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The term amphibolic ( Ancient Greek: ἀμφί, romanizedamphi, lit. 'both sides')[1] is used to describe a biochemical pathway that involves both catabolism and [2] and anabolism[3]. Catabolism is a degradative phase of metabolism in which large molecule are converted into smaller and simpler molecule, which involves two types of reactions. First, hydrolysis reactions, in which catabolism is the breaking apart of molecules to smaller molecules to release energy. An example of a catabolic reaction is digestion and cellular respiration, where you break apart sugars and fats for energy. Hydrolysis is how this is done and it is approximately the reverse of a dehydration reaction. Breaking down a protein into amino acids or a triglyceride into fatty acids or a disaccharide into monosaccharides are all hydrolysis or catabolic reactions. Second, oxidation reactions involve the removal of hydrogens and electrons from an organic molecule. [4][5] Anabolism is the biosynthesis phase of metabolism in which smaller simple precursor are converted to large and complex molecule of the cell. Anabolism has two classes of reactions, which are dehydration synthesis reaction, this type involves the joining of smaller molecules together to form larger, more complex molecules. This occurs through dehydration synthesis reactions. These are the most common ways smaller organic molecules can be formed into more complex ones and applies to the formation of carbs, proteins, lipids and nucleic acids. Other type called reduction reaction, which involves the adding of hydrogens and electrons to a molecule. Whenever you do that, it gains calories of energy because when you split a hydrocarbon bond, it releases energy).[2]

This term was proposed by B.Davis in 1961 to emphasise the dual metabolic role of such pathway.[6] These pathway consider to be central metabolic pathways which provide, from catabolic sequences, the intermediates which form the substrate of the metabolic processes.[7]

Contents

Reactions exist as amphibolic pathwayEdit

All the reactions associated with synthesis of biomolecule converge into the following pathway, viz, Glycolysis , Krebs cycle and electron transport chain ,exist as amphibolic pathway meaning that they can function anabolically as well as catabolically.

Other important amphibolic pathway are :Embden – Meyerhof pathway , Pentose phosphate pathway and Entner -Doudoroff pathway[7]

Embden- MeyerhoffEdit

The Embeden – Meyerhof pathway along with Krebs cycle are the centre of metabolism in nearly all bacteria and eukaryotes, they do not only provide energy they also provide precursors for biosynthesis of macromolecules that make up living system[7]

Citric acid cycleEdit

The citric acid cycle (The Krebs Cycle) is a good example of an amphibolic pathway because it functions in both the degradative (carbohydrate, protein, and fatty acid) and biosynthetic processes.[2] The cycle occurs on the cytosol of bacteria and within the mitochondria of Eukaryotic cells. The citric acid cycle provides electrons to the electron transport chain which is used to drive the production of ATP in oxidative phosphorylation. Intermediates of the citric acid cycle, such as oxaloacetate, are used to synthesize macromolecule constituents such as amino acids e.g. glutamate and aspartate.[8]

The first reaction of the cycle, in which oxaloacetate (a four carbon compound) condenses with acetate (a two carbon compound) to form citrate (a six carbon compound) is typically anabolic. The next few reactions, which are intramolecular rearrangements, produce isocitrate. The following two reactions, namely the conversion of D-isocitrate to α-Ketoglutarate followed by its conversion to Succinyl-CoA, are typically catabolic. COO is lost in each step and succinate (a four carbon compound) is produced.

There is an interesting and critical difference in the coenzymes used in catabolic and anabolic pathways; in catabolism NAD+ serves as an oxidizing agent when it is reduced to NADH. Whereas in anabolism the coenzyme NADPH serves as the reducing agent and is converted to its oxidized form NADP+.

