Conversion of PHB

FIGURE 3.2 The molecular weight conversion of PHB and PHBV films during... 

FIGURE 5.2 The molecular weight conversion of PHB and PHBV films during hydrolysis in phosphate buffer (PBS), pH = 7.4, 37 °C and 70... 

Mutational effects of PHB polymerase on PHB accumulation was reported by Taguchi et al. The assay system consists of a PCR-mediated random mutagenesis and two assay procedures based on plate assay and High Performance Liquid Chromatography (HPLC) assay based on the conversion of PHB to crotonic acid. The level of PHB accumulation, an activity estimation of the R. eutropha polymerase would be efficiendy achieved by monitoring the level of PHB accumulation using this in vivo assay system. 

Many researchers are investigating the conversion of these two feedstocks. It offers the possibility for a future technical process in which PHB is produced without side-products in an analogous way to the S3mthesis of poly(olefins). Moreover, the use of low-cost monomers from the established value-added chain generally avoids high-price renewable raw materials or discussion about food acreages. 

Inspired by these results, Nicolaou et al. used a temporary boronate tether to overcome the undesired regiochemical bias in the intermolecular Diels-Alder reaction of diene 56 and dienophile 57, required for their approach to the C-ring of taxol 58 [21a, b, 24], Thermolysis of a benzene solution of 56 and 57 in the presence of PhB(OH)2 under dehydrating conditions provided the cycloadduct 59 in 79% yield (77% conversion) after transacetalization of the boron tether with 2,2-dimethylpropane-l,3-diol and intramolecular acyl transfer to the less strained [4.3.0]bicyclic system. This was further elaborated to an advanced intermediate in the ultimately successful synthesis of taxol (Scheme 10-21) [21c]. 

The advantage of starch as a carbon source is that its price is lower than that of glucose. Choi and Lee (1999) estimated that on a production scale of 100,000 tons of PHB per year, production costs would decrease from US 4.91 to 3.72 kg if hydrolysed com starch (US 0.22 kg" ) were used instead of glucose (US 0.49 kg ). Most processes for PHA production based on starch require the conversion of starch to easily convertible substrates such as glucose by enzymatic or chemical hydrolysis (Chen et al. 2006 Huang et al. 2006). Alternatively, VFAs can be produced as fermentation substrates by acidogenesis (Yu et al. 2002). The production of P(3HB-co-3HV) by H. mediterranei on extruded starch in a pH-stat fed-batch mode was recently described by Chen et al. (2006). Here, an exogenous source of a-amylases was used. 

The first step of the biochemical pathway of PHB synthesis consists in the conversion of a selected carbon source to acetate. Then, an enzyme cofactor is attached via the formation of a thioester bond. The enzyme, called coenzyme A (CoA), is a universal carrier of acyl groups in biosynthesis and acetyl-CoA is a basic metabolic molecule found in all PHA-producing organisms. A dimer acetoacetyl-CoA is formed via reversible condensation and subsequently reduced to a monomer unit (R)-3-hydroxybutyryl-CoA. PHB is formed via the polymerization of the latter, maintaining the asymmetric centre [5]. The basic simplified process is shown in Scheme 22.1. 

The biochemical pathways of PHB biosynthesis are reasonably well understood and start with the conversion to acetate of an appropriate carbon substrate, such as low alcohols and acids, sugars or even gaseous mixtures of hydrogen and carbon dioxide. " The bacteria put a handle on the acetate molecules in the form of an enzyme cofactor linked by a thioester bond. Two molecules of this acetyl coenzyme-A can then be condensed to give acetoacetyl coenzyme-A which is subsequently reduced to 3-hydroxybutyryl coenzyme-A. The final polymerase enzyme joins these units together to form PHB, and recycles the cofactor as shown in Fig. 2. 

The final thermal transition in PHB is associated with degradation between 250 and 300°C. ° Thermogravimetric analysis shows complete weight loss in a single step between these temperatures and corresponds to quantitative conversion of the polymer to crotonic acid. ... 

The PHB synthesis is nothing but the conversion of acetyl-Co A to PHB as a mechanism for storing carbon. The P(3HB) biosynthesis, is the three step biosynthesis pathway, mainly consists of three enzymatic reactions catalyzed by three distinct enzymes. The enzymes are P-ketothiolase, acetoacetyl-CoA reductase and PHB synthase. 

On second thought, since glyceric acid is the reference molecule for carbohydrates, why should nature refrain from stereoregular conversion of simple hydroxyacids into polyesters Formula II shows the structural formula of poly-6-hydroxybutyrate (PHB) which is the main reserve material in bacteria when these are fed a nitrogen deficient carbohydrate rich diet. 

The most common bio-aliphatic polyesters are poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), polyfe-caprolactone) (PCL), and poly(3-hydroxybutyrate) (PHB). While PLA, PLGA, and PHB are bio-based, PCL is obtained from the chemical conversion of crude oil. But all of them are biodegradable and biocompatible. Thus, they attract increasing attention in biomedical applications. However, the biopolyesters present poor thermal stability with respect of traditional oil-based polyesters. 

Enzymatically hydrolyzed potato processing waste has been studied as a possible source of a fermentable substrate for the production of PHB by R. eutropha. The results indicated that potato starch waste could be converted with high yield to a concentrated glucose solution. The most economical process used barley malt as a source of amylase enzyme with an optimal ratio of 10 90 g g of potato waste. A conversion efficiency of 96% of the theoretical value was obtained with a final glucose concentration of 208 g L After dilution and addition of mineral salts the hydrolysate was converted by a batch culture to 5.0 g L of PHB, comprising 77% of the cell dry weight [241]. 

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