Detection of intermediates from the polymerization reaction catalyzed by a D302A mutant of class III polyhydroxyalkanoate (PHA) synthase.

Polyhydroxybutyrate (PHB) synthases catalyze the polymerization of (R)-3-hydroxybutyryl-CoA (HB-CoA) into high molecular weight PHB, biodegradable polymers. The class III PHB synthase from Allochromatium vinosum is composed of a 1:1 mixture of two approximately 40 kDa proteins: PhaC and PhaE. Previous studies using site-directed mutagenesis and a saturated trimer of hydroxybutyryl-CoA have suggested the importance of C149 (in covalent catalysis), H331 (in activation of C149), and D302 (in hydroxyl group activation for ester bond formation) in the polymerization process. All three residues are located on PhaC. We now report that incubation of D302A-PhaCPhaE with [14C]-HB-CoA results in detection, for the first time, of oligomeric HBs covalently bound to PhaC. The reaction products have been analyzed by SDS-PAGE, Westerns with PhaCPhaE antibodies, and autoradiography. Different migratory properties of D302A-PhaC on SDS-PAGE have been observed at [14C]-HB-CoA to enzyme (S/E) ratios between 5 and 100. Trypsin digestion and HPLC analysis of the D302A-PhaCPhaE (from a reaction with a S/E ratio of 5) allowed isolation of multiple radiolabeled peptides. N-Terminal sequencing, MALDI-TOF, and ESI mass spectrometric analysis of these peptides revealed that all of the peptides were identical but were modified by (HB)n ranging in size from n = 3 to n = 10. The in vitro results support the role of D302 in elongation rather than in activating the active site cysteine for acylation. This proposal has been further supported by our in vivo studies on a Wautersia eutropha strain in which the class I synthase gene has been replaced with the D302A-PhaCPhaE gene and the organism examined under PHB production conditions by transmission electron microscopy. Very small granules (<0.05 microm) were observed in contrast to the 0.2-0.5 microm granules observed with the wt strain. Use of the D302A synthase has allowed successful interrogation of the initiation and elongation steps catalyzed by the class III synthase.