In this article, we will consider one of the options for the oxidation of glucose - the pentose phosphate pathway. Variants of the course of this phenomenon, methods for its implementation, the need for enzymes, biological significance and the history of discovery will be analyzed and described.
Acquaintance with the phenomenon
The pentose phosphate pathway is one of the methods for the oxidation of C6H12O6 (glucose). It consists of an oxidizing and non-oxidizing stage.
General process equation:
3glucose-6-phosphate + 6NADP - à3CO 2 +6 (NADPH + H - ) + 2 fructose-6-phosphate + glyceraldehyde-3-phosphate.
After passing the oxidative pentose phosphate pathway, the molecule of glyceraldehyde-3-phosphate is converted to pyruvate and forms 2 molecules of adenosine triphosphoric acid.
Animals and plants among their subunits are widespread such a phenomenon, but microorganisms use it only as an auxiliary process. All pathway enzymes are located in the cell cytoplasm in animals and plant organisms. In addition, mammals also contain these substances in EPS, and plants in plastids, specifically in chloroplasts.
The pentose phosphate pathway for glucose oxidation is similar to the glycolysis process and has an extremely long evolutionary pathway. It is likely that in the Archean aquatic environment, before life appeared in its modern sense, reactions took place that had a pentose phosphate nature, but the catalyst for this cycle was not an enzyme, but metal ions.
Types of Existing Reactions
As noted earlier, the pentose phosphate pathway distinguishes two stages, or cycles: oxidative and non-oxidative. As a result, on the oxidizing part of the pathway, 6126 is oxidized from glucose-6-phosphate to ribulose-5-phosphate, and ultimately NADPH is reduced. The essence of the non-oxidative stage is to help synthesize pentose and include yourself in the reversible transfer of 2 to 3 carbon “pieces”. Then, pentoses can again be transferred to the state of hexoses, which is caused by an excess of pentose itself. The catalysts involved in this pathway are divided into 3 enzymatic systems:
- dehydriro – decarboxylation system;
- isomerizing type system;
- a system for reconfiguring sugars.
Reactions accompanied by and without oxidation
The oxidizing part of the path is presented in the following equation:
Glucose 6-phosphate + 2 NADP + + H2O àribulose 5-phosphate + 2 (NADPH + H + ) + CO2.
In the non-oxidative step, there are two catalysts in the form of transaldolase and transketolase. They accelerate the cleavage of the CC bond and the transfer of carbon chain fragments that are formed as a result of this break. Transketolase exploits the coenzyme of thiamine pyrophosphate (TPP), which is a diphosphor type vitamin ester (B 1 ).
The general form of the equation of the stage in a non-oxidative form:
3 ribulose 5phosphate 1 ribose 5 phosphate + 2 xylulose 5 phosphate 2 fructose 6 phosphate + glyceraldehyde 3 phosphate.
You can observe the oxidative variety of the path when NADPH is used by the cell or in other words, when ONADPH goes into its standard position in an unreduced form.
The use of the glycolysis reaction or the described pathway depends on the amount of NADP + concentration in the thickness of the cytosol.
Cycle path
Summing up the results obtained by analyzing the general equation of the pathway of the non-oxidative variant, we see that pentoses can be returned from hexoses to glucose monosaccharides using the pentose phosphate pathway. Subsequent conversion of pentose to hexose is a pentose phosphate cyclic process. The pathway under consideration and all its processes are concentrated, as a rule, in fatty tissues and liver. In total, the equation can be described as:
6 glucose-6-phosphate + 12nadph + 2H2Oà12 (NADPH + H + ) +5 glucose-6phosphate + 6 CO2.
Non-oxidative type of pentose phosphate pathway
The non-oxidative step of the pentose phosphate pathway can undergo glucose rearrangement without CO2 detachment, which is possible due to the enzymatic system (it transforms sugars and glycolytic enzymes of nature that convert glucose-6-phosphate to the state of glyceraldehyde-3-phosphate).
