Why histidine is positively charged

The large semi-transparent sphere indicates the location of the iron ion. Template:NatOrganicBox Histidine abbreviated as His or H [1] is one of the 20 most common natural amino acids present in proteins. Retrieved Categories : Pages with broken file links Proteinogenic amino acids Basic amino acids Essential amino acids Imidazoles.

Cookies help us deliver our services. By using our services, you agree to our use of cookies. Namespaces Home Page Discussion. Views Read View source View history Help. An amino acid has this ability because at a certain pH value different for each amino acid nearly all the amino acid molecules exist as zwitterions. The particular pH at which a given amino acid exists in solution as a zwitterion is called the isoelectric point pI.

At its pI, the positive and negative charges on the amino acid balance, and the molecule as a whole is electrically neutral. The amino acids whose side chains are always neutral have isoelectric points ranging from 5.

The basic amino acids which have positively charged side chains at neutral pH have relatively high examples. Amino acids undergo reactions characteristic of carboxylic acids and amines. All D's are -1; there are 2 D's, and so they contribute -2 to the total peptide charge. For a receptor to bind this peptide, its charge must be complementary; in other words, since the peptide has a net positive charge, you might expect the receptor to have a net negative charge.

So, the receptor should contain acidic residues acidic residues, like Asp and Glu, have a net negative charge at pH 7. We can use the Henderson-Hasselbach equation to determine the fraction of histidines which are protonated when the pKa is 7.