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BIOSYNTHETIC PLASTICS ARE ALSO BIODEGRADABLE
Plastics are polymers built from chains of monomer subunits, like proteins and nucleic acids. However, most plastics consist of the same monomer mindlessly repeated over and over again. Sometimes two or more closely related monomers may be mixed together and follow each other at random.
Certain bacteria make and store a group of related plastics known as polyhydroxyalkanoates (PHAs). Their composition is shown in Fig. 13.23. The bacteria accumulate PHAs when they have surplus carbon and energy but are running low on other essential nutrients, such as nitrogen or phosphorus. When conditions improve, the PHA is broken down and used as a source of energy. The most commonly found PHA has four-carbon (hydroxybutyrate, HB) subunits and is therefore called polyhydroxybutyrate (PHB). However, a plastic made by randomly mixing in 10% to 20% of five-carbon (hydroxyvalerate, HV) subunits has much better physical properties. Still other PHAs containing a proportion of subunits that are eight-carbon or longer give materials that are more rubbery.
A poly-HB/HV copolymer is manufactured by mutant bacteria of the species Alcaligenes eutrophus and is marketed by the Zeneca Corporation (United Kingdom) under the trade name of Biopol. It is more expensive than plastics made from oil, but it is completely biodegradable. Consequently, PHAs are restricted at present to specialized uses. For example, because they break down slowly inside the body to give natural, biochemical intermediates, they can be used for making slow-release capsules.
To make PHAs economically competitive, they will need to be produced cheaply and in bulk. One possible way to do this is to insert the genes for their synthesis into suitable crop Chloroplast plants. This is still in the experimental stage, but the genes for making PHA from Alcaligenes eutrophus have been successfully inserted into Arabidopsis, a plant widely used for genetic experiments. The engineered PHA genes were designed to be expressed inside the chloroplasts of the plant (Fig. 13.24). Because chloroplasts are the sites of photosynthesis, newly synthesized organic matter appears here first. Locating the PHA pathway in chloroplasts, rather than in the main compartment of the plant cells, gives a 100-fold increase in PHA yield. The next step will be to move the pathway into a genuine crop plant such as rapeseed or soybeans. Perhaps this will give new meaning to the phrase “plastic flowers”!
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