Calculating Isolectric Point based on pKa's

[nfg05]

When given the pKa of the COOH group, NH3+ group, and any side chains (whether basic OR acidic), how do you calculate an amino acid's pI?

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[161927]

When given the pKa of the COOH group, NH3+ group, and any side chains (whether basic OR acidic), how do you calculate an amino acid's pI?

take the two groups closest in pH and average them

 

[nfg05]

take the two groups closest in pH and average them

You mean closest in pKa? Not sure what you mean by the pH of a group.

 

[AsuKa]

the isoelectric point is the point where there is no net charge on the species.

let's take a look at a random amino acid. let's say glutamate

glutamate has 3 ionizable groups

NH3
COOH (carboxy terminal end)
and COOH (of the R group)

because i dont have them memorized, let's just say

NH3 pka = 10
COOH carboxy = 3.1
COOH R group = 4.1

when pKa > ph, it has a proton
when pKa < ph, it is deprotonated

from pH 1-3 what are the charges?
(nh3) 10 > 3 = + charge (nh3 with proton has a + charge)
4.1 and 3.1 > 3 = no charge (COOH is not charged)
net charge: +

now let's move up to a ph of 4

10>4 = + charge on the nh3
4.1 > 4 = 0 charge (cooh)
4 > 3.1 = - charge (coo- no proton)
net charge: 0

what happens at ph5 ?
10 > 5 = +
5 > 4.1 and 3.1 = 2x coo-
net charge: -1

so the pI or isoelectric point of this particular example is 3-4

well really, 3.2 to 4.0 but yea, you get the point. i hope this clears it up for you.

 

[161927]

You mean closest in pKa? Not sure what you mean by the pH of a group.

pKa obv yes

 

[pullandbear]

the isoelectric point is the point where there is no net charge on the species.

let's take a look at a random amino acid. let's say glutamate

glutamate has 3 ionizable groups

NH3
COOH (carboxy terminal end)
and COOH (of the R group)

because i dont have them memorized, let's just say

NH3 pka = 10
COOH carboxy = 3.1
COOH R group = 4.1

when pKa > ph, it has a proton
when pKa < ph, it is deprotonated

from pH 1-3 what are the charges?
(nh3) 10 > 3 = + charge (nh3 with proton has a + charge)
4.1 and 3.1 > 3 = no charge (COOH is not charged)
net charge: +

now let's move up to a ph of 4

10>4 = + charge on the nh3
4.1 > 4 = 0 charge (cooh)
4 > 3.1 = - charge (coo- no proton)
net charge: 0

what happens at ph5 ?
10 > 5 = +
5 > 4.1 and 3.1 = 2x coo-
net charge: -1

so the pI or isoelectric point of this particular example is 3-4

well really, 3.2 to 4.0 but yea, you get the point. i hope this clears it up for you.

wow very helpful.thanks!