Feedstocks for the Future - ACS Publications - American Chemical

Chapter 19

Polyhydroxyalkanoate Production in Crops Gregory M. Bohlmann

Downloaded by UNIV OF GUELPH LIBRARY on August 6, 2012 | http://pubs.acs.org Publication Date: January 12, 2006 | doi: 10.1021/bk-2006-0921.ch019

Process Economics Program, SRI Consulting, Menlo Park, C A 94025

Polyhydroxyalkanoates (PHAs) are biodegradable polyesters produced by microorganisms as intracellular energy reserves. The metabolic pathway from these microorganisms can be bioengineered into a variety of plants for making PHAs. While this new scheme is not yet commercial, it may have the potential for large scale manufacture at very low cost. Numerous challenges, both technical and non-technical, are associated with commercializing this technology. One is to achieve a high level of polymer production in the plant without a decrease in crop yield. Another is to economically recover the polymer from the plant biomass. There are also barriers associated with utilization of agricultural infrastructure for production of industrial products. This paper presents an analysis of the process economics for producing PHAs in agricultural crops such as soybean or switchgrass. The economics are compared to those for P H A production by E. coli fermentation.

Introduction Biodegradable polyhydroxyalkanoates (PHAs) are synthesized by a wide variety of bacteria as a carbon reserve and electron sink. Bacterial fermentation has been employed as a commercial means of producing PHAs since their introduction in 1981. The metabolic pathways from these microorganisms can be bioengineered into a variety of plants for making PHAs. This new scheme for producing PHAs is not yet commercial. However, there has been significant research undertaken by government, academic and commercial organizations

© 2006 American Chemical Society

In Feedstocks for the Future; Bozell, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

253


254 over the past decade to develop the technology. The main rationale for the synthesis of P H A in plants is the potential for producing the polymer on a large scale at a cost lower than bacterial fermentation. One company active in this field hopes to drive P H A cost down to under $l/lb using bioengineered plants (')• Over 100 different monomers have been found to be included in bacterial PHAs and the metabolic pathways involved in the synthesis of a variety of these pathways have been elucidated by research scientists. Through bioengineering, it is possible to modify plant cells to incorporate PHA metabolic pathways from bacteria. The chemical diversity of P H A translates into a wide spectrum of physical properties, ranging from stiff and brittle plastics to softer plastics, elastomers, rubbers and glues. The major diversity is in the length and presence of functional groups in the side chain of the polymer. Table 1 summarizes selected important PHAs.

Table I. Selected P H A s PHA Polyhydrobutyrate (PHB) Polyhydoxyvalerate (PHV) Polyhydroxybutyrate-valerate (PHBV, Biopol) PHBHx (Kaneka) PHBO (Nodax) PHBOd

Side Chain -CH -CH -CH -CH -CH -CH

3

3

3

3

3

2

CH and and and and

3

-CH CH -CH CH CH -(CH ) CH -(CH ) CH 2

3

2

2

2

4

2

14

3

3

3

Source: Reproduced with permission from reference 2. Copyright 2003 Wiley. PHB is a stiff and brittle polymer, so a variety of other copolymers have been developed that have physical properties more suitable for commercial applications. Copolymerization of 3-hydroxybutyrate with 3-hydroxyvalerate or with medium chain length hydroxyalkanoate units is effective for improving the brittleness. One of the first pioneer companies to develop a biodegradable polymer that was completely biodegradable and also have good properties was ICI in the United Kingdom with its P H A product known as BIOPOL™, commercialized in 1981. Monsanto purchased the BIOPOL™ business from Zeneca Bio Products (formerly ICI) in April of 1996. Monsanto manufactured P H B V at its Knowsley, England fermentation facility until 1999. A t that time Monsanto elected to exit the biodegradable polymers business after failing to make BIOPOL™ costs more competitive with petroleum based polymers. Monsanto indicated that it cost roughly $8.8 to make a kilogram ($4/lb) of P H B V through fermentation and



