Question

The isoelectric point (pI) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pI can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pI value of the following hexapeptide:

Trp-His-Glu-Tyr-Gly-Asp

Map Sapling Learning macmillan learning The isoelectric point (pl) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pl can be estimated fairly accurately (within O.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pl value of the following hexapeptide: Amino Acid Side Chain pK Arg 12.5 Asp 3.7 Cys 8.2 Glu 4.3 Trp-His-Glu-Tyr-Gly-Asp His 6.0 Number Lys 10.5 Тyr 10.5 pl = *peptide- NH3* 8.0 *peptide- СООН 3.4 *Amino and carboxy terminal values differ from the a amino and carboxy values of a single amino acid. EE EEE

Answer 1

QUESTION-1

ANSWER

ISOELECTRIC POINT-

The isoelectric point (pI) of a peptide is the pH at which the peptide does not migrate in an electric field and the number of positive charge equals the negative charge.

ZWITTER ION-

A zwitterion is the dipolar ion of amino acid with a spatially separated positive and negative charge.

When the amino acid is dissolved in water exist in Zwitterionic form that means it can act either as a base or an acid.

We need to calculate the pI of the following given hexapeptide.

(Hexapeptide- Made up of six amino acid with 5 peptide bond)

Trp-His-Glu-Tyr-Gly-Asp

In the given peptide the amino acids with charged side chain are

Histidine, tyrosine, Aspartate, Glutamate

Others do not have a charged side chain.

AMINO ACIDS	CHARGE ON SIDE GROUP
Histidine	6.0
Tyrosine	10.5
Aspartate	3.7
Glutamate	4.3
GLYCINE	0
TRYPTOPHAN	0
peptide- NH3+	8.0
peptide - COOH	3.4

Now calculate the pI

8.0-Trp-His-Glu-Tyr-Gly-Asp-3.4

STEPS

1. Draw the peptide structure at it's most protonated state.

2. Calculate the overall charge.

3. Calculate the overall charge as the pH rises.

4.Using two pKa values surrounding zwitterion calculate the pI.

Nig-T-GlTeG-App C0OH ovocall than- ta 1 CCH + OH CCCH NuTap HisGl- Tye - Gly -Asp- c RHT COOH +1 OH CODH 4t 3.9 N-Tcp -HiS-Gl (ly - Asp - Coo RH COOH ZHrtletioo - Tyt OH Coo NH-Ticp-Hs-Gle-Tye -Gly - Asp - Coo COO CH 6.0 N-TapHsGlee- TYE-Gly-Asp COD -C00 0H RCOO Coo NHTi-Hisale-Ty-Gly-As 3 too COO OH RCO0 4t 10.0 Tye-ly App- Co Coo Coaro N-Tp 7 R

So here the two pKa surrounding zwitterion are 3.7 and 4.3

So pI = 3.7+ 4.3/ 2

= 8/2

=4

SO the pI of given hexapeptide is 4.

GOOD LUCK.