Chapter 33: Problem 1

Find out why the net charge on a protein molecule varies with pH. Identify the amino acids primarily responsible for determining the net charge on a protein molecule and draw simple diagrams to represent the ionisation of their side chains, indicating how you would expect these side chains to be charged at acid, neutral and alkaline \(\mathrm{pH}\) values.

Short Answer

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The net charge on a protein molecule varies with pH because of the ionisation state changes of amino acids that contain ionisable side chains. These amino acids include arginine, aspartate, cysteine, glutamate, histidine, lysine, tyrosine and tryptophan. Depending on the pH, these amino acids can be positively charged, negatively charged or neutral.

Step by step solution

- Understand the Effect of pH on Protein Charge

The pH of a solution can affect the ionisation state of the acidic and basic groups present in the amino acids that make up the protein. When these groups ionise, they can carry a positive or negative charge. At lower pH (acidic conditions, less than 7), the protein will have a net positive charge as more hydrogen ions are available to protonate the ionisable groups. At higher pH (alkaline conditions, greater than 7), the protein will have a net negative charge due to the deprotonation of these groups.

- Identify Amino Acids Responsible for Net Charge

The amino acids primarily responsible for the protein charge are those that have ionisable side chains. These include arginine, aspartate, cysteine, glutamate, histidine, lysine, tyrosine and tryptophan.

- Draw Ionisation Diagrams

Draw simple diagrams representing the ionisable side chains of the above mentioned amino acids. At acidic pH, arginine, lysine would be positively charged due to the additional proton, Aspartate, glutamate, cysteine, tyrosine and tryptophan would be neutrally charged. At neutral pH, arginine, lysine would still carry a positive charge and aspartate and glutamate would be negatively charged. The others would be in their neutral state. At alkaline pH, arginine, lysine would be neutrally charged while aspartate and glutamate would be negatively charged. Histidine due its pKa value near to neutral pH would be positively charged at acidic pH, neutral at neutral pH and negatively charged at alkaline pH.

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Short Answer

Step by step solution

- Understand the Effect of pH on Protein Charge

- Identify Amino Acids Responsible for Net Charge

- Draw Ionisation Diagrams

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