Proteolytic modification

In addition, HIV-1 infection also induces the secretion of certain metallo-proteinases (MMP-2, MMP-9) that are capable of cleaving the N-terminus of SDF-1 but not that of the P-chemokines, and this decreases SDF-1 affinity for CXCR4 binding (227-229). Proteolytic modification of SDF-1 by a serum factor(s) that is different from CD26 and MMPs, which results in reduced anti-HIV activity, has been described as well (230). How SDF-1 proteolytic... 

The combination of this top-down proteomics approach, which generates information on the structure of the intact protein, with a bottom-up approach for protein identification (using MS/MS data of tryptic peptides from the collected fractions) has been particularly useful for identifying posttranslational modifications, cotransla-tional processing, and proteolytic modifications in a number of proteins. Examples from our work will be shown to illustrate this hybrid methodology for proteomics analysis. 

When viral replication occurs, the DNA segment corresponding to the viral genome is first transcribed by host cell enzymes (6). This gives rise not only to viral ssRNA, but also to transcription of mRNAs for precursors of the viral proteins (7). These precursors are integrated into the plasma membrane (8, 9) before undergoing proteolytic modification (10). The cycle is completed by the release of new virus particles (11). 

Highly coordinated chemical reactions, carried out simultaneously in response to the continuously changing cellular environment, are mediated by several enzymeregulating mechanisms. Enzymes are regulated by covalent (e.g., phosphorylation) and proteolytic modifications, binding to stimulatory and inhibitory proteins, and... 

The main entries in this field may be proteolytic modification, or side-chain modification, or no modification. The commentary will give details of the modifications e.g. ... 

Proteolytic modification has special importance for the improvement of solubility of proteins. This effect becomes significant even after very limited proteolysis. Hydrolysis of casein to DH of 2 and 6.7% with Staphylococcus aureus V8 protease increased the isoelectric solubility to 25 and 50%, respectively (Chobert et al., 1988a). However, it should be noted that the solubility profiles were not identical, due to a shift of the isoelectric point of the modified proteins. Solubility of a protein hydrolysate depends on the enzyme used (Adler-Nissen, 1986a). Protamex (a Bacillus proteinase complex) hydrolysates of sodium caseinate (DH 9 and 15%) displayed 85-90% solubility between pH 4 and 5 (Slattery and FitzGerald, 1998). 

An area of particularly attractive application is that of the new protein technologies. As relationships between protein structure and functionality become elucidated, selective proteolytic modification to yield a particular functionality should become feasible. 

Endopeptidases. Our expanding understanding of the relationship between structure and functionality of food proteins presents the opportunity for designing functionality into proteins by selective, specific proteolytic modification. Control of reaction and prevention of autolysis offered by immobilization are essential to establish the conditions for a highly selective modification. Hydrolysis at specific positions in the primary structure of proteins could be coupled with resynthesis of peptide bonds by selection of conditions, for example, as in the plastein reaction. By careful choice of enzymes and conditions according to the characteristics of the substrate proteins, it may be possible to design new structures from known food proteins. 

The processes of prepropolypeptide synthesis, translocation, proteolytic processing and non-proteolytic modification can be enzymatically defined. These definitions are continuing to be developed and clarified. There are limited reports on insect neuropeptide processing (101.102. but these investigations should increase rapidly with the identification of precursor sequences via molecular genetics. The identification of processing enzymes, both proteolytic and non-proteolytic, will further open whole new areas for exploration. 

Figure 3. Laboratory method for proteolytic modification of ultrafiltered soy protein isolate...

This chapter deals with protein structure-function relationships resulting from proteolytic modification, covalent amino acid incorporation, and the effects of these enzymatic modifications on sensory and nutritional quality, on the functional properties, and on the biological activities of proteins and peptides. 

Summarizing the above, proteolytic modification of proteins by covalent sulfur amino acid enrichment could be a suitable method for producing protein-based reducing agents, scavengers, and inducers of cellular detoxification, that is, for improvement in the safety of foods. 

The improvement of functional properties can be obtained by the help of enzyme modification with the directed change of the size and composition of peptides or of their terminal sequence. Several of these proteolytic modification reactions could be used even on a commercial scale in the food industry. 

Proteolytic cleavage. Proteolytic cleavage is one of the most prevalent types of protein posttranslational modifications. Proteolytic modification of proteins is catalyzed by various endo- and exo-peptidases (subsection 12.8.1 for catalytic mechanism) and play essential functions in ... 

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