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ILLUMINATIONS

ANALOGY FOR EXPLAINING INTERMEDIATE METABOLISM

Molecular Physiology Unit, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, Tlalpan 14000, Mexico City, Mexico E-mail: gamba{at}sni.conacyt.mx

This Illumination describes an analogy for explaining intermediate metabolism by comparing glucose with the monthly paycheck. Intermediate metabolism is depicted in Fig. 1A and its analogy in Fig. 1B. The body’s major energy source is carbohydrates. Glucose (Fig. 1A) is the most important monosaccharide. Similarly, the major income for most people is a monthly paycheck (Fig. 1B). Glucose can be used in several different ways, but it must first be converted to acetyl-CoA, the most important intermediate metabolite. Acetyl-CoA is the key molecule that allows an interchange between several biochemical pathways. In a similar way, a paycheck can be used in several ways, but it must be converted to currency, the key "metabolite" in our economy. Acetyl-CoA can enter the tricarboxylic acid (Krebs) cycle, generating NADH and FADH, which are used in oxidative phosphorylation to form ATP. Thus, to make energy, glucose is transformed into acetyl-CoA and traded for ATP. Similarly, a paycheck is transformed into currency, which "enters" into the shopping center and is ultimately traded for goods.

View larger version (15K): [in this window] [in a new window]   FIG. 1 A: intermediate metabolism. B: analogy.

When food consumption is consistently higher than required, glycogen stores are saturated, and glucose is then stored as fat. For this purpose, acetyl-CoA is converted into fat for storage. When necessary, fat may release fatty acids that are oxidized to form acetyl-CoA by the ß-oxidation pathway. In the analogy, when people have excess money, long-term savings is in order. This is a way to keep money safe for a long time; however, it is also more difficult to access it. An example is investments such a buying gold. When people gain weight, it is because food consumption is greater than energy expenditure, just as when people save money, it is because their income is greater than their expenses. To lose weight, the individual must decrease consumption (diet), increase expenditure (exercise), or both, just as the reduction in savings can be due to a decreased income, increased expenses, or both.

Note that, in Fig. 1A, another possibility for acetyl-CoA is to be converted into ketone bodies that might be released into the circulation to be used by other tissues for energy (especially by heart and kidney). Similarly, we can purchase money orders to send currency to friends and family. Ethanol is another source of energy. During the metabolism of ethanol, NADH is accumulated and used to produce ATP. For every ethanol molecule that is metabolized, one glucose molecule is no longer necessary and thus will be stored. In the analogy, ethanol represents food stamps that are good only to purchase food. Sometimes, in addition to a paycheck, people receive food stamps. Although food stamps cannot be converted to currency, every time a stamp is used to buy food there is currency that is not spent and thus will be stored.

Finally, if it is necessary, proteins can be degraded to amino acid residues that are converted to acetyl-CoA, which in turn can be used to produce energy or glucose (gluconeogenesis). This is, however, not a good idea for the body (catabolism). In the analogy, proteins represent a house or business, the most solid way to store money for a long time. If some day your income is seriously reduced, selling your home or business will produce the money that is needed for living. As with proteins, however, on the long-term basis this is not a good idea.

Students enjoy this analogy, which produces a clear picture of intermediate metabolism. I found this analogy also useful for the general public to assist them in understanding the mechanisms by which people gain or lose weight and how different diets work.

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