Deamidation reaction

Peptide stability in solids has been briefly presented in Sect. 63.2.5. It is important to note here that deamidation reactions can also play a major role in the degradation of peptides in solid matrixes. While deamidation in the solid state has received less attention than deamidation in solution, there is enough evidence to suggest that the mechanisms and pathways are comparable if not similar in the two types of media [8] [130],... 

S. Capasso, G. Balboni, P. Di Cerbo, Effect of Lysine Residues on the Deamidation Reaction of Asparagine Side-Chains , Biopolymers 2000, 53, 213 - 219. 

Deamidation, isomerization and racemization. These three reactions are common degradation pathways of proteins and peptides. These reactions are especially prevalent for peptides containing asparagine (Asn) and glutamine (Gin) residues. In the deamidation reaction, the Asn or Gin amide... 

Ammonia is formed in the cell or by bacteria during various deamidization reactions of amino acids. A certain quantity (ca. 25%) of ammonia is produced in the intestine by bacteria or through enzymatic action in the intestinal mucosa during protein degradation. This mean value of intestinal ammonia production is elevated under normal conditions as a result of the increased consumption of meat or fish and reduced during a predominantly lacto-vegetarian protein diet. Production of ammonia is raised by physical work a similar effect is possible in constipation. 

In the deamidation reaction, the side-chain amide linkage in a glutamine (Gin) or asparagine (Asn) residue is hydrolysed to form a free carboxylic acid the Asn peptides being more susceptible to deamidation than the Gin peptides. 

Two amino acid residues are involved in the deamidation reaction asparagyl and glutamyl. The conversion to the corresponding carboxylic acid residues results in... 

Asparagyl residues tend to be more susceptible to deamidation than glutamyl residues. Further, the deamidation reaction is strongly sequence-specific in model peptides with the half life of the -Asn-Pro- sequence being 100-fold greater than that of -Asn-Gly-. To some extent these observations can also be used on proteins that take the structural steiic factors and nearby amino acid residues into consideration. 

The ionic strength of the solution is kept low. At high ionic strength, the deamidation reaction can be fast even at neutral pH. 

Studies on the protein deamidation in food systems has recently been of great interest to the food industry. This is due to the fact that the deamidation reaction changes functional properties of protein such as solubility, emulsion property, and foaming ability... 

The deamidation reaction resulted in the release of ammonia fixim the residues of glutamine and asparagine in the wheat gluten hydrolysate. The ammonia released during... 

It was observed that the reacted mixture of WGH-G contained large amounts of dark colored products while the mixture fi om the reaction of DWGH with glucose contained only very small amounts of colored products. Investigation of this phenomena and the formation of volatile confounds fi om the WGH-G and DWGH-G systems demonstrated that the deamidation reaction of wheat gluten hydrolysates would increase the formation of aroma confounds and decrease the formation of brown color. 

Apart from protein sequence and structure, temperature, pH, and the type of buffers can all influence the deamidation and isoaspartate formation. The effect of temperature on the deamidation reaction rate generally follows the Arrhenius law. Deamidation activation energies around 21-22kcalmoH have been reported for two model peptides under alkaline conditions [19, 20]. The deamidation is also subject to general acid/base catalysis, as evidenced by an increase in deamidation rate with an increase in buffer concentration [2]. 

Deamidation in peptides and proteins generally require the participation of a water molecule to go to completion. In peptides, there are minimal obstructions to water access to the labile amide. However, the more stable protein structures may limit access of water to the amide groups and so influence the rates of any deamidation reactions. Deamidation rates of Asn and Gin residues on the surface of the proteins will not be limited by water access, while those that occur in the interior of proteins may be. Such a limitation will be determined by the static protein structure and by the frequency with which buried Asn and Gin are exposed to solvent during rapid dynamic changes in the structure due to thermal motion. 

A second type of deamidation reaction for glutamine and asparagine exists. Early work by Greenstein and co-workers demonstrated the existence of an a-keto acid-activated glutaminase and aspart nase in liver 43). It was own later by Meister and associates ( 44, 4 ) that these enzymes catalyze a transamination reaction between the amino acid amides and a-keto acids followed by the hydrolysis of the a-ketoglutaramic or a-ketosuccinamic acids with the release of ammoiua. These reactions will be considered in Section IV, 2. 

---