The hexose biooxidation process with pentose phosphate as the main intermediate is also known as the hexose phosphate branch. It is another way of aerobic degradation of carbohydrates besides glycolysis-tricarboxylic acid cycle. Through the pentose phosphate pathway, glucose-6-phosphate produced by phosphorylation of glucose is completely degraded to carbon dioxide, and at the same time, the oxidized nicotinamide adenine dinucleotide phosphate (NADP) is reduced to reduced nicotinamide adenine dinucleoside Acid Phosphoric Acid (NADPH).
This pathway proceeds in the cytosol, and several intermediate products are members of the Calvin cycle.
Chemical process the pentose phosphate pathway consists of an oxidative phase and a non-oxidative phase. The first step of the pentose phosphate pathway is the catalytic oxidation of glucose-6-phosphate-by-glucose-6-phosphate dehydrogenase to 6-phospho-D-glucono-delta-lactone, and a molecule of NADPH is produced at the same time. 6-phospho-D-gluconic acid-delta-lactone can be automatically hydrolyzed or converted into 6-phospho-gluconic acid by lactonase catalysis. 6-phospho-gluconic acid is then dehydrogenated and decarboxylated under the catalysis of phosphogluconate dehydrogenase to produce one molecule of D-ribulose-5-phosphate, one molecule of NADPH and one molecule of CO2. The above is the oxidation stage of the pentose phosphate pathway, and the overall reaction is:
Glucose-6-phosphate+2NADP++H2O→
D-ribulose-5-phosphate+CO2+2NADPH+2H+
Starting from ribulose-5-phosphate, it enters the non-oxidative phase of the pentose phosphate pathway. Transketolase carries a hydroxyacetaldehyde group from xylulose-5-phosphate to ribose-5-phosphate to form a molecule of sedum heptulose-7-phosphate and a molecule of glyceraldehyde-3-phosphate. The above seven-carbon sugar and three-carbon sugar are converted into one molecule of six-carbon sugar and one molecule of four-carbon sugar through the catalysis of transaldolase. In the non-oxidative stage of the pentose phosphate pathway, three, four, five, six, and seven carbon sugars undergo mutual conversion through the action of transketolase and transaldolase and the action of certain enzymes in the glycolysis pathway. The overall response is:
3D-ribulose-5-phosphate→glyceraldehyde-3-phosphate+2 fructose-6-phosphate
Therefore, the pentose phosphate pathway is that 6 molecules of glucose-6-phosphate are first oxidized to 6 molecules of ribulose-5-phosphate and 6 molecules of CO2. The 6 molecules of ribulose-5-phosphate are converted into 5 molecules of glucose-6-phosphate. The whole reaction can be expressed by the following formula:
6 glucose-6-phosphate+12NADP+→5 glucose-6-phosphate+6CO2 + 12NADPH+ 12H++Pi, the net result is:
6Glucose-6-phosphate+12NADP+→6CO2+12NADPH+12H++Pi
That is, glucose-6-phosphate can be completely oxidized to CO2 through the pentose phosphate pathway.
New developments in the non-oxidation stage the above-mentioned non-oxidation stage model has been generally accepted since Horecker proposed in 1954. However, this model is inconsistent with the results of some isotope tracer tests, and it is well known. In 1978, Williams et al. (J.F.Williams) proposed a new non-oxidation stage model (see figure). They use rat liver extract
For the test materials, five other intermediate products of the pentose phosphate pathway were found, namely D-mannoheptulose-7-phosphate, D-aloheptulose-1,7-diphosphate, and D-glycerol-D- Idulinulose-1,8-diphosphate, D-glycerol-D-Altro-octulose-1,8-diphosphate and D-arabinose. And identified a new enzyme phosphate arabinose-2-epimerase which can catalyze the interconversion of D-arabinose-5-phosphate and D-ribose-5-phosphate. Compared with the old model, the new model has no transaldolase and requires higher aldolase activity. The significance of the new model in plants has yet to be evaluated, but the Gibb’s effect in plant photosynthetic the new model can explain carbon assimilation (14C asymmetric marker in the glucose molecules of phosphate and starch). Although more evidence is needed to fully accept the new model, it is clearly incorrect to regard the non-oxidation phase as a fixed mechanism that converts 3 molecules of pentose phosphate into one molecule of triose phosphate and 2 molecules of hexose phosphate. There are many possible reaction networks in the non-oxidation stage, and the focus of the reaction can be changed due to different tissues or tissue physiological states.
Regulation and physiological significance it is known that both the pentose phosphate pathway and glycolysis are carried out in the cytoplasm. The pathway through which glucose-6-phosphate is degraded depends on the concentration and activity of various enzymes, and is affected by the cell Control of the ratio of nicotinamide adenine dinucleotide (NAD) and NADP. Because glycolysis-the tricarboxylic acid cycle requires NAD, and the pentose phosphate pathway must utilize NADP. If the concentration of NADP is greater than the concentration of NAD, the pentose phosphate pathway predominates. On the contrary, glycolysis is mainly based on the tricarboxylic acid cycle. Many factors affect the ratio of NAD to NADP concentration. For example, when the oxygen concentration is high, NADH is oxidized by mitochondria faster, while NADPH is oxidized by the cytochrome system slowly, which is not conducive to the pentose phosphate pathway. For another example, the enhancement of certain synthesis reactions that require the consumption of NADPH will increase the concentration of NADH, thereby benefiting the pentose phosphate pathway.
The physiological significance of the pentose phosphate pathway mainly includes the following four aspects:
①The NADPH produced is a specific electron donor in many biosynthetic reactions. For example, the synthesis of fatty acids and sterols must be supplied with NADPH. ②Some intermediate products are important raw materials for biosynthesis. For example, ribulose-5-phosphate is the raw material for the synthesis of nucleic acids; erythrose-4-phosphate is the raw material for the synthesis of lignin and other aromatic compounds, such as phenols and other substances related to plant disease resistance. ③NADPH can re-oxidize to produce energy through the action of transhydrogenase system or NADPH-cytochrome C reductase. Although the main purpose of the pentose phosphate pathway is not to produce energy. It is related to the disease resistance of plants. Many anti-disease substances are known to belong to phenolic compounds. The synthesis of these substances requires erythrose-4-phosphate, an intermediate product of the pentose phosphate pathway, as a raw material. In addition, phenol oxidase and ascorbate oxidase, which are related to disease resistance, are related to NADPH produced by the pentose phosphate pathway. The pathway of pentose phosphate in plants is significantly enhanced after disease.