3.5: Chemical Reactions
- Page ID
- 63594
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One substance is made from another by making and breaking covalent bonds. Ordinarily, these chemical reactions are extremely slow. The graphite in a pencil won’t become a diamond no matter how long it sits on your desk. Labs and kitchens use heat, pressure, and changes in acidity to speed up chemical reactions. But in living things, severe changes in heat, pressure, and acidity are fatal.
Using catalysts is another way to speed up chemical reactions. Enzymes are biological catalysts that accelerate chemical reactions that occur in living things. Without enzymes, most reactions necessary for life would occur too slowly. Enzymes increase the speed of biochemical reactions by factors of about a million to ten billion. In other words, a reaction that would spontaneously occur once in 1000 years could occur in 1 second with the aid of an enzyme.
Enzymes are very specialized. Each chemical reaction is catalyzed by a specific enzyme. Thousands of enzymes catalyze the thousands of chemical reactions that occur in the human body.
We see the importance of even a single enzyme in the disease phenylketonuria (PKU), which occurs in about 1 of every 13,000 newborns in the U.S. It’s one of many genetic diseases caused by a lack of a single enzyme, in this case a severe deficiency of an enzyme needed to make a simple change in the structure of the amino acid phenylalanine (Figure 3.3). Lacking this enzyme, phenylalanine can’t be processed normally. If the disease isn’t diagnosed and treated immediately, this amino acid accumulates in large amounts, causing severe mental disability.
Newborns in the U.S. are routinely screened for PKU so that affected babies can be diagnosed and treated within the first few days of life. Before birth, the necessary enzyme is provided by the mother. Phenylalanine (found in all proteins) is an essential nutrient. Newborns found to have PKU are immediately taken off milk and put on a special infant formula that has minimal amounts of phenylalanine (barely enough for growth and development) to prevent the abnormal buildup that causes brain damage.
Some people with PKU stay on a low-phenylalanine diet throughout their life. NutraSweet (aspartame) contains phenylalanine; products sweetened with this artificial sweetener are labeled with a warning: Phenylketonurics: contains phenylalanine.