Question

The isoelectric point, pl, of the protein trypsin is 10.5, while that of clostripain is 4.8 . What is the net charge of trypsin at pH 5.1 ? What is the net charge of clostripain at pH 2.5 ?! The isoelectric point of cysteine is 5.07 ; histidine , 7.59. During paper electrophoresis at pH 4.3 , toward which electrode does cysteine migrate? During paper electrophoresis at pH 7.1 , toward which electrode does histidine migrate?

Answer 1

The pI or the isoelectric point is the pH when a protein or an amino acid has no charge and so it cannot move. The net charge of amino acids below the pI value is positive and the charge of the amino acids above the pI value is negative. It is due to the protonation of amino acids when H+ concentration is more at acidic pH and deprotonation of the amino acids when the H+ concentration is less.

so if pI of trypsin is 10.5, the net charge below the pH of 10.5 will be positive and above will be negative. (same applies with clostripain)

Net charge of trypsin at pH- 5.1- Positive

Net charge of clostripain at pH- 2.5- Positive

The isoelctric point of cysteine is 5.07 and hsitidine is 7.59

Like the before question, when the amino acids are placed in paper electrophoresis when the pH is less than the isoelecric point, the amino acids will move to the cathode (postive moves towards negative) and when the pH is more than the pI value, the amino acids will move towards the anode (positive electrode)

At the pH of 4.3, the cysteine will migrate towards the cathode. But it will be migrating slowly as it is nearer to the pI value

At the pH of 7.1, histidine will migrate towards the cathode but very slowly  because it will have only 25% of the positive charge.