Modification of carboxyl groups at the active site of ribulose-1,5-bisphosphate carboxylase/oxygenase

Mechanism of acylphosphatase inactivation by Woodward's reagent K

The organ common-type (CT) isoenzyme of acylphosphatase is inactivated by Woodward's reagent K (WRK) (N-ethyl-5-phenylisoxazolium-3'-sulphonate) at pH6.0. The inactivation reaction follows apparent pseudo first-order kinetics. The dependence of the reciprocal of the pseudo first-order kinetic constant (kobs) on the reciprocal WRK concentration reveals saturation kinetics, suggesting that the WRK forms a reversible complex with the enzyme before causing inactivation. Competitive inhibitors, such as inorganic phosphate and ATP, protect the enzyme from WRK inactivation, suggesting that this reagent acts at or near to the enzyme active site. The reagent-enzyme adduct, which elicits a strong absorption band with lambdamax at 346 nm, was separated from unreacted enzyme by reverse phase HPLC and the modified protein was cleaved with endoproteinase Glu-C to produce fragments. The HPLC fractionation gave two reagent-labelled peptides (peak 1 and peak 2) that were analysed by ion-spray MS and sequenced. The former is VFFRKHTQAE (residues 20-29 of human CT acylphosphatase) and the latter IFGKVQGVFFRKHTQAE (residues 13-29). MS demonstrated that both peptides are WRK adducts. A fragment ion with m/z of 1171, which is present in the mass spectrum of peak 1, has been identified as a WRK adduct of the peptide fragment 20-26. The lambdamax at 346 nm of WRK adduct suggests that the modified residue is His-25. Five recombinant enzymes mutated in residues included in the 20-29 polypeptide stretch have been produced. Analysis of their reactivities with WRK demonstrates that His-25 is the molecular target of the reagent as its modification causes the inactivation of the enzyme. Since both His-25-->Gln and His-25-->Phe mutants maintain high catalytic activity, we suggest that the observed enzyme inactivation is caused by the reagent (covalently bound to His-25), which shields the active site.

Woodward's reagent K inactivation of Escherichia coli L-threonine dehydrogenase: increased absorbance at 340-350 nm is due to modification of cysteine and histidine residues, not aspartate or glutamate carboxyl groups

L-Threonine dehydrogenase (TDH) from Escherichia coli is rapidly inactivated and develops a new absorbance peak at 347 nm when incubated with N-ethyl-5-phenylisoxazolium-3'-sulfonate (Woodward's reagent K, WRK). The cofactors, NAD+ or NADH (1.5 mM), provide complete protection against inactivation; L-threonine (60 mM) is approximately 50% as effective. Tryptic digestion of WRK-modified TDH followed by HPLC fractionation (pH 6.2) yields four 340-nm-absorbing peptides, two of which are absent from enzyme incubated with WRK and NAD+. Peptide I has the sequence TAICGTDVH (TDH residues 35-43), whereas peptide II is TAICGTDVHIY (residues 35-45). Peptides not protected are TMLDTMNHGGR (III, residues 248-258) and NCRGGRTHLCR (IV, residues 98-108). Absorbance spectra of these WRK-peptides were compared with WRK adducts of imidazole, 2-hydroxyethanethiolate, and acetate. Peptides III and IV have pH-dependent lambda max values (340-350 nm), consistent with histidine modification. Peptide I has pH-independent lambda max (350 nm) indicating that a thiol is modified. WRK, therefore, does not react specifically with carboxyl groups in this enzyme, but rather modifies Cys-38 in the active site of TDH; modification of His-105 and His-255 does not affect enzyme activity. These results are the first definitive proof of WRK modifying cysteine and histidine residues of a protein and show that enzyme inactivation by WRK associated with the appearance of new absorptivity at 340-350 nm does not establish modification of aspartate or glutamate residues, as has been assumed in numerous earlier reports.

Modification of maize phosphoenolpyruvate carboxylase by Woodward's reagent K

Maize leaf phosphoenolpyruvate carboxylase was completely and irreversibly inactivated by treatment with micromolar concentrations of Woodward's reagent K (WRK) for about 1 min. The inactivation followed pseudo-first-order reaction kinetics. The order of reaction with respect to WRK showed that the reagent causes formation of reversible enzyme inhibitor complex before resulting in irreversible inactivation. The loss of activity was correlated to the modification of a single carboxyl group per subunit, even though the reagent reacted with 2 carboxyl groups per protomer. Substrate PEP and PEP+Mg2+ offered substantial protection against inactivation by WRK. The modified enzyme showed a characteristic absorbance at 346 nm due to carboxyl group modification. The modified enzyme exhibited altered surface charge as seen from the elution profile on FPLC Mono Q anion exchange column. The modified enzyme was desensitized to positive and negative effectors like glucose-6-phosphate and malate. Pretreatment of PEP carboxylase with diethylpyrocarbonate prevented WRK incorporation into the enzyme, suggesting that both histidine and carboxyl groups may be closely physically related. The carboxyl groups might be involved in metal binding during catalysis by the enzyme.