Asparagine isoelectric point

PSA is a single-chain glycoprotein that is 7% carbohydrate. It has 237 amino acid residues and four carbohydrate side chains with linkages at amino acid 45 (asparagine), 69 (serine), 70 (threonine), and 71 (serine). The N-terminal amino acid is isoleucine, and the C-terminal residue is proline. Its MW is 28,430, and it has isoelectric points from 6.8 to 7.2 because of its various isoforms. The three-dimensional structure and the antigenic domain of PSA have been characterized. ... 

Blood serum (separated from fibrinogen and/or fibrin by ultracentrifugation) contains serum albumins. These possess a molar mass of 67 500 and an isoelectric point at pH 4.8-5.0. The serum albumins of higher mammals differ particularly in the amino acid residues at the carboxyl end of the protein chain, while the N-terminal amino acid residue is always asparagine ... 

With aspartate (Asp, D) in hand, the conversion to asparagine (Asn, N) has been shown to be straightforward. As expressed in US. Patent 5326908, when a slurry of aspartic acid (Asp, D) in methanol (CH3OH) and sulfuric acid (H2SO4) is prepared, the 4-carboxylate (that furthest from the amino group) is preferentially methylated and, without isolation, treatment of the reaction mixture with at least a fivefold excess of ammonia (NH3) produces asparagine (Asn, N). After the removal of excess ammonia, adjustment of the pH to the isoelectric point of the amino add (pi 5.4) yielded a crystalline material (Scheme 12.38). 

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