Iso-electric point of proteins?

[philip041]

I'm crystallizing apoferritin, and still don't understand what an iso-electric point is, despite reading a whole chapter of a book about it. What happens if the surrounding buffer solution is at the iso-electric point?

Cheers!

 

[Ygggdrasil]

The isoelectric point of a protein is the pH at which the protein has no net charge. At pH values lower than the isoelectric point, more basic side chains of amino acids become protonated to give the protein a net positive charge. Similarly, at pH values higher than the isoelectric point, more acidic side chains become deprotonated, giving the protein a net negative charge. It should be noted that the protein still contains charged side chains at its isoelectric point; however, at the isoelectric point, the number of positively-charged side chains is equal to the number of negatively-charged side chains.

The concept of isoelectric point is important when crystallizing proteins because proteins may have an easier time packing into a crystal lattice when there have no net charge (although this is not always the case).

 

[yerpo]

The situation can be a bit more complicated in such large molecules as proteins, because it can have regions with sequences of differently charged aminoacids.

 

[unab]

Heya,

I´m wondering if all protein rich in beta-structure have a low pI but proteins rich in alpha structure have high pI but can´t find any references regarding that. Any comments or references regarding this are highly appreciated.

Cheers, Una

 

[Ygggdrasil]

I don't see any reasons why this should be the case. The pI of a protein depends on only its primary sequence and secondary structure should have no effect on the pI. It is possible that your statement could be true, however. For example, if the sequences favoring beta sheet formation tend to have more acidic residues that sequences that favor alpha helix formation, then your statement would be true. However, I have not heard of this rule before.

 

[unab]

yeah that was what I was thinking I was just looking at a ladder used for IEF gels and the proteins with low pI were all rich in beta-strands and the ones with high pI all have a higher pI. I was just wondering if this was only a coincidence!

Thanks for your reply, Una