Modeling inhibitors in the active site of aspartate aminotransferase

Aspartate aminotransferase complexed with erythro-beta-hydroxyaspartate: crystallographic and spectroscopic identification of the carbinolamine intermediate

The crystal structure of mitochondrial aspartate aminotransferase (mAAT) of chicken complexed with erythro-beta-hydroxyaspartate has been determined at 2.4 A resolution. Pregrown crystals of mAAT complexed with the inhibitor maleate (closed enzyme conformation, orthorhombic space group C222(1)) were soaked in solutions of erythro-beta-hydroxyaspartate. The ligand exchange was monitored by microspectrophotometry. The active site turned out to be predominantly occupied by the carbinolamine intermediate. The carbinolamine is a true intermediate of the catalytic cycle forming the last covalently bound enzyme:substrate complex before release of the keto acid product. Occupancies of approximately 80% for the carbinolamine and of approximately 20% for the quinonoid intermediate were obtained. Two hydrogen bonds were identified that are potentially relevant for the accumulation of the carbinolamine intermediate: one to the hydroxyl group of Tyr 70* and the other to the epsilon-NH2 group of Lys 258.

Reaction of aspartate aminotransferase with L-erythro-3-hydroxyaspartate: involvement of Tyr70 in stabilization of the catalytic intermediates

The reaction of Escherichia coli aspartate aminotransferase (AspAT) with L-erythro-3-hydroxyaspartate (HOAsp) produces an intense absorption at 494 nm (epsilon = 13,650 M-1 cm-1), which is ascribed to the quinonoid intermediate. However, when Tyr70 of AspAT has been replaced by Phe, the enzyme shows only a faint absorption at 494 nm (epsilon = 522 M-1 cm-1) on the reaction with HOAsp. This indicates the involvement of the hydroxy group of Tyr70 in stabilizing the quinonoid intermediate formed from HOAsp and pyridoxal 5'-phosphate at the AspAT active site. Kinetic analysis of the absorption changes of the wild-type and Y70F mutant AspATs has shown that the reactions with HOAsp conform to the equation, EL + S<-->ES1<-->ES2<-->ES3<-->EM + P, in which there is a rapid formation of the quinonoid intermediate (ES2) from ES1, followed by a slow equilibrium between ES2 and ES3. ES3 absorbs primarily at 330 nm. The kinetic parameters for individual steps have been determined, and free energy profiles for the reactions of the two enzymes with HOAsp have been obtained. The stability of the quinonoid intermediates of the two enzymes in the normal catalytic reactions with aspartate has been assessed by static measurement of the spectra in the presence of both aspartate and oxalacetate, and the free energy profiles for the reactions have been similarly obtained. Comparison of the free energy levels in the profiles showed that the interaction of the beta-hydroxy group of HOAsp with the hydroxy group of Tyr70 accounts for 8.7 kJ mol-1 of the 18.5 kJ mol-1 stabilization of the quinonoid intermediate by the beta-hydroxy group. Model building of the active site of AspAT complexed with HOAsp suggests that the rest of the stabilization is mediated through the interaction of the beta-hydroxy group of HOAsp with the protonated epsilon-amino group of Lys258. This interaction is expected to strengthen the hydrogen-bonding network involving Tyr70, HOAsp, and the coenzyme phosphate. A similar network is possibly formed in the carbinolamine intermediate, suggesting ES3 to be the carbinolamine. A mechanism for the reaction of AspAT with HOAsp, which conforms to all the kinetic and spectroscopic data presented here, is proposed. This study provides a basis for subsequent spectroscopic characterization of the HOAsp-AspAT complex, which is a good model for the critical intermediate (quinonoid) structure of the AspAT-catalyzed reactions.

Site-directed mutagenesis of Escherichia coli aspartate aminotransferase: role of Tyr70 in the catalytic processes

Site-directed mutagenesis of Tyr70 in the active site of Escherichia coli aspartate aminotransferase (AspAT) followed by kinetic studies has elucidated the roles of the hydroxyl group and benzene ring of Tyr70. X-ray crystallographic analysis showed that replacement of Tyr70 by Phe did not alter the active-site conformation of the enzyme. Comparison of the kinetic parameters of the four half-transamination reactions (the pyridoxal 5'-phosphate form of the enzyme with L-aspartate or L-glutamate and the pyridoxamine 5'-phosphate form with oxalacetate or 2-oxoglutarate) between the wild-type and [Tyr70----Phe]AspATs showed that the mutation increases the energy level of the transition state by 2 kcal.mol-1 for all the four substrates, suggesting some contribution of the hydroxyl group of Tyr70 to the transition state. When Phe70 was further replaced by Ser, the energy level of the transition state for L-glutamate or 2-oxoglutarate, but not for L-aspartate or oxalacetate, was further increased by 2-3 kcal.mol-1, suggesting that the presence of a benzene ring at position 70 is essential for recognizing the L-glutamate-2-oxoglutarate pair as substrates.