Amino Acid PI (isoelectric point) vs pKa values

[GomerPyle]

Every amino acid has an isoelectric point (PI) where the carboxy end is deprotonated (COO-) and the amine end is protonated (NH3+), resulting in a net neutral charge.

If the amino acid is in a solution where the PH is above the isoelectric point, than both ends of the amino acid will be in the protonated form (net charge +1), whereas if the PH is below isoelectric point, both ends will be in deprotonated form and thus net charge of -1.

Now how do we relate this to the 2 pKa values of the amino acid? We know the carboxyl end has a pKa that is considerably lower than the pKa at the amine end (since the amine end is basic and the carboxyl end is acidic). If we are in PH solution lower than the carboxyl end's pKa, then that means the carboxyl end is deprotonated and also the amine end is deprotonated as well. If we are at a PH above the pka of the amine end, then the amine end will be protonated (and obviously the carboxyl end will be protonated as well) giving the amino acid a net charge of +1. What about if you are between the 2 pKa's? Is that where the isoelectric point is? If you are at a PH above the pKa of the carboxyl end but lower then the pKa of the amine end, the carboxyl end will be protonated and the amine end will be deprotonated, which gives it a net charge of '0', but this is not the isoelectric state...

I am CONFUSED! Where is isoelectric point relative to the two pKa's?

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[FeinMS]

pI for simple amino acids (i.e. non acidic, non basic) is just average between pKa of carboxyl and amine end. At pH=pI, carboxyl group is deprotonated (-1) and amine group is protonated (+1) resulting in 0 charge.
You have misunderstanding between pKa and pH. When pH<pKa, there will be more of protonated form. When pH>pKa, there will be more of deprotonated form. This can be seen through Henderson-Hasselbach Eq. pH=pKa+log(A-/HA). If pH<pKa, there are more HA than A. and vice versa.

 

[GomerPyle]

Geez, someone told me the opposite and it totally messed me up. Thanks for the clarification. PI is the average between the two pKa's, and if PH is less than the pKa, you are protonated and if you are greater, than you are deprotonated.

 

[gettheleadout]

Geez, someone told me the opposite and it totally messed me up. Thanks for the clarification. PI is the average between the two pKa's, and if PH is less than the pKa, you are protonated and if you are greater, than you are deprotonated.

I think about it like this, I find this is intuitive:

pKa is a measure of acidity, right? If the solution pH is lower than the pKa of a solute, then the solution is effectively more acidic than that solute, and so it protonates the solute. If the solution pH is higher than the pKa of some solute, then the solution is effectively basic compared to that solute, and the solute protonates the solution (and is thus, deprotonated).

 

[sps27]

pI for simple amino acids (i.e. non acidic, non basic) is just average between pKa of carboxyl and amine end. At pH=pI, carboxyl group is deprotonated (-1) and amine group is protonated (+1) resulting in 0 charge.
You have misunderstanding between pKa and pH. When pH<pKa, there will be more of protonated form. When pH>pKa, there will be more of deprotonated form. This can be seen through Henderson-Hasselbach Eq. pH=pKa+log(A-/HA). If pH<pKa, there are more HA than A. and vice versa.

To add to all of the above, what about amino acids with basic (Lysine, Arginine, Histidine) and acidic side chains (glutamic and aspartic acid). What is their pI? In that case the pI is calculated slightly differently. It is not simply an average. There will be three pKa values in each of these cases so it is not simply an average of all 3. With acidic amino acids such as Glutamatic acid, their pI is calculated by averaging the pKa of two acidic groups, carboxyl 1 and carboxyl 2. And, if the amino acid side chain is basic, pI will then be average of pKa of amino 1 and amino 2, So these are some caveats of pI of amino acids. Remember that Glutamic acid pI will be more towards the acidic side i.e., 5 or less, whereas Histidine pI will be more towards the basic side i.e., 9 or above.