THE ROLE OF BETA-OXIDATION OF SHORT-CHAIN ALKANOATES IN POLYHYDROXYALKANOATE COPOLYMER SYNTHESIS IN AZOTOBACTER-VINELANDII UWD

Valerate and other short-chain, uneven-length fatty acids promoted the formation of the polyhydroxyalkanoate (PHA) copolymer poly(3-hydroxyb utyrate-co-3-hydroxyvalerate) in Azotobacter vinelandii UWD growing in glucose medium. The uptake of valerate was inducible, being repressed by acetate but not by glucose. A likely route that would direct valer ate into PHA synthesis involved the beta-oxidation pathway. The short- chain fatty acids butyrate, valerate, trans-2-pentenoate, crotonate, h exanoate, heptanoate, and octanoate induced the coordinate production of the beta-oxidation enzymes enoyl-CoA hydratase (ECH) and L-(+)-3-hy droxybutyryl-CoA dehydrogenase (HAD). trans-3-Pentenoate was the best inducer of these activities. which suggested that the isomerase of the beta-oxidation complex also was present. However, 3-hydroxyacyl-CoA e pimerase activity of the beta-oxidation complex was not detected. 3-Ke toacyl-CoA thiolase activity was constitutive in A. vinelandii and app eared to associate only loosely with the 73 000 Da ECH-HAD complex. Th us, 3-ketoacyl-CoA, the end product of HAD activity, could be directed into PHA synthesis through acetoacetyl-CoA reductase generating the 3 -hydroxyvalerate subunit of the polymer. When valerate was the sole ca rbon source, the incorporation of valerate into the polymer was normal , but most of the valerate was directed into metabolism and very littl e PHA was formed. When glucose also was present, the beta-oxidation of short-chain alkanoates inhibited the specific activity of acetoacetyl -CoA reductase and 3-ketothiolase and the PHA yield, A model for PHA s ynthesis was developed that suggests that the use of fatty acids to pr omote PHA copolymer formation in A. vinelandii will inevitably result in decreased PHA yield.