When studying the metabolism of yeast forming lipids (which lack phosphofructokinase, which does not allow them to oxidize C6H12O6 monosaccharides using glycolysis), it turned out that glucose in the amount of 20% is oxidized by the pentose phosphate pathway, and the remaining 80% undergoes readjustment to the non-oxidative step of the pathway . Currently, the answer to the question of how specifically a 3-carbon compound is formed, which can only be created by glycolysis, remains unknown.
Function for living organisms
The value of the pentose phosphate pathway in animals and plant organisms, as well as microorganisms, is almost the same. All cells perform this process in order to form a reduced version of NADPH, which will be used as a hydrogen donor in a reduction type reaction and hydroxylation. Another function is to provide cells with ribose-5-phosphate. Despite the fact that NADPH can also be formed as a result of the oxidation of malate with the formation of pyruvate and CO2, and in the case of isocitrate dehydrogenation, the production of equivalents of a reducing nature occurs due to the pentose phosphate process. Another of the intermediates of this pathway is erythrozo-4-phosphate, which, when condensed with phosphoenolpyruvates, initiates the formation of tryptophan, phenylalanine and tyrosine.
The functioning of the pentose phosphate pathway is observed in animals in the liver, mammary glands during lactation, testes, adrenal cortex, as well as in red blood cells and adipose tissues. This is due to the presence of actively undergoing hydroxylation and regeneration reactions, for example, during the synthesis of fatty acids, is also observed during the destruction of xenobiotics in the liver tissues and the active oxygen form in red blood cells and other tissues. Such processes cause a high demand for a variety of equivalents, including NADPH.
Consider the example of red blood cells. In these molecules, glutathione (tripeptide) is involved in the neutralization of the active oxygen form. This compound, undergoing oxidation, converts hydrogen peroxide to 2, but the reverse transition from glutathione to the reduced variation is possible in the presence of NADPH + H + . If the cell has a defect in glucose-6-phosphate dehydrogenase, then aggregation of hemoglobin promoters can be observed, as a result of which the red blood cell loses its plasticity. Their normal functioning is possible only with the full operation of the pentose phosphate pathway.
The reverse pentose phosphate phosphate pathway forms the basis for the dark phase of photosynthesis. In addition, some plant groups are largely dependent on this phenomenon, which may determine, for example, the rapid interconversion of sugars, etc.
The role of the pentose phosphate pathway for bacteria lies in the reactions of gluconate metabolism. Cyanobacteria use this process due to the lack of a full Krebs cycle. Other bacteria exploit this phenomenon to oxidize various sugars.
Regulatory processes
The regulation of the pentose phosphate pathway depends on the need for glucose-6-phosphate by the cell and the level of NADP + concentration in the cytosol liquid. It is these two factors that will determine whether the aforementioned molecule will enter into glycolysis reactions or on the path of the pentose phosphate type. The absence of electronic acceptors will not allow the first stages of the path to proceed. With the rapid transfer of NADPH to NADP + , the concentration level of the latter rises. Allosteric stimulation of glucose-6-phosphate dehydrogenase occurs and, as a result, the amount of glucose-6-phosphate flux increases using the pentose phosphate type pathway. Slowing down the intake of NADPH leads to a decrease in the level of NADP + , and glucose-6-phosphate is disposed of.
Historical data
The pentose phosphate pathway began its research path due to the fact that attention was drawn to the absence of a change in glucose intake by general glycolysis inhibitors. Almost simultaneously with this event, O. Warburg made the discovery of NADPH and began to describe the oxidation of glucose-6-phosphates to 6-phosphogluconic acids. In addition, it was proved that C6H12O6, marked with 14 C isotopes (labeled C-1), converted to 14 CO2 relatively sooner than the same molecule, but marked C-6. This has shown the importance of glucose utilization through alternative routes. These data were published by I.K. Hansalus in 1995.
Conclusion
And so, we see that the pathway under consideration is used by cells as an alternative way to oxidize glucose and is divided into two options in which it can occur. This phenomenon is observed in all forms of multicellular organisms and even in many microorganisms. The choice of oxidation methods depends on various factors, the presence of certain substances in the cell at the time of the reaction.