Crystallization and preliminary X-ray diffraction studies of dialkylglycine decarboxylase, a decarboxylating transaminase

Aminophosphonate inhibitors of dialkylglycine decarboxylase: structural basis for slow binding inhibition

The kinetics of inhibition of dialkylglycine decarboxylase by five aminophosphonate inhibitors are presented. Two of these [(R)-1-amino-1-methylpropanephosphonate and (S)-1-aminoethanephosphonate] are slow binding inhibitors. The inhibitors follow a mechanism in which a weak complex is rapidly formed, followed by slow isomerization to the tight complex. Here, the tight complexes are bound 10-fold more tightly than the weak, initial complexes. The slow onset inhibition occurs with t(1/2) values of 1.3 and 0.55 min at saturating inhibitor concentrations for the AMPP and S-AEP inhibitors, respectively, while dissociation of these inhibitor complexes occurs with t(1/2) values of 13 and 4.6 min, respectively. The X-ray structures of four of the inhibitors in complex with dialkylglycine decarboxylase have been determined to resolutions ranging from 2.6 to 2.0 A, and refined to R-factors of 14.5-19.5%. These structures show variation in the active site structure with inhibitor side chain size and slow binding character. It is proposed that the slow binding behavior originates in an isomerization from an initial complex in which the PLP pyridine nitrogen-D243 OD2 distance is approximately 2.9 A to one in which it is approximately 2.7 A. The angles that the C-P bonds make with the p orbitals of the aldimine pi system are correlated with the reactivities of the analogous amino acid substrates, suggesting a role for stereoelectronic effects in Schiff base reactivity.

Similarity between serine hydroxymethyltransferase and other pyridoxal phosphate-dependent enzymes

A structural homology of the pyridoxal-5'-phosphate (PLP)-dependent enzyme serine hydroxymethyltransferase (SHMT) with aspartate aminotransferase (AAT) is proposed. Although the two sequences are very dissimilar, a reasonable alignment was obtained using the profile analysis method. Sequences of AAT and dialkylglycine decarboxylase (DGD), for which crystal structure data are available, have been aligned on the basis of their structure superposition. A profile was then calculated and SHMT sequence aligned to it. Three of the four residues conserved in all aminotransferases (including the PLP-binding lysine) are matched. A profile search with DGD-AAT-SHMT profile is more selective and sensitive than individual sequence profiles for PLP-dependent enzyme detection. Potential homologies with the eryC1 gene product involved in erythromycin biosynthesis and with amino acid decarboxylases were observed. Homology with AAT will be used as a guideline for planning site-directed mutagenesis experiments on SHMT.

Dialkylglycine decarboxylase structure: bifunctional active site and alkali metal sites

The structure of the bifunctional, pyridoxal phosphate-dependent enzyme dialkylglycine decarboxylase was determined to 2.1-angstrom resolution. Model building suggests that a single cleavage site catalyzes both decarboxylation and transamination by maximizing stereoelectronic advantages and providing electrostatic and general base catalysis. The enzyme contains two binding sites for alkali metal ions. One is located near the active site and accounts for the dependence of activity on potassium ions. The other is located at the carboxyl terminus of an alpha helix. These sites help show how proteins can specifically bind alkali metals and how these ions can exert functional effects.