A LITTLE LIGHTBULB MOMENT
Amino acids, proteins and enzymes
Zwitterion and pI
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The amino acid wears both charge stickers at once
The idea behind the cue
In water, amino acids often exist as zwitterions: the amino group is protonated and the carboxyl group is deprotonated. At the isoelectric point, the net charge is zero. This matters for solubility and electrophoretic behavior.
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Short & simple
The amino acid wears both charge stickers at once
Acronym
ZPCI — Zany Pancakes Chase Icebergs.
Story
At a costume party, one sticker says plus on the amino group and another says minus on the carboxyl group. The molecule looks embarrassed, but the total bill balances to zero.
Connect the pieces
- Z (Zany) → Zwitterion: In water, amino acids often carry both a positive and a negative charge at once.
- P (Pancakes) → Protonated amino: The amino group is usually protonated in water, giving it a positive charge.
- C (Chase) → Carboxylate: The carboxyl group is usually deprotonated in water, giving it a negative charge.
- I (Icebergs) → Isoelectric point: At the pI, the net charge is zero; charged side chains can shift the pI.
- Zany · Zwitterion: In water, amino acids often carry both a positive and a negative charge at once.
- Pancakes · Protonated amino: The amino group is usually protonated in water, giving it a positive charge.
- Chase · Carboxylate: The carboxyl group is usually deprotonated in water, giving it a negative charge.
- Icebergs · Isoelectric point: At the pI, the net charge is zero; charged side chains can shift the pI.
Know the limits
Net charge depends on side-chain ionization too; charged R groups shift pI.
Further reading · OpenStax science textbooks ↗Check your understanding
What best describes an amino acid at its isoelectric point? A. It has no ionic groups at all B. Its net charge is zero C. It must be fully protonated D. It must be a nonpolar amino acid
A. It must be fully protonated
B. It must be a nonpolar amino acid
C. It has no ionic groups at all
D. Its net charge is zero
Reveal explanation
Its net charge is zero
At the isoelectric point, positive and negative charges balance so the molecule’s net charge is zero, even though internal charges can still be present.