Terminators: Wordplays for Ideas & Insights
Galactic Interplay
Lactose is milk sugar. It's a disaccharide composed of two linked
sugars: glucose and galactose. "Lactose" is derived from the '-ose'
ending meaning "sugar," and the Latin 'lac' meaning milk, from the
Greek 'gala,' also meaning milk. Gala is also the source of the word
"galaxy" which literally means "The Milky Way." Originally "galaxy"
meant only the swirling cluster of stars we live in. Now galaxy also
means any such cluster.
Now shift from the Milky Way to the molecular. The lac operon is a
classic model system for gene regulation and expression. The lac
operon of E. coli has a promoter followed by three structural genes
that encode for three proteins: a B-galactosidase, a lactose
permease, and a transacetylase. The galactosidase catalyzes the
splitting of lactose into glucose and galactose. Bacteriologists
observed that E. Coli fed both lactose and glucose at first use only
the glucose. But when all the glucose is gone, the bacterium starts
using the lactose. Using the lactose involves turning on
(derepressing) a set of linked genes--the lac operon. Making
B-galactosidase allows the bacterium to convert lactose to glucose and
galactose.
Why isn't the enzyme called "lactase" instead of "galactosidase"?
Here it's helpful to note the difference between "galactose" and
"galactoside." Galactose is a six-carbon sugar. A galactoside is any
compound such as lactose that when hydrolyzed yields a galactose
molecule. So a galactosidase is an enzyme that catalyzes the splitting
off of galactose from a larger compound. In this case, the larger
compound is lactose. And in this case, if the galactosidase acts on
lactose, it is also then a lactase.
Some adults lack lactase. This can lead to "milk intolerance." The
main symptoms include gas and flatulence. One approach to managing
milk intolerance is to avoid dairy foods. Another approach is to get
extra lactase. Lactase is the active enzyme in products such as
"Lactaid" that may be used by people who are lactose-intolerant.
Lactase can be added to milk or it can be swallowed by the person.
The idea is to convert lactose to glucose and galactose early in
digestion. Without the enzyme, the lactose passes into the large
intestine where bacteria digest it and make gas.
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lac operon
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