Amphibolic
The term amphibolism (
The term amphibolic was proposed by B. Davis in 1961 to emphasise the dual metabolic role of such pathways.[6] These pathways are considered to be central metabolic pathways which provide, from catabolic sequences, the intermediates which form the substrate of the metabolic processes.[7]
Reactions exist as amphibolic pathway
All the reactions associated with synthesis of biomolecule converge into the following pathway, viz., glycolysis, the Krebs cycle and the electron transport chain, exist as an amphibolic pathway, meaning that they can function anabolically as well as catabolically.[clarification needed]
Other important amphibolic pathways are the
Embden-Meyerhoff
The Embeden–Meyerhof pathway and the Krebs cycle are the centre of metabolism in nearly all bacteria and eukaryotes. They provide not only energy but also precursors for biosynthesis of macromolecules that make up living systems.[7]
Citric acid cycle
The
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
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 has two modes that play two roles, the first being energy production produced by the oxidative mode, as the acetyl group of acetyl-coA is fully oxidized to CO2. This produces 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 intermediates are removed for biosynthesis.[9]
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Figure 1: Citric acid cycle
Pentose phosphate pathway
The pentose phosphate pathway gets its name because it involves several intermediates that are phosphorylated five-carbon sugars (
Entner-Doudoroff pathway
The Entner-Doudoroff pathway is a glycolytic pathway that is considered the second pathway used for carbohydrates used by certain microbes. In this process, glucose-6-phosphate is oxidized through 6-phosphogluconate to pyruvate and glyceraldehyde 3-phosphate, with the concomitant reduction of NADP. By conventional glyceraldehyde-3-phosphate oxidation to pyruvate, one NAD is reduced and a net one[clarification needed] ATP is formed. In that pathway, for every glucose molecule there is an "investment" of one ATP molecule and a yield of two ATP and two pyruvate molecules and one NADH. The difference between the glycolytic[word missing] used by humans and this pathway is that the latter requires one ATP to yield two ATP and two pyruvates as a net of only one NADPH produced and one ATP result (from substrate-level phosphorylation), and the former requires two ATP molecules to yield four ATP and two pyruvate molecules per glucose as a net of two ATP molecules.[11]
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Figure 2: Entner- Doudoroff pathway
Regulation
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 uses 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]
References
- ^ Chambers Dictionary,11th edition; Liddell & Scott lexicon, 1963
- ^ ISBN 9781329546264.
- ^ "Amphibolic Pathway". Archived from the original on 2018-08-27.
- ^ "Lehninger's Principles of Biochemistry", 4th edition, pp. 616, 2004.
- ^ "Voet's Biochemistry", 2nd edition, pp. 538, 1995.
- PMID 4301881.
- ^ a b c d Pandey, Dr P. S. Verma & Dr B. P. ISC Biology Book I for Class XI. S. Chand Publishing.
- ^ "tricarboxylic acid cycle". Academic Dictionaries and Encyclopedias. Retrieved 2018-05-21.
- ISBN 9782710811350.
- ^ ISBN 9780763721343.
- ISBN 9783131084118.