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The isoenzyme glutathione S-transferase (GST) I from maize (Zea mays) was cloned and expressed in Escherichia coli, and its catalytic mechanism was investigated by site-directed mutagenesis and dynamic studies. The results showed that the enzyme promotes proton dissociation from the GSH thiol and creates a thiolate anion with high nucleophilic reactivity by lowering the pK(a) of the thiol from 8.7 to 6.2. Steady-state kinetics fit well to a rapid equilibrium, random sequential Bi Bi mechanism, with intrasubunit modulation between the GSH binding site (G-site) and the electrophile binding site (H-site). The rate-limiting step of the reaction is viscosity-dependent, and thermodynamic data suggest that product release is rate-limiting. Five residues of GST I (Ser(11), His(40), Lys(41), Gln(53) and Ser(67)), which are located in the G-site, were individually replaced with alanine and their structural and functional roles in the 1-chloro-2,4-dinitrobenzene (CDNB) conjugation reaction were investigated. On the basis of steady-state kinetics, difference spectroscopy and limited proteolysis studies it is concluded that these residues: (1) contribute to the affinity of the G-site for GSH, as they are involved in side-chain interaction with GSH; (2) influence GSH thiol ionization, and thus its reactivity; (3) participate in k(cat) regulation by affecting the rate-limiting step of the reaction; and (4) in the cases of His(40), Lys(41) and Gln(53) play an important role in the structural integrity of, and probably in the flexibility of, the highly mobile short 3(10)-helical segment of alpha-helix 2 (residues 35-46), as shown by limited proteolysis experiments. These structural perturbations are probably transmitted to the H-site through changes in Phe(35) conformation. This accounts for the modulation of K(CDNB)(m) by His(40), Lys(41) and Gln(53), and also for the intrasubunit communication between the G- and H-sites. Computer simulations using CONCOORD were applied to maize GST I monomer and dimer structures, each with bound lactoylglutathione, and the results were analysed by the essential dynamics technique. Differences in dynamics were found between the monomer and the dimer simulations showing the importance of using the whole structure in dynamic analysis. The results obtained confirm that the short 3(10)-helical segment of alpha-helix 2 (residues 35-46) undergoes the most significant structural rearrangements. These rearrangements are discussed in terms of enzyme catalytic mechanism.

A number of reactive dichlorotriazine dyes specifically and irreversibly inactivate pig heart lactate dehydrogenase, yeast glucose 6-phosphate dehydrogenase and yeast hexokinase at sites competitive with NAD+, NADP+, and ATP respectively. Monochlorotriazine dyes, including Cibacron Blue F3G-A, do not inactivate lactate dehydrogenase but display high affinity and thus inhibit the inactivation by dichlorotriazine dyes. These data are interpreted in terms of the ability of nucleotide-binding enzymes to bind polysulphonated aromatic chromophores.

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Formate dehydrogenase (FDH, EC 1.2.1.2) was purified from Candida boidinii cells in a single step by biomimetic-dye affinity chromatography. For this purpose, seven' biomimetic analogues of the monochlorotriazine dye, Cibacron(R) Blue 3GA (CB3GA), and parent dichloro-triazine dye, Vilmafix Blue A-R (VBAR), bearing a car-boxylated structure as their terminal biomimetic moiety, were immobilized on crosslinked agarose gel, Ultrogel A6R. The corresponding new biomimetic-dye adsorbents, along with nonbiomimetic adsorbents bearing CB3GA and VBAR, were evaluated for their ability to purify FDH from extracts obtained after press-disintegration of C. boidinii cells. Optimal conditions for maximizing specific activity of FDH in starting extracts (1.8 U/mg) were realized when cell growth was performed on 4% methanol, and press disintegration proceeded in four consecutive passages before the homogenate was left to stand for 1 h (4 degrees C). When compared to nonbiomimetic adsorbents, biomimetic adsorbents exhibited higher purifying ability. Furthermore, one immobilized biomimetic dye, bearing as its terminal biomimetic moiety mercap-topyruvic acid linked on the chlorotriazine ring (BM6), displayed the highest purifying ability. Adsorption equilibrium data which were obtained for the BM6 adsorbent in a batch system corresponded well to the Langmuir isotherm and, in addition, breakthrough curves were taken for protein and FDH adsorption in a fixed bed of BM6 adsorbent. The dissociation constant ( K(D)) of the complex between immobilized BM6 and FDH was found to equal 0.05 microM. Adsorbent BM6 was employed in the purification of FDH from a 18-L culture of C. boidinii in a single step (60% overall yield of FDH). The purified FDH afforded a single-band on sodium dodecyl sulphate poly-acrylamide gel electrophoresis, and a specific activity of 7,0 U/mg (30 degrees C).