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Unit 23: Amino Acids, Peptides, and Proteins

A molecule with both a positive and a negative charge at once, linked to its neighbors by the same amide bond from the carboxylic acid unit.

5 topics · 17 questions

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Amino Acid Structure and the Zwitterion

  1. How it worksHow does the carboxylic acid's proton transfer onto the amine's nitrogen to form a zwitterion with no net motion of the molecule?
  2. Worked exampleFind the net charge on glycine's zwitterion at physiological pH, roughly 7.4.
  3. Common mistakeWhy isn't a zwitterion the same thing as a molecule that's simply been fully deprotonated or fully protonated?
Your turn1 of 2
What is the net charge on glycine's zwitterion at physiological pH, roughly 7.4?

Isoelectric Point and Electrophoresis

  1. How it worksHow does the balance between the carboxylic acid's pKa and the amine's pKa set the exact pH where net charge hits zero?
  2. Worked exampleFind the isoelectric point of glycine using its two pKa values, 2.3 and 9.6.
  3. Going furtherWhat happens to an amino acid's isoelectric point when its side chain carries an extra acidic or basic group, like aspartate?
Your turn1 of 2
An amino acid has a carboxylic acid pKa of 2.496 and an amine pKa of 9.99. Using pI = (pKa1 + pKa2)/2, what is its isoelectric point?

Peptide Bond Formation

  1. How it worksHow does one amino acid's carboxylic acid condense with another's amine, losing water to form the new amide peptide bond?
  2. Worked exampleFind the dipeptide formed when the carboxylic acid of alanine reacts with the amine of glycine, forming one new amide bond.
  3. Common mistakeWhy doesn't a peptide bond rotate freely the way an ordinary carbon-carbon single bond does?
  4. Ties togetherHow does this peptide bond reuse the exact nucleophilic acyl substitution mechanism from the carboxylic acid derivatives unit?
Your turn1 of 2
The carboxylic acid of alanine reacts with the amine of glycine, forming one new amide bond. What is the resulting dipeptide?

Primary Through Quaternary Protein Structure

  1. How it worksHow does hydrogen bonding along a protein's backbone fold a plain amino acid sequence into an alpha helix or a beta sheet?
  2. Worked exampleFind which level of protein structure describes hemoglobin's four separate polypeptide chains coming together as one unit.
  3. Going furtherWhat happens to a protein's tertiary structure when a disulfide bond forms between two cysteine side chains far apart in sequence?
Your turn1 of 2
Which level of protein structure describes hemoglobin's four separate polypeptide chains coming together as one functional unit?

Protein Folding and Denaturation

  1. How it worksHow does heat or extreme pH disrupt the hydrogen bonds and other weak interactions holding a protein's folded shape together?
  2. Worked exampleFind which types of interactions, covalent or noncovalent, actually break when a protein denatures under heat.
  3. Common mistakeWhy doesn't denaturing a protein break any of its actual peptide bonds running along the backbone?
  4. Going furtherWhat happens to some denatured proteins when the denaturing condition is removed and the protein is allowed to cool slowly?
Your turn1 of 2
What types of interactions, covalent or noncovalent, actually break when a protein denatures under heat?