Protein during enzymatic modification

Table I. Application of Membrane Processes During Enzymatic Modification of Proteins...

During enzymatic modification under appropriate reaction conditions, L-amino acids (generally in ester form) are partially covalently incorporated into the peptide chains of a protein hydrolysate. Thus these enzymatic modification reactions with amino acid enrichment would be expected to be more important for health aspects than other modification processes without covalent amino acid enrichment. 

Formaldehyde fixes proteins in tissue by reacting with basic amino acids— such as lysine,5 7—to form methylol adducts. These adducts can form crosslinks through Schiff base formation. Both intra- and intermolecular cross-links are formed,8 which may destroy enzymatic activity and often immunoreactiv-ity. These formaldehyde-induced modifications reduce protein extraction efficiency and may also lead to the misidentification of proteins during proteomic analysis. 

In addition to the 20 proteinogenic amino acids (see p. 60), there are also many more compounds of the same type in nature. These arise during metabolic reactions (A) or as a result of enzymatic modifications of amino acid residues in peptides or proteins (B). The biogenic amines (C) are synthesized from a-amino acids by decarboxylation. 

Cnzyme-catalyzed modifications of proteins deserve much more work and therefore must be considered as an important field for further scientific investigations. In spite of the prolific research carried out in this area during the past three decades, a number of enzymatic modifications not only are poorly investigated or understood but attempts to apply them to food protein systems are nonexistent. The fundamental aspects of enzymatic modification of proteins are of interest since potential applications for nutritional and functional improvements of food proteins appear to be numerous and promising. Enzymatic and chemical modifi-... 

Numerous undesirable reactions that result in organoleptic, nutritional and functional deterioration may occur in food proteins during processing and storage. These include the non-enzymatic or Maillard reactions, transamidation condensation reactions with dehydroalanine forming crosslinks, and carbonyl amine interactions, all of which may involve the free e-amino group of lysine (11,23). To minimize these reactions a significant volume of work has been done on the protective modification of the e-NH2 of lysine by formylation, acetylation, propionylation (26) or reductive dimethylation (10,11). 

Application of membrane processes during production of purified food proteins is a mild treatment which ensures that the functional properties of the native proteins are retained. (1 ) These properties are mostly found to be superior to those of denatured proteins. However, not all possible needs of the modern food industry are fulfilled by using native proteins instead of denatured ones. Therefore, enzymatic modification of proteins has been demonstrated as a possible means of meeting the needs of the food industry for high-quality protein ingredients ( ), (13), (14). 

Aso et al. [95] studied a model system in order to obtain basic information on the mechanism of amino acid incorporation during an enzymatic modification reaction in the presence of papain. They found that the amino acid ester reacted as a nucleophile in the aminolysis of the acyl-enzyme intermediate to result in the formation of new peptides. Several proteases used in enzymatic peptide bond synthesis are known to form transitory acyl-enzyme intermediates during the hydrolysis of proteins. However, the acyl groups can be transferred to other nucleophiles (amino terminals of peptides or amino acids), synthesizing new peptide bonds [71]. With full knowledge of the above-mentioned facts, covalent amino acid enrichment of proteins can result in... 

Successful oral delivery of protein involves overcoming the barriers of enzymatic degradation, achieving epithelial permeability, and taking steps to conserve bioactivity during formulation processing. The coadministration of enzyme inhibitors and permeation enhancers is an approach used to enhance the bioavailability of oral protein formulations. Chemical modification of peptides and use of polymeric systems as carriers have also been attempted to overcome the inherent barriers. 

Many of radioactive isotopes are very useful for the following biochemical processes (Table 6.1). The radioactive label is introduced into macromolecules, especially proteins, either during biosynthesis, e.g., during translation in the presence of S-methionine, or enzymatically, e.g., by use of P-labeled ATP during protein phosphorylation by protein kinases, or chemically by modification of amino acid side chains. Examples for reagents used in chemical radiolabeling of proteins are given in Table 6.2. 

T15. Traverso, N., Menini, S., Cottalasso, D., Odetti, P., Marinari, U. M., and Pronzato, M. A., Mutual interaction between glycation and oxidation during non-enzymatic protein modification. Biochim. Biophys. Acta 1336, 409-418 (1997). 

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