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We carried out chymotryptic digestion of multimeric ATP-dependent Lon protease from Escherichia coli. Four regions sensitive to proteolytic digestion were located in the enzyme and several fragments corresponding to the individual structural domains of the enzyme or their combinations were isolated. It was shown that (i) unlike the known AAA+ proteins, the ATPase fragment (A) of Lon has no ATPase activity in spite of its ability to bind nucleotides, and it is monomeric in solution regardless of the presence of any effectors; (ii) the monomeric proteolytic domain (P) does not display proteolytic activity; (iii) in contrast to the inactive counterparts, the AP fragment is an oligomer and exhibits both the ATPase and proteolytic activities. However, unlike the full-length Lon, its AP fragment oligomerizes into a dimer or a tetramer only, exhibits the properties of a non-processive protease, and undergoes self-degradation upon ATP hydrolysis. These results reveal the crucial role played by the non-catalytic N fragment of Lon (including its coiled-coil region), as well as the contribution of individual domains to creation of the quaternary structure of the full-length enzyme, empowering its function as a processive protease
Enzymatic modifications are applied in many branches of the food and pharmaceutical industry. They are undoubtedly an attractive alternative for physical and chemical modifications. Many proteolytic enzymes can be used for modifying of functional properties of proteins. The differences in functional properties among proteins result from the differences in their structural properties. Enzymatic modifications affect conformation of protein molecules and, consequently, their physico-chemical properties. Enzymatic hydrolysis of proteins results mainly in the loss of their molecular weight, increase in the number of functional groups capable to ionize and exposition of hydrophobic sites. Limited proteolysis of milk proteins may improve their emulsifying and foaming properties. It is suggested that the necessary condition for good emulsifying and foaming properties is amphiphatilicity of their structures and certain minimum molecular weight of the peptides formed.
Limited proteolysis experiments can be successfully used to probe conformational features of proteins. In a number of studies it has been demonstrated that the sites of limited proteolysis along the polypeptide chain of a protein are characterized by enhanced backbone flexibility, implying that proteolytic probes can pinpoint the sites of local unfolding in a protein chain. Limited proteolysis was used to analyze the partly folded (molten globule) states of several proteins, such as apomyoglobin, a-lactalbumin, calcium-binding lysozymes, cytochrome c and human growth hor­mone. These proteins were induced to acquire the molten globule state under spe­cific solvent conditions, such as low pH. In general, the protein conformational fea­tures deduced from limited proteolysis experiments nicely correlate with those deriv­ing from other biophysical and spectroscopic techniques. Limited proteolysis is also most useful for isolating protein fragments that can fold autonomously and thus be­have as protein domains. Moreover, the technique can be used to identify and pre­pare protein fragments that are able to associate into a native-like and often func­tional protein complex. Overall, our results underscore the utility of the limited pro- teolysis approach for unravelling molecular features of proteins and appear to prompt its systematic use as a simple first step in the elucidation of structure-dynamics- function relationships of a novel and rare protein, especially if available in minute amounts.
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