Proteins chemically-modified

Prior to these structural studies, work using chemically modified proteins had shown the requirement for at least four histidine ligands per subunit together with a glutamic acid residue and Tyr-109 (Tyr-114 in met). The most recent X-ray work has eliminated the possibility that Tyr-109 is a ligand, a possibility proposed in earlier X-ray work. Other work on chemical modification of residues has been reviewed.1277... 

HI. Habeeb, A. F., Chemical evaluation of conformational differences in native and chemically modified proteins. Biochim. Biophys. Acta 115, 440 154 (1966). 

Electron-transfer (ET) reactions, the simplest chemical transformations, play vital roles in a diverse ensemble of biological processes. Biological electron transfer is an extraordinarily vibrant field of inquiry, responsible for thousands of original research articles during the past decade. This chapter will focus on studies of ET in chemically modified proteins, protein-protein complexes, and two key biological energy transduction pathways, photosynthesis (see Photosynthesis) and respiration (see Cytochrome Oxidase). 

A typical isotherm, for lysozyme, is shown in Fig. 1. There is a knee at 0.05—0.1 h (g of water per g of protein, mass ratio) and a strong upswing to the isotherm near 0.25 h. Sorption measurements on model polymers and chemically modified proteins (see Watt and D Arcy, 1976 ... 

The development of EPL has facilitated the production of large protein targets, but the requirement of specific N-terminal amino acids at the ligation site (cysteine [7], selenocysteine ]66J) reduces the general utilization of this method. Recently, we introduced a novel approach that we named expressed enzymatic ligation (EEL) for the semisynthesis of larger and chemically modified proteins that combines the advantages of the EPL with those of the substrate mimetic... 

Machova, Z., von Eggelkraut-Gottanka, R., Wehofsky, N., Bordusa, E., and Beck-Sickinger, A. G. (2003) Expressed enzymatic ligation a new approach for the synthesis of chemically modified proteins, Angew. Chem. Int. Ed. 42, 4916-4918. 

Although the emphasis in this review has been on the manipulation of synthetic polymers it would not be appropriate to omit entirely the complementary work on biopolymers. Two important strategies are evolving here, both of which involve molecular imprinting. The first concerns specifically chemically modified proteins, and the second the generation of catalytic antibodies with functions similar to enzymes, but potentially with much greater scope in terms of the reactions catalysed. 

As with most heterogeneous catalysts, it is often difficult to characterize immobilized enzymes at a molecular level. Most immobilized preparations are often complex mixtures with a distribution of chemically modified protein species. The gross catalytic properties observed are a composite of those of a range of differentially modified individual proteins, often irregularly distributed within the sample. Mass transfer limitations and microenvironment effects further complicate characterization. 

Kinstler, O. B., Gabriel, N.E., Farrar, C. E., De-Prince, R.B., N-terminally chemically modified protein compositions and methods, US patent 5985265, 1999, and N-terminally chemically modified protein compositions and methods, US patent 5824784, 1998. 

Direct Determination of the Contents of Amino Acids in Proteins. Amino acid contents of proteins are presently determined, with few exceptions, on an acid hydrolysate of the protein. In some instances, however, a particular amino acid may be at least partially destroyed by the conditions for acid hydrolyses. In other instances, a particular derivative of an amino acid of the protein may be hydrolyzed by the acid condition to regenerate the original amino acid. An analysis of such a chemically modified protein would thus need to be done on the protein before acid hydrolysis. In still other instances, there may be a need to perform a large number of analyses of different proteins for a single amino acid. The availability of a simple, direct chemical method for determination of such a single amino acid would be desirable. 

Selection of Methods. The purification and analysis of a chemically modified protein can be as formidable a task as the chemical modification itself (10). An important general rule to follow in the selection of the proper procedures for the purification and fractionation of modified proteins is that the selections must be on an operational basis. Any method of purification should be selected with regard to the type of chemical modification, the manner in which modification is done, and the purpose of the modification. It is frequently necessary to change the original chemical modification to give a product which can be purified more easily. Similar reasonings may also be applied to the selection of techniques for analyses of the products. 

Problems Encountered in the Purification and Analysis of Chemically Modified Proteins. One of the main problems plaguing the chemical modification expert is the heterogeneities of the products. With chemically modified proteins heterogeneity may make purification nearly impossible and analysis merely a reflection of an average value for the heterogeneous population of molecules. Heterogeneity can be caused by incomplete modifications as well as by side reactions of either a physical or chemical nature. Careful considerations of these as well as other possible problems is mandatory in achieving satisfactory purifications and analyses. A recent review (10) should be consulted for a more comprehensive discussion of the purification and analysis of chemically modified proteins. 

Formation of toxic products by chemical modification is perhaps the area which will receive the most attention in future investigations. The possibility of forming toxic products frightens commercial research directors and higher administrators in industry. Toxicity in a protein is not a simple matter because of the complexities of protein structure and protein digestion in the digestive tract. A chemically modified protein might be toxic as ... 

This selective review, which deals primarily with the chemical modification of soy proteins, is further limited to nondestructive chemical reactions which alter physical and biochemical properties of importance in food systems. Soy protein products have been modified by various chemical reactions including (a) treatment with alkalies and adds, (b) acylation, (c) alkylation and esterification, and (d) oxidation and reduction. In most instances these reactions have been applied to heterogeneous protein mixtures containing nonprotein impurities, and often to proteins of unknown prior history. Nonetheless, these reactions indicate that protein functional properties of value in food fabrication can be altered significantly through reaction with chemical reagents. It is recognized that chemically modified proteins must be critically evaluated for food safety. 

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