Plastic-degrading enzyme

Recently, the production of a biodegradable plastic-degrading enzyme from cheese whey by the phyllosphere yeast Pseudozyma antartica GB-4(1)W was studied [25]. It was reported that this yeast produces a cutinase-like enzyme, PaE, which has the ability to degrade biodegradable plastics. This enzyme was reported to have the ability to degrade several bioplastics such as PBS, polybutylene succinate-co-adipate, poly(e-caprolactone) and polylactic acid. Fed-batch cultivation of this yeast in xylose resulted in the production of PaE with high productivity. 

Maeda H, Yamagata Y, Abe K, Hasegawa F, Machida M, Ishioka R, Gomi K, Nakajima T (2005) Purification and characterization of a biodegradable plastic-degrading enzyme from Aspergillus oryzae. Appl Microbiol Biotechnol 67 778-788... 

Most man-made polymers are resistant to biological degradation because their carbon components cannot be broken down by the enzymes of microorganisms. In addition, the hydrophobic character of plastics inhibits enzyme activity, and the low surface area of plastics along with their inherent high molecular weight (MW), further compounds their resistance to microbial attack [1]. In the past two decades, biodegradable polymers have... 

Compared with tar, which has a relatively short lifetime in the marine environment, the residence times of plastic, glass and non-corrodible metallic debris are indefinite. Most plastic articles are fabricated from polyethylene, polystyrene or polyvinyl chloride. With molecular weights ranging to over 500,000, the only chemical reactivity of these polymers is derived from any residual unsaturation and, therefore, they are essentially inert chemically and photochemically. Further, since indigenous microflora lack the enzyme systems necessary to degrade most of these polymers, articles manufactured from them are highly resistant or virtually immune to biodegradation. That is, the properties that render plastics so durable... 

For the reasons stated above, deep intrusion of degrading microbes into polysaccharide-plastic films is demonstrably and theoretically improbable. Since starch removal does occur when the films are buried in soil, the primary mechanism must be microbial production of amylase in or near a pore, diffusion of the enzyme into pores and diffusion of soluble digestion products back to the surface where they are metabolized (Figure 3). This mechanism would be the only choice when the pore diameter is too small to admit a microbial cell (i.e., at diameters < 0.5 /im). An alternative mechanism could be diffusion of a water-soluble polysaccharide to the film surface, at which point degradation would occur. None of the materials used in these investigations showed loss of starch even when soaked in water for extended periods with microbial inhibitors present. Therefore, diffusion of amylase to the substrate rather than diffusion of the substrate to the film surface is the more likely mechanism. 

Because enzymes are too big to diffuse into the bulk of a polymer, biodegradation is an erosion process that takes place at the surface of the plastic article [5]. Therefore, the thickness of a plastic article is a decisive parameter in determining the time needed for complete degradation. 

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