I'm looking at amino acid abbreviations and on every site I visit, asparagine and glutamine have two different abbreviations. Is there a reason for this? Do they represent different forms of the amino acid that may have different properties?
I'm looking at the abbreviations on this site as an example: http://www.hgmd.cf.ac.uk/docs/cd_amino.html
On the page, asparagine has an abbreviation of 'N' and an abbreviation of 'B'. The abbreviation 'B' doesn't correspond to a codon so why is it there?
Glutamine also has two abbreviations ('Q' and 'Z'). The abbreviation 'Z' doesn't correspond to a codon either.