Why Can’t Animals Use All the Same Energy Sources as Plants and Bacteria?

Why Can’t Animals Use All the Same Energy Sources as Plants and Bacteria?

The citric acid cycle is important not only as a source of energy during aerobic metabolism but also as a key pathway in the synthesis of important metabolic intermediates. We shall see in subsequent that it is a source of starting materials for the production of amino acids, carbohydrates, vitamins, nucleotides, and heme. However, if these intermediates are used for the synthesis of other molecules, then they must be replenished to maintain the catalytic nature of this cycle. The term anaplerotic means “filling up,” and the reactions that replenish the citric acid cycle are called anaplerotic reactions. One source of needed compounds, available to all organisms, is the group of amino acids that can be converted to citric acid cycle intermediates in a single reaction. A simple reaction available to all organisms is to add carbon dioxide to the pyruvate and phosphoenolpyruvate generated from metabolism of sugars. Another source, important in bacteria and plants, is the glyoxylate cycle. This source is vital to the ability of plants to fix carbon dioxide to carbohydrates.

Some anaerobic organisms have developed only parts of the citric acid cycle, which they use exclusively to make the important precursors. These simple yet important reactions emphasize the truly connected nature of what we often artificially separate into “pathways.” They also illustrate the convergence of evolution to a few key molecules and metabolic steps.

Which molecule is arguably the most important metabolic intermediate? Acetyl-CoA is perhaps the central molecule of metabolism. When one plots a chart of all known metabolic path-ways, acetyl-CoA ends up close to the center of that chart.

The reasons are quite simple. This important compound really links the metabolism of the three major classes of nutrients to each other. All sugars, all fatty acids, and many amino acids pass through acetyl-CoA on their way to becoming water and carbon dioxide. Equally important is the key use of this intermediate in the synthesis of the major biomolecules. Some, but not all, organisms can carry out all these conversions. Bacteria provide an example of organisms that can do so, whereas humans are an example of ones that cannot. Many bacteria can live off acetic acid as their sole carbon source; however, it is first converted to acetyl-CoA. Acetyl-CoA is converted to fatty acids, terpenes, and steroids. More important is the conversion of two molecules of acetyl-CoA to malate in plants and bacteria via the glyoxylate pathway. This key compound is the starting point for the synthesis of both amino acids and carbohydrates.