7. 1, 3-bisphosphoglyceratc is converted to 3-phosphoglycerate. In this reaction the enzyme phosphoglycerokina.se transfers the high-energy phosphoryl group from 1,3-bisphosphoglycerate to ADP yielding ATP and 3-phosphoglycerate. The formation of ATP by phosphoryl group transfer from a substrate (1,3-bisphosphoglycerate) is called substrate level phosphorylation.

8. 3-phosphoglycerate is now converted to 2-phosphoglycerate. In this reaction the enzyme phosphoglycerate mutase catalyses a reversible shift of the phosphoryl group between C-2 and C-3 of glycerate; Mg²⁺ is essential for this reaction.

9. In this step the enzyme enalase promotes reversible removal of a molecule of water from 2-phosphoglycerate to yield phosphoenolpyruvate.

10. This is the last step in glycolysis. Phosphoryl group from phosphoenolpyruvate is transferred to ADP by enzyme pyruvate kinase to yield ATP and pyruvate via substrate level phosphorylation. The enzyme pyruvate kinase requires K and cither Mg²⁺ or Mn²⁺ for its activity.

The whole of glycolysis can be represented by the following simple equation:

Glucose + 2ADP + 2Pi + 2NAD⁺ = 2 pyruvate + 2ATP + 2NADH + 2H⁺

Metabolic Pathway # 2. Pentose Phosphate Pathway or Hexose Monophosphate Pathway (HMP Pathway):

Pentose phosphate pathway or hexose monophosphate pathway (HMP pathway) is the other common pathway to break down glucose to pyruvate and operates in both aerobic and anaerobic conditions.

This pathway produces NADPH, which carries chemical energy in the form of reducing power and is used almost universally as the reductant in anabolic (energy utilization) pathways (e.g., fatty acid biosynthesis, cholesterol biosynthesis, nucleotide biosynthesis) and detoxification pathways (e.g., reduction of oxidized glutathione, cytochrome P450 monooxygenases).

Also, the pentose phosphate pathway generates pentose sugar ribose and its derivatives, which are necessary for the biosynthesis of nucleic acids (DNA and RNA) as well as ATP, NADH, FAD, and coenzyme A. In this way, though the pentose phosphate pathway may be a source of energy in many microorganisms, it is more often of greater importance in various biosynthetic pathways.

Pentose phosphate pathway (Fig. 24.2.) consists of two phases: the oxidative phase and the non-oxidative phase. In oxidative phase, there is generation of NADPH when glucose 6-phosphate is oxidised to ribose 5-phosphate.

In non-oxidative phase, the pathway catalyzes the inter conversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions that can result in the synthesis of five-carbon sugars for nucleotide biosynthesis or the conversion of excessive five-carbon sugars into intermediates of glycolysis. All the reactions of non-oxidative phase take place in the cytoplasm of the cell.

Oxidative Phase:

The oxidative phase of the pentose phosphate pathway initiates with the conversion of glucose 6-phosphate to 6-Phosphogluconate. NADP⁺ is the electron acceptor yielding NADPH during this reaction. 6-Phosphogluconate, a six-carbon sugar, is then oxidatively decarboxylated to yield ribulose 5-phosphate, a five-carbon sugar. NADP⁺ is again the electron acceptor yielding NADPH.

In the final step of oxidative phase, there is isomerisation of ribulose 5-phosphatc to ribose 5-phosphate by phosphopentose isomerase and the conversion of ribulose 5-phosphate into its epimer xylulose 5-phosphate by phosphopentose epimerase for the transketolase reaction in non-oxidative phase.

Nonoxidative Phase:

In the non-oxidative phase, enzyme transketolase catalyzes the transfer of a two carbon fragment of xylulose 5-phosphate to ribose 5-phosphate forming the seven-carbon sedoheptulose 7-phosphate and three-carbon glyceraldehyde 3-phosphate.

Enzyme transaldolase then catalyses the transfer of a three-carbon fragment from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate resulting in six-carbon fructose 6-phosphate and four carbon erythrose 4-phosphate.

Now transketolase acts again, forming fructose 6-phosphate and glyceraldehyde 3-phosphate from erythrose 4-phosphate and xylulose 5-phosphate. Two molecules of glyceraldehyde 3-phosphate formed by two interations of these reactions can be converted into a molecule of fructose 1, 6-bisphosphate.

The overall result of pentose phosphate pathway is that 3 glucose 6-phosphates are converted to two fructose 6-phosphates, glyceraldehyde 3-phosphate, and three CO₂ molecules, as shown in the following equation:

3 glucose 6-phosphate + 6 NADP⁺ + 3H₂O → 2 fructose 6-phosphate + glyceraldehyde 3-phosphate + 3CO₂ + 6 NADPH + 6H⁺

Fructose 6-phosphate and glyceraldehyde 3-phosphate intermediates are used in two ways. The fructose 6-phosphate can be converted back to glucose 6-phosphate, while glyceraldehyde 3-phosphate is converted to pyruvate by glycolysis-enzymes.

The glyceraldehyde 3-phosphate also may be returned to pentose phosphate pathway through glucose 6-phosphate formation. This results in the complete degradation of glucose 6-phosphate to CO₂ and the production of great deal of NADPH.

Metabolic Pathway # 3. Entner-Doudoroff Pathway (ED Pathway):

Entner-Doudoroff pathway (ED pathway) is another pathway utilised by bacteria to convert glucose to pyruvate. Although most bacteria have the glycolytic pathway (glycolysis) and pentose phosphate pathway (hexose monophosphate pathway), some substitute ED pathway for glycolytic pathway. The bacteria that use this pathway are mostly gram-negative and rarely gram-positive.

Two key enzymes of the ED pathway are 6-phosphogluconate dehydrase and 2-keto-3-deoxyglucosephosphate aldolase (KGDP-aldolase).

A survey for the presence of these enzymes in a variety of bacteria has revealed that they are generally present in bacteria of genera Pseudomonas, Rhizobium, Azotobacter, Agrobacterium, Zymomonas, and several other gram negative bacteria but are absent from gram-positive bacteria (except for a few Nocardia isolates and Enterococcus faecalis).

The Entner-Doudoroff pathway (Fig. 24.3.) begins with the same reactions as the pentose phosphate pathway. Glucose is phosphorylated, like pentose phosphate pathway, to glucoses- phosphate which then oxidized to 6- phosphogluconate. The latter, instead of being further oxidized, is dehydrated to form 2-keto-3- deoxy-6-phosphogluconate, the key intermediate compound in this pathway.

2-keto-3-deoxy-6- phosphogluconate (KDPG) is then cleaved to pyruvate and glyceraldehyde 3-phosphate by the enzyme KDPG-aldolose. Glyceraldehyde-3-phosphate enters into the glycolytic pathway and is converted, finally, to pyruvate. This pathway yields one ATP, one NADH, and one NADPH per glucose metabolized.