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Isoelectric Point of Amino Acids with Ionizable R-Groups quiz

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  • Why is calculating the isoelectric point (pI) of amino acids with ionizable R groups more challenging than for those with non-ionizable R groups?
    Amino acids with ionizable R groups have three pKa values, making it harder to determine which two to average for the pI calculation.
  • How many pKa values do amino acids with ionizable R groups have?
    They have three pKa values: one for the amino group, one for the carboxyl group, and one for the R group.
  • What mnemonic helps identify the seven amino acids with ionizable R groups?
    The mnemonic is 'yucky crazy dragons eat knights riding horses.'
  • What is the first step in calculating the pI of amino acids with ionizable R groups?
    Familiarize yourself with the mnemonic to identify the amino acids and their charge states when ionized.
  • What is the second step in calculating the pI for these amino acids?
    Order the three pKa values from smallest to largest.
  • What is the third step in determining the pI of amino acids with ionizable R groups?
    Determine the net charge of the predominant amino acid structure at any pH between adjacent pKa values.
  • Which two pKa values are averaged to calculate the pI for amino acids with ionizable R groups?
    The middle pKa and the pKa that gives a neutral net charge are averaged.
  • For tyrosine, what is the nature of its ionizable R group when ionized?
    Tyrosine's R group has a negative charge when ionized.
  • What are the typical pKa ranges for the alpha amino and carboxyl groups?
    Alpha amino groups have pKa values around 9–10.5, and carboxyl groups around 2.
  • How do you determine the predominant structure at a given pH between two pKa values?
    Compare the pH to each pKa; if pH < pKa, the conjugate acid form predominates, and if pH > pKa, the conjugate base form predominates.
  • What is the net charge of tyrosine at pH 5, between its carboxyl and amino group pKa values?
    The net charge is zero because the positive charge of the amino group cancels the negative charge of the carboxyl group, and the R group is neutral.
  • What happens to the amino group when the pH is greater than its pKa?
    The amino group is in its conjugate base form (NH2), which is neutral.
  • What is the net charge of tyrosine at pH 9.5, between its amino and R group pKa values?
    The net charge is negative one because the amino group is neutral, the carboxyl group is negative, and the R group is neutral.
  • How is the isoelectric point (pI) of tyrosine calculated?
    By averaging the pKa of the carboxyl group (2.2) and the amino group (9.1), resulting in a pI of 5.65.
  • Why is it important to identify the region where the net charge is zero when calculating pI?
    Because the pI is the pH at which the amino acid has no net charge, so only the pKa values sandwiching this region are averaged.