Environmental Engineering Reference
In-Depth Information
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C.
P
OLYHYDROXYALKANOATES
1. Introduction
A variety of bacteria synthesize PHAs as intracellular carbon and energy storage
materials, much as higher organisms synthesize starch, glycogen, or fat for energy storage.
PHAs were first observed as dark-staining bodies within microbes observed under
microscopes, and in 1925, poly(3HB), was first isolated from Bacillus megaterium by
Lemoigne [1, 2]. Because they are bacterial storage polymers, PHAs are of necessity
biodegradable, and further study revealing their plasticity spurred commercial interest. In
1974, Wallen and Rohwedder predicted that novel hydroxyalkanoate (HA) subunits could
become important to further development of the microbial polyester, due to the ability of
varying structural units within poly(3HB) to modify polymer properties [3].
Research investigating the identities and characteristics of various units within microbial
PHAs began in earnest during the 1980s, when a number of bacteria were found to synthesize
optically active homopolymers and copolymers of (R)-3-hydroxyalkanoates [(R)-3HAs]
ranging from 4 to 14 carbon atoms [4-9]. Saturated, unsaturated, halogenated, branched, and
aromatic side chains in (R)-3HA monomeric units have now been found within microbial
PHAs; at present, at least 140 different monomeric units have been found as constituents of
microbial PHAs [10-12]. In addition, certain bacteria also produce copolymers containing
hydroxyalkanoate repeat units with side chains such as 3-hydroxypropionate and 4-
hydroxybutyrate [13, 14]. Subsequently, many different approaches have been taken to
produce PHA materials with desirable physical properties [15-17].
2. PHA Biosynthesis, Biodegradation, and Environmental Impact: Overview
The biosynthetic routes to PHA monomer synthesis are interconnected with several
central metabolic pathways: the tricarboxylic acid (TCA) cycle, fatty acid degradation (β-
