Catalytic activity of non-cross-linked microcrystals of aspartate aminotransferase in poly(ethylene glycol)

Catalytic competence of O-acetylserine sulfhydrylase in the crystal probed by polarized absorption microspectrophotometry

The reactions of the pyridoxal 5'-phosphate-dependent enzyme O-acetylserine sulfhydrylase with the substrate O-acetyl-L-serine and substrate analogs have been investigated in the crystalline state by single-crystal polarized absorption microspectrophotometry. This approach has allowed us to examine the catalytic competence of the enzyme in different crystalline states, one of which was used to determine the three-dimensional structure; experimental conditions were defined for the accumulation of catalytic intermediates in the crystal suitable for crystallographic analyses.O-Acetyl-L-serine reacts with the enzyme in one of the crystal forms leading via a beta-elimination reaction to the accumulation of the alpha-aminoacrylate Schiff base, absorbing maximally at 320 and 470 nm, as in solution. The dissociation constant for the alpha-aminoacrylate Schiff base is in the millimolar range, 500-fold higher than in solution, suggesting that crystal lattice interactions may oppose functionally relevant conformational changes. The dissociation constant exhibits a bell-shaped dependence on pH centered at pH 7. At this pH the alpha-aminoacrylate species slowly decays with time (30% decrease in 24 hours). The alpha-aminoacrylate intermediate readily reacts with sodium azide, an analog of sulfide, the natural nucleophilic agent, to give a new amino acid and the native enzyme, indicating that the crystalline enzyme catalyzes the overall beta-replacement reaction as in solution. In other crystal forms, including that used for the X-ray investigation, O-acetyl-L-serine either has an even higher dissociation constant or causes crystal damage upon binding. When the crystalline enzyme reacts with either L-cysteine or L-serine, the external aldimine intermediate is formed. The dissociation constants for both substrate analogs are closer to those observed in solution and are modulated by pH as in solution. Findings demonstrate that O-acetylserine sulfhydrylase is catalytically competent in the crystal although some regions of the molecule, likely involved in an open-closed transition induced by O-acetyl-L-serine binding, may have a limited flexibility. The accumulation in the crystal of both the external aldimine and the alpha-aminoacrylate intermediate makes feasible their structural determination and, therefore, the elucidation of the catalytic pathway at the molecular level.

Microcrystals of tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium are catalytically active

An improved and efficient method has been developed for the purification of the tryptophan synthase alpha 2 beta 2 complex (EC 4.2.1.20) from Salmonella typhimurium containing a multicopy plasmid. Microcrystals prepared in 12% poly(ethylene glycol) 8000 containing 2.5 mM spermine are shown by scanning electron microscopy to have the same crystal habit as the larger crystals that are being used for structural analysis by X-ray crystallography. The average dimensions of the crystals are 33 microns (length) X 9 microns (width) X 3 microns (maximum thickness). Our finding that suspensions of microcrystals are active in several reactions catalyzed by the active sites of the alpha and beta 2 subunits demonstrates that both active sites are functional in the crystal and accessible to substrates. Thus the larger crystals being used for X-ray crystallographic studies should form complexes with substrates and analogues at both active sites and should yield functionally relevant structural information. A comparison of the reaction rates of suspensions of microcrystals with those of the soluble enzyme shows that the maximum rate of the crystalline enzyme is 0.8 that of the soluble enzyme in the cleavage of indole-3-glycerol phosphate (alpha reaction), 0.3 that of the soluble enzyme in the synthesis of L-tryptophan by the beta reaction or the coupled alpha beta reaction, and 2.7 that of the soluble enzyme in the serine deaminase reaction. These small differences in rates probably reflect functional differences between the crystalline and soluble enzymes since the reaction rates of the microcrystals are calculated to be virtually free of diffusional limitation under these reaction conditions.(ABSTRACT TRUNCATED AT 250 WORDS)