In 1920 Maurice Lemoigne discovered Poly 3-hydroxybutyrate (or PHB) which is biodegradable.This polymer is special type of Polyhydroxyalkanoates(PHA).They are used in the manufacture of the bio degradable plastic. The bacteria called Alcaligenes eutrophus naturally synthesizes PHB by series of enzymatic reactions.
Alcaligenes eutrophus is an aerobic ,hydrogen oxidizing,gram negative and non –spore forming bacillus.Their optimum growth temperature is 30 degree Celsius.There are few more names for this bacteria.They are Ralstonia eutrophus,Wusteria eutrophus,Ralstonia metallidurans,and Cuprivadus necator.Often reffered as Ralstonia metallidurans and Alcaligenes eutrophus.This bacteria resistant to the small concentrations of some toxic metals such as Zn,Cd,Co,Pb,Hg,Ni,and Cr.
Polyhydroxybutyrate
There are three regulatory enzymes important for the PHB producing process in A.eutrophus.3-ketothiolase,acetoacetyl-CoA reductase,and PHA synthatase.In bacteria genome this enzymes encoded by the genes phbA,phbB,phbC respectively. According to molecular genetic experiments which done by using E.coli,Greater than 50% of PHB production was observed only when these three genes present in E.coli. PHB is a type of Polyhydroxyalkanoate.(PHA) a polymer belonging to the polyester class.PHB rigid,fragile thermoplastic polymer.It is highly crystalline unlike many other biodegradable polymers. It’s resistant to moisture,impermeable to oxygen and insoluable in water,like most polymers.It has poor resistance for acids and bases and good ultraviolet resistance. PHB’s melting point is 175 degree celsius.It is very close to degrading temperature(185 degree celsius). This is the reason for PHB is so fragile.This fragility can be lowered by mixing PHB with more elastic polymer or integrading co-monomer 3-hydroxyvalerate, which is one of the developed forms of PHB.When 3-hydroxyvalerate units increasing in the structure,increases PHB’s elongation to break , become more elasticity,tougher.Melting point is also lowers by more 3-hydroxyvalerate units.
Production of PHB in Alcaligenes eutrophus
There are three regulatory enzymes important for the PHB producing process in A.eutrophus.3-ketothiolase,acetoacetyl-CoA reductase,and PHA synthatase.In bacteria genome this enzymes encoded by the genes phbA,phbB,phbC respectively. According to molecular genetic experiments which done by using E.coli,Greater than 50% of PHB production was observed only when these three genes present in E.coli.
In Alcaligenes eutrophus the synthesis of PHB involves glucose as a carbon source,three enzymes and NADPH as areducing agent.Firstly acetyl-CoA converts into acetoacetyl-CoA by 3-ketothiolase.Then acetoacetyl-CoA reductase reduces acetoacetyl-CoA into R-3-hydroxybutynl-CoA.In this reaction NADPH act as the reducing agent.Then R-3-hydroxybutynl-CoA polymerized into PHB by PHB synthatase. Alcaligenes eutrophus can produce an amount of PHB equal to 80% of their dry body weight when grown in sugar rich but nitrogen and phosphate deprived media. Alcaligenes eutrophus can produce different kinds of polymers by changing carbon source.
Today PHB synthesized by using genetically modified E.coli more efficiently than A.eutrophus .PHB produced by recombinant E.coli has 95% purity and 96% recovery,about 16% higher than the recovery of A.eutrophus.
