Class IV polyhydroxyalkanoate (PHA) synthases and PHA-producing Bacillus

Abstract

This review highlights the recent investigations of class IV polyhydroxyalkanoate (PHA) synthases, the newest classification of PHA synthases. Class IV synthases are prevalent in organisms of the Bacillus genus and are composed of a catalytic subunit PhaC (approximately 40 kDa), which has a PhaC box sequence ([GS]-X-C-X-[GA]-G) at the active site, and a second subunit PhaR (approximately 20 kDa). The representative PHA-producing Bacillus strains are Bacillus megaterium and Bacillus cereus; the nucleotide sequence of phaC and the genetic organization of the PHA biosynthesis gene locus are somewhat different between these two strains. It is generally considered that class IV synthases favor short-chain-length monomers such as 3-hydroxybutyrate (C4) and 3-hydroxyvalerate (C5) for polymerization, but can polymerize some unusual monomers as minor components. In Escherichia coli expressing PhaRC from B. cereus YB-4, the biosynthesized PHA undergoes synthase-catalyzed alcoholytic cleavage using endogenous and exogenous alcohols. This alcoholysis is thought to be shared among class IV synthases, and this reaction is useful not only for the regulation of PHA molecular weight but also for the modification of the PHA carboxy terminus. The novel properties of class IV synthases will open up the possibility for the design of new PHA materials.

Draft Genome Sequence of Bacillus aryabhattai Strain PHB10, a Poly(3-Hydroxybutyrate)-Accumulating Bacterium Isolated from Domestic Sewerage

“…In the present study, we investigated the whole-genome sequence of strain PHB10 for novel enzymes involved in PHB biosynthetic pathways. A typical PHB biosynthetic gene cluster with a class IV polyhydroxyalkanoate (PHA) synthase, reported previously in Bacillus megaterium , was identified in this strain (3, 4). …”

Genome Structure ofBacillus cereustsu1 and Genes Involved in Cellulose Degradation and Poly-3-Hydroxybutyrate Synthesis

“…PhaC is the key enzyme involved in the polymerization process; it determines the types of monomers (R-hydroxyacyl-CoAs) incorporated into the PHA polymer chain based on the enzyme's substrate specificity, as well as controlling PHA chain length and polydispersity. Previous researches consistently indicate that class IV synthases favor short-chain-length monomers such as 3-hydroxybutyrate (C4) and 3-hydroxyvalerate (C5) for polymerization, but it can also polymerize some unusual monomers as minor components [52][53][54]. There is an increasing interest in class IV PHA synthase, due to the possible alcoholysis activity as an inherent feature among these enzymes [55].…”

Study of Class I and Class III Polyhydroxyalkanoate (PHA) Synthases with Substrates Containing a Modified Side Chain

“…While class II synthases polymerize medium-chain-length (MCL) monomers (C 6 –C 14 ), all other classes prefer short-chain-length (SCL) monomers (C 3 –C 5 ) as the substrates. 18 Due to limited access to PhaC substrates, studying PHA synthases has been hampered significantly. Thus far, large-scale chemical synthesis of PhaC substrates was reported only for 3-( R )-hydroxybutyryl coenzyme A (HBCoA, R’ = Me in Scheme 1) and 3-( R )-hydroxyvaleryl coenzyme A (HVCoA, R’ = Et).…”