Enzyme Kinetics in Ordinary Life

From the New England Journal of Medicine:

A 68-year-old man presented with unilateral ptosis [i.e., one droopy eyelid] and no other symptoms […] Myasthenia gravis was suspected, and the ice-pack test was performed with the placement of an instant cold pack over the left eye. After 2 minutes, the ptosis was substantially diminished (>5 mm), indicating a positive test […] The inhibition of acetylcholinesterase activity at a reduced muscle temperature is thought to underlie the observed clinical improvement.

A bit of background:

  • Neurons secrete a neurotransmitter called acetylcholine onto muscle cells which binds to the muscle cells’ acetylcholine receptors. This leads to muscle contraction and is the way neurons cause muscles to contract. Acetylcholinesterase is an enzyme present in the junction between neuron and muscle cell, which speeds up the breakdown of acetylcholine so that its effect on the muscle isn’t too long-lasting.
  • In the disease known as myasthenia gravis, the body produces antibodies which bind to the muscle’s acetylcholine receptors, blocking acetylcholine from binding. The result is muscle weakness (one manifestation is droopy eyelids), as the nerves’ ability to stimulate muscle contraction is diminished.
  • Enzymes, including acetylcholinesterase, work less efficiently at lower temperatures, because at lower temperature molecules move slower, so are less likely to bump into the enzyme, and they are less likely to bump into the enzyme with sufficient energy to bring the reaction to completion.
  • One way to treat myasthenia gravis is by administering a drug which inhibits acetylcholinesterase, so that it cannot break down acetylcholine as quick, therefore leaving acetylcholine to act for longer. Now, decreasing temperature also has the same effect, which is why myasthenia gravis is alleviated in the presence of cold. (Note: inhibition of acetylcholinesterase is not the only reason for low temperature helping myasthenia gravis – there’s a couple others – just one major one).

I’m sharing this because I was really impressed by a basic principle of biochemistry (that low temperature reduces enzyme efficiency) having such a simple visual demonstration.

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