Fructose-1,6-bisphosphatase from snake muscle (Zaocys dhumnades)

Attempts to rationalize protein crystallization using relative crystallizability

Protein crystal growth (PCG) remains the bottleneck of crystallography despite many decades of study. The nucleation zone in the two-dimensional-phase diagram has been used to evaluate the relative crystallizability of proteins, which is expressed as a percentage over the phase area delineated by experimental protein and precipitating agent concentration ranges. For protein-salts which are subject to a direct temperature effect on solubility, as represented by Egg Lysozyme, a decrease in temperature augments the nucleation zone percentage whereas for those with retrograde solubility as a function of temperature, for example fructose-1,6-bisphosphatase in the presence and absence of AMP, an increase in temperature can significantly enhance the relative crystallizability. These results have been confirmed by the number of "hits" using PEGs as precipitating agents in Sparse Matrix Screen experiments for different proteins and are in excellent agreement with the relative crystallizability. The relationship between solubility dependence, relative crystallizability and crystallization success, has been evidenced. Such crystallizability can become a guide to identify efficient crystallization regions, providing a rational approach to PCG and structural biology.

Association and activation of fructose 1,6-bisphosphase during unfolding and refolding: spectroscopic and enzymatic studies

Fructose 1,6-biphosphase is a well-characterized oligomer enzyme, and many effectors allosterically control its activity. In this report, we compared the activity, allosteric properties, and conformational changes in its denaturant-induced unfolding processes. In addition, a trpytophan residue has been introduced into the interface between the C1 and C2 subunits to investigate conformational changes during unfolding. Results show that the denaturation curves of WT FruP2ase detected by various methods do not agree, and the dissociation occurs first with a monomeric form existing around 0.4 M GdmCl as shown by gel filtration. The dissociation of all mutants is accompanied by changes in fluorescence intensity. The results suggest that the unfolding of FruP2ase is a complicated, multiphase process. The activation of FruP2ase by GdmCl at low concentrations can be interpreted as a consequence of the effect of monovalent cation. In the refolding experiments, it is found that Mg2+ is not only essential for enzyme activity, but also can assist the enzyme in refolding and association by preventing the formation of aggregates.

Magnet used for protein crystallization: novel attempts to improve the crystal quality

The accuracy of the structures of biological macromolecules determined by X-ray crystallography is of fundamental importance, both for the understanding of life processes and for medical applications. The resolution of the structure is thus critical, and is largely determined by the quality of single crystals. Here we report the results of applying a magnetic field and a magnetization force during growth of the snake muscle fructose-1,6-bisphosphatase and human estrogenic 17beta-hydroxysteroid dehydrogenase crystals. For both enzyme proteins, the quality of the crystals improved with repeated assay, and their data sets were collected at significantly higher resolutions. These results coincide with a mechanism involving the reduction of convection, due to both the hydrodynamics within a magnet and the partially reduced gravity induced by a magnetization force. The density difference between the crystal and solution becomes less significant, and the sedimentation speed of the crystals is also reduced in the presence of the magnetization force.

AMP activation of snake muscle fructose 1,6-bisphosphatase at alkaline pH

AMP, an allosteric inhibitor at neutral pH, activates snakes muscle fructose 1,6-bisphosphatase at pH 9.2. The activation is virtually unique for the snake muscle enzyme: activation was not observed for the enzymes from either human and rabbit liver or porcine kidney. The activation is Mg(2+)-dependent but was not observed until the concentration of Mg2+ reaches 1 mM. It is known that subtilisin, trypsin, or lysosomal proteases hydrolyse the N-terminal loop of fructose-1,6-bisphosphatase in the vicinity of amino acid residue 60 generating a form of the enzyme with a pH optimum at 9.2. In the presence of AMP, the pH profile of the native snake muscle enzyme resembles that of the alkaline form and modification of the highly reactive sulfhydryl group abolishes AMP activation. The fact that AMP has a dual function at different pH levels suggests that pH might be an important factor in regulating the activity of the enzyme upon binding of AMP at the allosteric site. Indeed, the mode of AMP binding to the allosteric site may differ at neutral and alkaline pH levels. A residue that ionizes with a pKa of 8.9 might be involved in this process.

Inactivation kinetics of the reduced spinach chloroplast fructose-1,6-bisphosphatase by subtilisin

The course of inactivation of the reduced spinach chloroplast fructose-1,6-bisphosphatase by digestion with subtilisin has been followed by the progress curve method [Tsou, C. L. (1988) Adv. Enzymol. 61, 381-436] and found to follow first-order kinetics. On the basis of the hydrolysis of the substrate, fructose 1,6-bisphosphate, at different concentrations during proteolysis by subtilisin, the first-order inactivation rate constants for the free enzyme and the enzyme-substrate complex can both be determined. The ratio between the inactivation rate constants for the free enzyme and the enzyme-substrate complex indicates strong protection against subtilisin proteolysis by the substrate. It is proposed that the above ratio can be used as a quantitative measure of substrate protection for enzyme inactivation generally. As it has been found that the site of proteolysis is located in a loop region near the N-terminus and well away from the active site, the substrate protection indicates a conformation change of the enzyme away from the substrate binding site.

Inactivation precedes changes in allosteric properties and conformation of D-glyceraldehyde-3-phosphate dehydrogenase and fructose-1,6-bisphosphatase during denaturation by guanidinium chloride

It has been shown that inactivation of several enzymes precedes overall conformational changes of the enzyme molecules as a whole during denaturation [Tsou (1993) Science, 262, 380-381]. However, the relation between inactivation, loss of allosteric properties of oligomeric enzymes and unfolding of the enzyme molecule during denaturation remain little explored. These have now been compared for D-glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and fructose-1,6-bisphosphatase (FruP2ase) during denaturation by guanidinium chloride (GdmCl). GAPDH is completely inactivated at 0.3 M GdmCl but at this GdmCl concentration it still binds NAD+ with negative co-operativity. At 0.4 M GdmCl, inactivation of FruP2ase reaches completion whereas its allosteric properties, including the heterotropic effect of AMP inhibition and K+ activation with positive co-operativity, are only partially affected. Much higher GdmCl concentrations are required to bring about unfolding of the overall structures of both enzymes.