3-Hydroxybutyrate dimer hydrolase

Hydroxybutyrate Dimer-Hydrolases and 3-Hydroxybutyrate Oligomer-Hydrolases... 

Fig. 3. (a) Biosynthesis of SCL PHA 1,3-ketothiolase 2, NADPH-dependent acetoacetyl-CoA reductase 3, SCL PHA polymerase 4, SCL PHA depolymerase 5, d(-)-3-hydroxybutyrate-dimer hydrolase, (b) Biosynthesis of P(HB-co-HV), 1, 3-ketothiolase 2a, NADPH-dependent acetoacetyl-CoA reductase 2b, NADH-dependent acetoacetyl-CoA reductase 3, SCL PHA polymerase 4, fatty acyl-CoA dehydrogenase 5, enoyl-CoA hydratase. 

Intracellular PHB Depolymerase of Rhodospirillum rubrum. R. rubrum possesses a putative intracellular PHB depolymerase system consisting of a soluble PHB depolymerase, a heat-stable inactivator and a 3-hydroxybutyrate dimer hydrolase. It consists of one polypeptide of around 35 kDa and has a pH optima of 9 and temperature optima of 55 °C. The purified enzyme was inactive with dPHB. It does not show lipase, protease or esterase activity with p-nitrophenyl fatty acid esters. It is highly substrate specific. ... 

Tanaka Y, Saito T, Fukui T, Tanio T, Tomita K (1981), Purification and properties of D(-)-3-hydroxybutyrate-dimer hydrolase Ifom Zoogloea-ramigera I-16-M , Eur J Biochem, 118, 177-182. 

PhaZ Poly(3-hydroxybutyrate) depolymerase PhaZc Hydroxybutyrate-dimer hydrolase... 

The first products of enzymatic hydrolysis of poly(3HB) by purified poly(3HB) depolymerases are a mixture of monomeric and/or oligomeric 3-hydroxybuty-rate esters. Some enzymes are able to hydrolyze oligomers and dimers to monomeric 3-hydroxybutyrate after prolonged time of hydrolysis in the presence of an excess of the appropriate depolymerase. These poly(3HB) depolymerases have high endogenous dimer-hydrolase activities (e.g., the poly(3HB) depolymerases of Comamonas strains, P. stutzeri, S. exfoliatus, and the depolymerases... 

So far, biosynthesis of PHA can be summarized in eight pathways (Fig. 4, Table 1). The first pathway involves the three key enzymes (3-ketothiolase, NADPH-dependent acetoacetyl-CoA reductase, and PHA synthase encoded by genes phaA, phaB, and phaC, respectively. Ralstonia eutropha is the representative of this pathway. An associated pathway involving PHA degradation catalyzed by PHA depolymerase, dimer hydrolase, 3-hydroxybutyrate dehydrogenase, and acetoacetyl-CoA synthase helps regulate PHA synthesis and degradation. The associated pathway was found in strains of Aeromonas hydrophila. Pseudomonas stutzeri, R. eutropha, and Pseudomonas oleovorans (Sudesh et al. 2000). 

Biodegradation in the environment involves extracellular enzymes secreted by microorganisms, and these enzymes have been collectively described as PHA depolymerases. Of the characterized depolymerases, most of them hydrolyze the PHA polymer into dimers, and the dimers are further hydrolyzed to the monomers by an extracellular or intracellular dimer hydrolase. The depolymerase from a Comamonas species, however, appears to hydrolyze poly(3HB) directly to 3-hydroxybutyrate (reviewed by Hocking and Marchessault 1994). Not surprisingly, the degree of crystallinity significantly affects the rate of PHA degradation, and it was concluded that poly(3HB) molecules in the amorphous state are more easily hydrolyzed than poly(3HB) in the crystalline state (reviewed by Steinbiichel 1996). 

---