The Breakdown of Glucose to Pyruvate by a Cell Is an Example of What Type of Reaction?
The following points highlight the 3 master metabolic pathways to intermission glucose into pyruvate. The pathways are: one. Glycolysis two. Pentose Phosphate Pathway or Hexose Monophosphate Pathway 3. Entner-Doudoroff Pathway.
Metabolic Pathway # 1. Glycolysis :
Glycolysis (Gk. glykys = sweetness, lysis = splitting), besides 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 internet production of ii molecules of ATP.
Glycolysis is about 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 absenteeism 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 2 molecules of the 3-carbon pyruvate) occurs in ten steps (Fig. 24.i). The offset 5-steps constitute the preparatory phase while the residual live-steps represent the payoff phase (oxidation phase).
In preparatory stage at that place 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 stage.
The step-wise concise account of glycolysis is the following:
1. Glucose (hexose saccharide) 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 Mgii+ for its action.
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 Mgtwo+ and is specific for glucose 6-phosphate and fructose half-dozen-phosphate.
three. Enzyme phosphofructokinase catalyses the transfer of a phosphoryl grouping from ATP to fructose 6-phosphate to yield fructose i, 6-bisphosphate. This reaction is substantially irreversible under cellular conditions. Phosphofructokinase besides requires Mg2+ for its activeness.
4. The enzyme fructose 1, 6-bisphosphate aldolase, often called only aldolase catalyses the cleavage of fructose 1,half dozen-bisphosphate to yield ii different triose sugar phosphates, glyceraldehyde 3-phosphate (an aldose) and dihydroxyacetone phosphate (a ketose).
v. 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 past the enzyme triose phosphate isomerase. This reaction completes the preparatory stage of glycolysis.
6. This step is the start footstep of payoff phase of glycolysis, Glyceraldehyde three-phosphate oxidises to i, three- bisphosphoglycerate with the interest of enzyme glyceraldehyde iii- phosphate dehydrogenase. During this reaction NAD+ is reduced yielding NADH (oxidative phosphorylation).
7. one, three-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 past phosphoryl group transfer from a substrate (1,3-bisphosphoglycerate) is called substrate level phosphorylation.
viii. iii-phosphoglycerate is now converted to 2-phosphoglycerate. In this reaction the enzyme phosphoglycerate mutase catalyses a reversible shift of the phosphoryl group betwixt C-ii and C-3 of glycerate; Mg2+ is essential for this reaction.
ix. 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 activeness.
The whole of glycolysis tin be represented by the post-obit 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 downwards glucose to pyruvate and operates in both aerobic and anaerobic atmospheric condition.
This pathway produces NADPH, which carries chemic energy in the form of reducing ability and is used near universally as the reductant in anabolic (energy utilization) pathways (e.g., fat acid biosynthesis, cholesterol biosynthesis, nucleotide biosynthesis) and detoxification pathways (east.chiliad., reduction of oxidized glutathione, cytochrome P450 monooxygenases).
Besides, the pentose phosphate pathway generates pentose sugar ribose and its derivatives, which are necessary for the biosynthesis of nucleic acids (Deoxyribonucleic acid 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, information technology 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 not-oxidative stage. In oxidative phase, in that location is generation of NADPH when glucose vi-phosphate is oxidised to ribose v-phosphate.
In non-oxidative phase, the pathway catalyzes the inter conversion of three-, four-, v-, half-dozen-, and seven-carbon sugars in a series of non-oxidative reactions that tin 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 six-phosphate to half dozen-Phosphogluconate. NADP+ is the electron acceptor yielding NADPH during this reaction. half dozen-Phosphogluconate, a six-carbon sugar, is so oxidatively decarboxylated to yield ribulose 5-phosphate, a 5-carbon sugar. NADP+ is once more the electron acceptor yielding NADPH.
In the final step of oxidative stage, there is isomerisation of ribulose five-phosphatc to ribose v-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 v-phosphate forming the seven-carbon sedoheptulose 7-phosphate and 3-carbon glyceraldehyde iii-phosphate.
Enzyme transaldolase and so catalyses the transfer of a three-carbon fragment from sedoheptulose vii-phosphate to glyceraldehyde iii-phosphate resulting in six-carbon fructose six-phosphate and four carbon erythrose four-phosphate.
At present transketolase acts again, forming fructose 6-phosphate and glyceraldehyde iii-phosphate from erythrose iv-phosphate and xylulose v-phosphate. Two molecules of glyceraldehyde 3-phosphate formed past two interations of these reactions can be converted into a molecule of fructose 1, vi-bisphosphate.
The overall outcome of pentose phosphate pathway is that 3 glucose six-phosphates are converted to two fructose 6-phosphates, glyceraldehyde three-phosphate, and three CO2 molecules, as shown in the following equation:
three glucose 6-phosphate + half dozen NADP+ + 3H2O → 2 fructose half-dozen-phosphate + glyceraldehyde 3-phosphate + 3CO2 + half-dozen NADPH + 6H+
Fructose half dozen-phosphate and glyceraldehyde 3-phosphate intermediates are used in two ways. The fructose half dozen-phosphate tin be converted dorsum to glucose 6-phosphate, while glyceraldehyde 3-phosphate is converted to pyruvate past glycolysis-enzymes.
The glyceraldehyde iii-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 some other pathway utilised by bacteria to convert glucose to pyruvate. Although most leaner take 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 central enzymes of the ED pathway are vi-phosphogluconate dehydrase and ii-keto-iii-deoxyglucosephosphate aldolase (KGDP-aldolase).
A survey for the presence of these enzymes in a multifariousness of bacteria has revealed that they are generally nowadays in bacteria of genera Pseudomonas, Rhizobium, Azotobacter, Agrobacterium, Zymomonas, and several other gram negative bacteria simply 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 aforementioned reactions as the pentose phosphate pathway. Glucose is phosphorylated, like pentose phosphate pathway, to glucoses- phosphate which so oxidized to 6- phosphogluconate. The latter, instead of beingness further oxidized, is dehydrated to form 2-keto-3- deoxy-6-phosphogluconate, the central intermediate compound in this pathway.
two-keto-iii-deoxy-6- phosphogluconate (KDPG) is then broken 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, ane NADH, and one NADPH per glucose metabolized.
Source: https://www.biologydiscussion.com/microbiology-2/microbial-respiration/metabolic-pathways-to-break-glucose-into-pyruvate/55278
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