Is lysine positive or negative

Acid and basic amino acids

Acid Amino Acids

The acidic amino acids aspartic acid and glutamic acid (structural formulas)

The amino acids aspartic acid and glutamic acid belong to the group of acidic amino acids. Why?

If one examines the side chains of the two amino acids more closely, one finds that both residues contain a carboxyl group -COOH. But as we have seen earlier, a carboxyl group behaves like an acid; it can donate a proton to a base and thereby acquire a negative charge:

The carboxyl group can donate a proton

Aspartic acid donates a proton of the side chain carboxyl group to a base

The fact that there are acidic amino acids is less important for the structure of proteins than the fact that acidic amino acids can have a negative charge in the side chain. Therefore we remember:

Acidic amino acids have a carboxyl group in the amino acid residue.
If this carboxyl group gives a proton to a base: B-,
so one is formed negatively charged side chain.