Analysis of catalytic properties of hen egg white lysozyme during renaturation from denatured and reduced material

Effect of the structure of the denatured state of lysozyme on the aggregation reaction at the early stages of folding from the reduced form

We previously demonstrated that the hydrophobic clusters present in hen lysozyme under denaturing conditions were disrupted by the mutation of Trp62 to Gly (W62G). In order to examine the effects of the structure of the denatured state of W62G lysozyme on folding, we analyzed the early events in the folding of reduced W62G lysozyme in detail. From the exchange measurements of disulfide bonds using the variants containing a pair of cysteine residues (1SS), it was found that the formation of disulfide bond in the W62G1SS lysozyme was not accompanied by a prominent interaction between amino acid residues, indicating that the disruption of the hydrophobic core led to the random folding at the early stages in the process of folding of the reduced lysozyme. On the other hand, analyses of the oxidative-renaturation of reduced W62G lysozymes, as well as measurements of the extent of aggregation of the reduced and carboxy amido methylated W62G lysozyme, indicated that the formation of an aggregate is more prominent in the reduced W62G lysozyme than in the reduced wild-type lysozyme. Moreover, a lag phase was detected in the oxidative-renaturation of reduced W62G lysozyme, as based on observations of the recovery of activity. The simulation of the folding process indicated that intermediates were present at the early stages in the folding of the reduced W62G lysozyme. These results suggest that the presence of the intermediates was derived from the random folding at the early stages in the folding process of reduced W62G lysozyme due to the disruption of the structure of the denatured state. Folding thus appears to have been kinetically delayed by these processes, which then led to the significant aggregation of reduced lysozyme. Moreover, from the analysis of amyloid aggregation of the reduced lysozymes, it was suggested that the disruption of the residual structure in denatured state by W62G mutation deterred the formation of the amyloid fibrils of lysozyme.

In vitro selection of nucleoprotein enzymes

Natural nucleic acids frequently rely on proteins for stabilization or catalytic activity. In contrast, nucleic acids selected in vitro can catalyze a wide range of reactions even in the absence of proteins. To augment selected nucleic acids with protein functionalities, we have developed a technique for the selection of protein-dependent ribozyme ligases. After randomizing a previously selected ribozyme ligase, L1, we selected variants that required one of two protein cofactors, a tyrosyl transfer RNA (tRNA) synthetase (Cyt18) or hen egg white lysozyme. The resulting nucleoprotein enzymes were activated several thousand fold by their cognate protein effectors, and could specifically recognize the structures of the native proteins. Protein-dependent ribozymes can potentially be adapted to novel assays for detecting target proteins, and the selection method's generality may allow the high-throughput identification of ribozymes capable of recognizing a sizable fraction of a proteome.

Refolding of denatured and denatured/reduced lysozyme at high concentrations

Refolding of proteins at high concentrations often results in aggregation. To gain insight into the molecular aspects of refolding and to improve the yield of active protein, we have studied the refolding of lysozyme either from its denatured state or from its denatured/reduced state. Refolding of denatured lysozyme, even at 1 mg/ml, yields fully active enzyme without aggregation. However, refolding of denatured/reduced lysozyme into buffer that lacks thiol/disulfide reagents leads to aggregation. Thiol/disulfide redox reagents such as cysteine/cystine and reduced/oxidized glutathione facilitate the renaturation, with the yield depending on their absolute concentrations. We have obtained an approximately 70% renaturation yield upon refolding of lysozyme at 150 microgram/ml. The cysteine/cystine redox system is more efficient compared with the glutathione redox system. When lysozyme is refolded in the absence of redox reagents, a transient intermediate that has regained a significant amount of secondary structure is formed. The tryptophans in this intermediate are as exposed to water as in the fully unfolded protein. It shows increased exposure of hydrophobic surfaces compared with the native or completely unfolded enzyme. This aggregation-prone intermediate folds to active enzyme upon addition of oxidized glutathione before the aggregation process starts. These properties of the intermediate in the refolding pathway of lysozyme are similar to those proposed for the molten globule.

Folding of lysozyme

Due to the detailed knowledge of the three-dimensional structure, chemistry and catalytic mechanism of hen egg white lysozyme, this enzyme has become a major model for the analysis of the folding pathway of globular proteins. Unfolding and folding of lysozyme are reversible processes. Unfolding is a highly cooperative event; under physiological conditions only the native and the unfolded states are stable. Folding of lysozyme involves both a cooperative and a parallel pathway. The complexities in the folding pathway arise from the collapsed state which is formed within a burst-phase in the first milliseconds of folding. In a second, fast folding phase, major parts of the secondary structures both in the alpha-domain and the beta-domain are formed. During the slow folding phase, formation of secondary structure is completed and native tertiary structure is formed in less than 1 second. Folding of reduced lysozyme combines both secondary and tertiary structure organization, as well as formation of four disulphide bonds. Analysis of formation of disulphide bonds showed that there exists a restricted search of structures in the formation of the native conformation and a nucleation in the folding pathway. The transition from a two-disulphide bond intermediate to a three-disulphide bond form appears to be the rate-limiting step in this pathway. Native-like catalytic properties depend on the correct generation of all four disulphide bonds. Folding of both denatured and denatured/reduced lysozyme is characterized by transient folding species possessing structural properties of the molten globule state: high content of secondary structure, no tertiary fold, and the appearance of hydrophobic structures.