Wj. Page et J. Manchak, THE ROLE OF BETA-OXIDATION OF SHORT-CHAIN ALKANOATES IN POLYHYDROXYALKANOATE COPOLYMER SYNTHESIS IN AZOTOBACTER-VINELANDII UWD, Canadian journal of microbiology, 41, 1995, pp. 106-114
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.