Amino acids negatively charged

The protein isoelectric point (pi) value is the pH at which the sum of acidic amino-add negative charges and of basic amino-add positive charges equals zero. It ranges between less than 3 and more than 12. 

The first DAAO studied mechanistically was from pig kidney (pkDAAO) many kinetic and mechanistic studies have been performed on this enzyme. More recently, yeast DAAOs from Rhodotorula gracilis (RgDAAO) and Trigonopsis variabilis (TvDAAO) have also been studied. Each has different substrate specificities. The best substrate for pkDAAO is D-proline, followed by hydrophobic and neutral amino acids. Positively charged amino acids are bad substrates, while negatively charged D-amino acids are not substrates.In contrast, methionine and valine are the best substrates for RgDAAO. ... 

At the surfaces of proteins are amino acid residues that interact with water. The amino acids are referred to as hydrophilic amino acids and include arginine, lysine, aspartic acid, and glutamic acid. At pH 7 the side chains of these amino acids carry charges—positive for arginine and lysine, negative... 

Scheme 1 Low-energy structures for the complex of and Trp. Structures can be classified as salt-bridge (SB interaction between the positive metal ion and the negative carboxylate of the zwitterionic amino acid) or charge solvation (CS interaction of the metal ion with Lewis-basic sites of the canonical amino acid). Nomenclature of the various structures further includes the main binding sites of the amino acid...

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