Citric acid cycle play two mode that play two roles, the first role is energy production that is produced by oxidative mode, as the acetyl group of acetyl -COA is fully oxidized to CO2, this mode produce most of the ATP in the metabolism of aerobic heterotrophic metabolism, as this energy conversion in the membrane structure (cytoplasmic membrane in bacteria and mitochondria in eukaryotes ) by oxidative phosphorylation by moving electron from donor (NADH and FADH2) to the acceptor O2 .Every cycle give 3 NADH, 1 FADH2, CO2 and GTP .The second role is biosynthetic, as citric acid cycle regenerate oxaloacetate when cycle intermediate are removed for biosynthesis[9]

Pentose phosphate pathwayEdit

The Pentose phosphate pathway get its name because it involve several intermediate that are phosphorylated five carbon sugars (pentoses). The pentose phosphate pathway provide monomers for many metabolic pathways by transform glucose into four carbon sugar ( erythrose) and five carbon sugar ( ribose), that are important monomers in many metabolic pathways .Many of the reactants in the pentose phosphate pathway are those similar in glycolysis, also both occurs in cytosol .[10][11] The ribose -5-phosphate can be transport into nucleic acid metabolism, producing the basis of DNA and RNA monomers, the nucleotides . In meristematic cells, large amounts of DNA must be produce during the S-phase of a short cell cycle, the pentose phosphate pathway is an extremely important part of the metabolism of these cells. In these cell, the pentose phosphate pathway is active and shifted in favor of ribose production .[11]

Entner- Doudoroff pathwayEdit

Entner-Doudoroff pathway is a glycolytic pathway that is considered the second pathway used for carbohy used by certain microbes. In this process, Glucose6-phosphate is oxidize through 6-phosphogluconate to pyruvate and glyceraldehyde 3-phosphate, with the concomitant reduction of NADP. By conventional glyceraldehyde3-phosphate oxidation to pyruvate, one NAD is reduced and a net one ATP is formed In that pathway, as for every glucose molecule there is investment of one ATP molecule and a yield of 2 ATP and 2 pyruvate molecules also 1 NADH .This show the difference between the glycolytic used by humans and this pathway,that in this pathway, it require an investment of one ATP to yield a 2ATP and 2 pyruvate as a net of only one NADPH produced and one ATP result (from substrate level phosphorylation ), and the other pathway involve an investment of 2 ATP molecules to yield 4 ATP and 2 pyruvate molecules per glucose as a net of 2 ATP molecule .[12][13]

RegulationEdit

The cell determines whether the amphibolic pathway will function as an anabolic or catabolic pathway by enzyme –mediated regulation at a transcriptional and post transcriptional level . As many reactions in amphibolic pathways are freely reversible or can be bypassed, irreversible steps that facilitate their dual function are necessary. The pathway will use a different enzyme for each direction for the irreversible step in the pathway, allowing independent regulation of catabolism and anabolism . Due their inherent duality, amphibolic pathways represent the regulation modes of both anabolic by its negative feedback end product and catabolic by feedback by energy indicator sequences .[7]

ReferencesEdit

  1. ^ "the definition of amphi-". Dictionary.com. Retrieved 2018-05-21.
  2. ^ a b c Amabye, Teklit Gebregiorgis. Biochemistry for college students. Lulu.com. ISBN 9781329546264.
  3. ^ "Amphibolic Pathway".
  4. ^ "Lehninger's Principles of Biochemistry", 4th edition, pp. 616, 2004.
  5. ^ "Voet's Biochemistry", 2nd edition, pp. 538, 1995.
  6. ^ Shen, Laura; Fall, Lana; Walton, Gordon; Atkinson, Daniel (1968). "Interaction between energy charge and metabolite modulation in the regulation of enzymes of amphibolic sequences. Phosphofructokinase and pyruvate dehydrogenase". Biochemistry. 7 (11): 4041–4045. doi:10.1021/bi00851a035.
  7. ^ a b c d Pandey, Dr P. S. Verma & Dr B. P. ISC Biology Book I for Class XI. S. Chand Publishing.
  8. ^ "tricarboxylic acid cycle". Academic Dictionaries and Encyclopedias. Retrieved 2018-05-21.
  9. ^ Jones, Trevor; Vandecasteele, Jean-Paul. Petroleum Microbiology. Editions OPHRYS. ISBN 9782710811350.
  10. ^ Mauseth, James D. (2003). Botany: An Introduction to Plant Biology. Jones & Bartlett Learning. ISBN 9780763721343.
  11. ^ a b Mauseth, James D. (2003). Botany: An Introduction to Plant Biology. Jones & Bartlett Learning. ISBN 9780763721343.
  12. ^ Lengeler, Joseph W.; Drews, Gerhart; Schlegel, Hans Günter (1999). Biology of the Prokaryotes. Georg Thieme Verlag. ISBN 9783131084118.
  13. ^ "Entner-Doudoroff Pathway - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2018-05-10.