255 that its goal of using bio-engineered crops to lower production costs was at least 5-7 years away (3). In 2001 Metabolix purchased Monsanto's Biopol patent assets to add to its P H A technology portfolio. Metabolix is a research company that does not own or operate commercial facilities for P H A production. In 2002 the company's fermentation technology was demonstrated at the 50,000 liter scale to produce PHA. Metabolix indicates that potential production costs are well under $ 1 /lb for its fermentation process (4). Metabolix also has R & D efforts towards bioengineering plants to produce PHAs. The efforts are partly funded by the DOE. In 2003 B A S F and Metabolix entered into a research collaboration agreement on PHAs. Metabolix will produce P H A from sugar using fermentation technology and supply B A S F with pilot scale quantities to investigate the material properties. Metabolix indicates that planting of large field acreages for production from crops is likely to be at least five years away (5). Metabolix hopes to drive P H B V cost down to under $ 1/lb using bioengineered plants (/). Procter & Gamble has developed a family of PHAs known as Nodax™. P & G plans to license different aspects of the production and application of Nodax. P & G and Kaneka Corp. in Japan are jointly developing PHAs, including poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH). The companies are producing P H B H at a rate of 50 kg/week at two locations in the United States and are considering sites in Europe and in Asia for a 30,000 metric tons per year plant (r5).

Metabolic Pathways Over 40 bacteria harbor the genes which are related to P H A biosynthesis. In addition, more than 200 microorganisms can use PHAs as an energy source, which makes P H A entirely biodegradable in a wide range of environmental conditions ranging from soil to sea water. The wealth of knowledge concerning the various bacteria which can biosynthesize P H A is a valuable starting point for bioengineering plants towards this end. Bacteria synthesizing PHAs have been broadly subdivided into two groups based on monomer chain length. Metabolic pathways have been identified for both major categories of PHAs: •

•

Short chain length (SCL) P H A s that contain 3-hydroxyacid monomers ranging from three to five carbons in length. P H B and P H B V are both included in this category. The most prominent bacteria with a S C L metabolic pathway is Ralstonia eutropha. Medium chain length ( M C L ) P H A s that contain 3-hydroxyacid monomers ranging from six to sixteen carbons in length. A number of Pseudomonads have M C L metabolic pathways.



256 The division between S C L and M C L P H A is mainly determined by the substrate specificity of the polymerization enzyme in the organism. The division is not strict since several bacteria have been found that can synthesize a "hybrid" P H A that can include monomers from four to eight carbons (7). A number of enzymes and metabolic pathways have been identified by scientists. P H A synthases are the key enzymes of P H A biosynthesis in all of these pathways. These enzymes catalyze the covalent linkage between the hydroxyl group of one and the carboxyl group of another hydroxyalkanoic acid. The substrates of P H A synthases are the coenzyme A thioesters of hydroxyalkanoic acids. There is no evidence that P H A synthases can utilize either free hydroxyalkanoic acids or other derivatives. Most P H A synthases incorporate either short carbon chain length hydroxyalkanoic acids with 3-5 carbon atoms or medium carbon chain length hydroxyalkanoic acids with 6-16 carbon atoms. There are only a few P H A synthases which can incorporate both short chain and medium chain monomers. Examples are the P H A synthases of T. pfennigii and Aeromonas caviae (8). PHB is the most widespread and thoroughly characterized P H A found in bacteria. A large part of the current knowledge on P H B biosynthesis has been obtained from Ralstonia eutropha (formerly Alcaligenes eutrophus) (7). In this bacterium, P H B is synthesized from acetyl-coenzyme A (CoA) by the sequential action of three enzymes (see Figure 1): •

3-Ketothiolase catalyzes the reversible condensation of two acetyl-CoA moieties to form acetoacetyl-CoA. The acetyl-CoA metabolite is naturally available in many plants and bacteria.

•

Acetoacetyl-CoA reductase subsequently reduces acetoacetyl-CoA to R-(-)3-hydroxybutyryl-CoA.

•

P H A synthase polymerizes R-(-)-3-hydroxybutyryl-CoA to form P H B .

Transgenic Plants Advances in plant genetic engineering, combined with growing concerns about the environment and decreasing petroleum reserves have created new interest to use plants for the large scale production of renewable chemicals and polymers. Good examples of commodity chemicals produced efficiently in plants are starch and oils, which both can be harvested for food and non-food uses. Recent progress in molecular biology and plant transformation has enabled the creation of plants that have the capacity to produce new industrially useful products that are not naturally found in plants.


257

PHB PATHWAY acetyl-CoA


CH

S-CoA

3

C o A S H

Feedstocks for the Future - ACS Publications - American Chemical

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