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chaeyeon chaeyeon
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6 years ago
In 1918, an epidemic of sleeping sickness caused an unusual rigid paralysis in some survivors, similar to symptoms of advanced Parkinson’s disease. Years later, L-dopa (below, left), a chemical used to treat Parkinson’s disease, was given to some of these patients, as dramatized in the movie Awakenings, starring Robin Williams. L-dopa was remarkably effective at eliminating the paralysis, at least temporarily. However, its enantiomer, D-dopa (right), was subsequently shown to have no effect at all, as is the case for Parkinson’s disease. In a short essay (100–150 words), discuss how the effectiveness of one enantiomer and not the other illustrates the theme of structure and function.
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6 years ago
This is because the paralysis mentioned occurs due to a protein receptor not attaching to dopamine strongly. Proteins are L-enantiomeric (i.e. all amino-acids are L-enantiomers) leading to an L-enantiomeric tertiary structure. Because of this, it only responds to L-Dopa because D-Dopa lacks the specific chirality needed to engage the active site of the protein receptor implicated in the sleeping sickness. It must be remembered that proteins are very specific with their active sites and cannot adapt themselves readily to changes in chirality.
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6 years ago
A racemate or racemic mixture includes both enantiomers of a compound that are present in equal amounts (50:50 ratio). These two compounds that are enantiomers of each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. As a result, different enantiomers of a compound may have substantially different biological effects.

Enantiomers are two molecules that are identical in composition and connectivity. They both have the same molecular formula and the same molecules connected to each other--but one is a mirror image of the other. It's like your right hand vs. your left hand. Same fingers, connected in the same way, but they're not actually identical. They're reversed in space.

This becomes important because many biological enzymes and receptors are like gloves--they fit a molecule as precisely as a glove. And you can't fit a left hand into a right-handed glove. So even though they consist of the same atoms connected in the same way, enantiomers may have very different effects on the body--because of their differing structure.

A prime example of this is the way enantiomers can smell drastically different from each other. The right- versus right-handed version of a compound called limonene, for example, smells fresh and like oranges, while the left-handed version smells harsh and lemony. This is because the receptors in our nose fit the limonene molecules like a glove--and different receptors fit the right and left-handed versions.
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