The following points highlight the two main pathways in intermeniary metabolism of carbohydrates.
Pathway # 1. Gluconeogenesis:
In this process glucose is formed from non-carbohydrate precursors. Thus, citric acid cycle and glycolysis are the main pathways of gluconeogenesis. The substrates can be some amino acids, (e.g., proline) lactic acid, glycerol, etc. Gluconeogenesis from amino acids and some organic acids is reported in the germinating seeds and some pollen grains. Here the glycolysis is reversed (Fig. 17-12).
Pathway # 2. Hexose Monophosphate Shunt or the Pentose Phosphate Pathway (PPP):
In addition to glycolysis as the major pathway of oxidation of glucose, (Figs. 17-12, 13, 14, 15) and the Krebs cycle, there exists another respiratory pathway in higher organisms. This is most commonly known as higher hexose monophosphate shunt, or Warburg-Dickens pathway or phosphogluconate pathway.
It is often called the pentose phosphate pathway (PPP), since it involves five-carbon sugar phosphates as intermediates. It is fascinating to note that in this pathway only one molecule of CO2 is produced from one molecule of glucose after oxidation and the rest of the carbons participate in a complex reorganization.
Higher plants as well as other organisms have at their disposal pentose phosphate pathway as an alternative for the degradation of hexose sugars to pyruvate.
The pathway starts with the phosphorylation of 6 molecules of glucose in the presence of an enzyme hexokinase, yielding 6 glucose 6-P, which is immediately oxidized by glucose 6-phosphate dehydrogenase to 6-phosphogluconic acid and NADP is reduced.
Further, dehydrogenation and decarboxylation of 6-phosphogluconic acid is catalyzed by glucose 6-phosphogluconic acid dehydrogenase to yield 5-carbon compound, ribulose 5-phosphate.
This reaction is not reversible and, as in the first step, NADP is required. Carbon dioxide is produced only during this conversion at the rate of one molecule of CO2 per glucose molecule. Glucose 6-phosphate can also arise from starch breakdown through starch phosphorylase followed by phosphoglucomutase action in glycolysis.
A comparison of Calvin cycle and PPP would reveal that many of the compounds are similar for sugar phosphates synthesis in chloroplasts.
The chief difference between the two pathways is that during PPP sugar phosphates are oxidized rather than synthesized, and thus the reactions of PPP resemble glycolysis. In fact, the two pathways appear to be interwoven with each other. Pentose phosphate pathway differs from glycolysis since in the former NADP+ accepts the electrons from sugar phosphates. However, in glycolysis NAD+ is the acceptor.
Contrary to the earlier belief that glycolysis and PPP occur only in the cytoplasm, outside any organelle, it has been shown now that PPP does occur in the chloroplasts but in the dark. In fact, light inactivates glucose 6-phosphate dehydrogenase enzyme while those of Calvin cycle are activated.
In these two steps of oxidation, 6 molecules of glucose 6-P are oxidized to 6 molecules of ribulose 5-P and 6 CO2 and 12 NADH2 formed. Twelve molecules of NADPH are then re-oxidized to 12 NADP in the presence of the cytochrome system and oxygen of air.
The main function of the subsequent reactions is to cycle back 6 molecules of ribulose 5-P into 6 molecules of glucose 6-P.
Regeneration of Glucose 6-Phosphate from Ribulose 5-Phosphate:
First of all two molecules of ribulose 5-phosphate (Ketose) are converted to their two molecules of isomers ribose 5-phosphate (aldose) and the reaction is catalyzed by isomerase.
Another four molecules of ribulose phosphate are converted to their epimerxylulose 5-phosphate and the reaction is catalyzed by epimerase.
Two molecules of ribose 5-phosphate and two molecules of xylulose 5-phosphate, in the presence of the thiamine-requiring enzyme transketolase react to form two molecules of 7 carbon, sedoheptulose 7-phosphate and two molecules of three-carbon glyceraldehyde 3-phosphate.
Next the two molecules of sedoheptulose 7-phosphate and the two molecules of glyceraldehyde 3-phosphate react under the influence of the enzyme transaldolase to form 2 molecules of fructose 6-phosphate and two molecules of four carbon erythrose 4-phosphate.
Two molecules of erythrose 4-phosphate then, react with two remaining molecules of xylulose 5-phosphate in the presence of the enzyme transketolase to form two molecules of fructose 6-phosphate and 2 molecules of glyceraldehyde 3-phosphate.
One of the molecules of glyceraldehyde-3-phosphate changes into its isomer dihydroxyacetone-phosphate and then the two isomers react to form fructose 1,6- bisphosphate. The latter loses a phosphate group and changes to fructose 6-phosphate.
All the 5 molecules of fructose 6-phosphate, formed in the reactions 7,8 and 9 directly converted to 5 molecules of glucose 6-phosphate and the oxidation then may start again.
Some of the nine Reactions may be summed up as under
The 5-carbon phosphorylated sugar can be imported into cell wall polymers (pentosans).
The Main Features of PPP:
i. Oxygen is required from the very outset.
ii. The hexose is broken down to CO2 + H2 O without the participation of glycolysis and the Krebs cycle.
iii. The reaction takes place in the cytoplasm and chloroplast in the darkness.
iv. NADPH+ produced is used for synthetic reactions.
v. It is a source of ribose 5-phosphate which is used for nucleotides and nucleic acids synthesis.
vi. In the oxidation of one molecule of glucose, 12 NADPH+ are formed from which 36 ATP molecules are synthesized. Thus this pathway is almost as efficient as the glycolytic Krebs cycle pathway in trapping the energy released in the oxidation of glucose molecule.
vii. Erythrose 4-phosphate is used in aromatic compounds and lignin.