Chemistry: Life at the Molecular Level

In metabolism, complex molecules are degraded into simpler products including amino acids, glucose, and fatty acids. These simpler molecules can subsequently be broken down into the Acetyl CoA intermediate (Voet, D., Voet, J., Pratt, C. 2006. p. 397). Acetyl CoA then enters the citric acid cycle (TCA cycle), and is oxidized into carbon dioxide, CO2. During the TCA cycle, NAD+ and FADH are reduced to produce high transfer potential electrons, NADH and FADH2. These NADH and FADH2 molecules are oxidized during oxidation phosphorylation and the electron transport chain and generate water, H2O and ATP (Voet et al. 2006. p. 397).
Intermediates formed from the citric acid cycle are important precursors and building blocks for producing important materials in an organism. These intermediates are drained from the TCA cycle in cataplerotic reactions to synthesize important products such as glucose, fatty acids, and amino acids. For example, gluconeogenesis, the synthesis of glucose, requires oxaloacetate that has been converted to malate, while fatty acid biosynthesis utilizes acetyl CoA, and amino acid biosynthesis utilizes oxaloacetate and α -ketoglutarate (Tymoczko, J. L., Berg, J. M., & Stryer, L. 2013. p. 339).
During the TCA cycle, pyruvate is oxidized to acetyl CoA, which undergoes a condensation reaction catalyzed by citrate synthase to form citrate. Citrate can then be isomerized to form isocitrate, which undergoes oxidative carboxylation that is catalyzed by isocitrate dehydrogenase, to form α -ketoglutarate. Succinyl CoA is then formed from the decarboxylation/oxidation of α -ketoglutarate, which is catalyzed by α -ketoglutarate dehydrogenase. Succinyl CoA can be used to form products including chlorophyll, heme, and prophyr...

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...409). In order to synthesize a sugar from the hexose monophosphate pool, two molecules of dihydroxyacetone phosphate (DHAP) are also required. Six rounds of the Calvin cycle must occur to synthesize one hexose sugar. For each CO2 molecule, three ATP and two NADPH are used in converting the CO2 into a hexose.
Overall, 12 ATP are used to phosphorylate 12 molecules of 3-phosphoglycerate into 1,3-bisphosphoglycerate (1,3-BPG). 12 NADH are subsequently used to reduce the 12 molecules of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate (Tymoczko et al. 2013. p. 410-412).

Works Cited
Tymoczko, J. L., Berg, J. M., & Stryer, L. (2013). Biochemistry: A Short Course, 2nd Edition. New York, NY: W. H. Freeman and Co.

Voet, D., Voet, J., & Pratt, C. (2006). Fundamentals of Biochemistry: Life at the Molecular Level, 2nd edition. Hoboken, NJ: John Wiley & Sons, Inc.