0195369637.pdf by Unknown

Author:Unknown
Language: eng
Format: mobi
ISBN: 0195369637
Published: 2009-08-12T08:50:12+00:00

CARBOHYDRATES

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Figure 16.1 A schematic representation illustrating the chemical reactions that interconvert small molecules in cells. Each dot represents a compound and the lines that connect them represent reactions. The heavy dots and lines down the center of the maze reflect the glycolytic pathway and the heavy circle near the bottom of the maze reflects the citric acid cycle. (© 2007

From Molecular Biology of the Cell, 5th Edition, by Alberts et al. Reproduced by permission of Garland Science/Taylor & Francis, LLC.)

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THE TAO OF CHEMISTRY AND LIFE

the next chapter. The circular reactions shown in boldface near the bottom of the figure constitute the citric acid cycle, also considered in the next chapter. These are arrays of enzyme-catalyzed chemical reactions that bring about major transformations of organic compounds of importance to the life of an organism. The compounds that are encountered along these metabolic pathways are termed metabolites.

To summarize these general considerations: the metabolism of a cell or organism is a complex and carefully controlled set of interconnected chemical reactions, each under the control of a specific enzyme, that are organized into pathways. We begin the next chapter with a description of a catabolic pathway that degrades glucose and generates energy in the form of ATP.

Not all sugars are sweet; not all sweets are sugars One of the consistent messages of this book is that biological properties of molecules are a sensitive function of molecular structure. Although molecular properties in biological systems generally change with structure in an orderly way, small structural changes can result in major property changes. So it is with sugars. When we think of sugars, we usually think of the property of sweetness. We have sucrose on the table to add sweetness to coffee, breakfast cereal, or whatever. The sweetness of honey comes from its high content of the sugar fructose. However, not all sugars are sweet.

And there are molecules that are very sweet which are not sugars. The molecular basis of taste is developed in chapter 25.

Let’s look at several examples of the sensitivity of taste to molecular structure.

d-glucose is quite sweet; in contrast, its enantiomer (nonsuperimposable mirror image) l-glucose is salty. The stereochemistry of d-glucose and l-glucose is different at four carbon atoms. Even more striking is the observation that -d-mannose is sweet but -d-mannose is bitter. These two isomers differ in stereochemistry at only one carbon atom. This simple, single change has altered their affinity for the receptors for sweet and bitter.

Similarly, d-isoleucine is sweet but l-isoleucine is bitter. Both d-leucine and d-valine are sweet but their l isomers are tasteless to bitter.

We generally associate saltiness with salts. But not all salts are salty. For example, most salts of lead are sweet. The Romans were accustomed to adding lead salts to their wine to add sweetness. Unhappily, lead salts are also toxic. To what extent lead poisoning from their wines contributed to the downfall of the Roman Empire is not known. But it certainly did nothing positive for the health of the Romans of the time.

Finally, many compounds that are neither sugars nor salts are sweet.

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