Consider a protein in which a negatively charged glutamic acid side chain (pKa=4.2) makes a salt bridge (ion-ion interaction) with a positively charged histidine side chain (pKa=6.5). Part C: Justify your answer with calculations of partial charges on these amino acid side chains at pH=7.9.(Hint: Consider lessons from Coulomb's law, and the Henderson-Hasselbalch equation.)
Consider a protein in which a negatively charged glutamic acid side chain (pKa=4.2) makes a salt bridge (ion-ion interaction) with a positively charged histidine side chain (pKa=6.5).
Part C: Justify your answer with calculations of partial charges on these amino acid side chains at pH=7.9.(Hint: Consider lessons from Coulomb's law, and the Henderson-Hasselbalch equation.)
Salt bridges are attractive interactions between two oppositely charged groups. So, more oppositely charged the two groups are, more strong the salt bridge will be.
If the negatively charged group is extremely negatively charged and the positively charged group is extremely positively charged, then the salt bridge will be formed very strongly.
But, if the intensity of the negative and positive charges decreases (i.e. the magnitude of the partial charges decreases), then the salt bridge becomes weaker.
The fraction of amino acid side chain that exists in its protonated and deprotonated form changes with change in pH. As, the pH goes above the pKa of an ionizable group, the group exists predominantly (more than 50%) in its deprotonated state. When the pH is below the pKa of an ionizable group, the group exists predominantly in its protonated state.
When the pH is equal to the pKa of the ionizable group, half of all the molecules will have the group in protonated state and the rest half in deprotonated state.
When the pH is greater or less than the pKa by 2 degrees or more, the ionization will be at least 99% complete. For example, consider the side chain of glutamic acid which has a pKa of 4.2. At pH values 6.2 and above, more than 99% of all glutamic acid will exist in its deprotonated state. At pH values 2.2 and below, more than 99% of all glutamic acid will exist in its protonated state.
Step by step
Solved in 4 steps with 5 images
