Microbial Degradation of Xenobiotic Polymers

What does ''biodegradability of polymers'' mean? The first criterion for biodegradation is enzymatic processing. In particular, hydrolyzable polymers such as polyesters and polyamides are enzymatically degraded into monomers (depoly-merization process) that can easily enter central metabolic processes unless they are xenobiotic compounds. Most monomers are naturally occurring compounds, such as organic acids, alcohols/glycols, and amide compounds. Proteases, lipases, and esterases originating from animals, plants, and microbes can hydrolyze xenobiotic polymers such as polyamides and polyesters, and PLA, although their original substrates are proteinaceous compounds, lipids, and esters. This is well explained by the fact that enzymes cannot discriminate between original substrates and substrates analogous to them. For example, proteinase K can degrade PLA because lactic acid is analogous to alanine. The second and most important criterion for biodegradation is microbial degradation of target polymers, which are often assimilated by microorganisms as sole carbon and energy sources.

The biodegradability of many polymers has been investigated. The results are summarized in Table 16.1 (Kawai 2010b). Previous studies have revealed that depolymerization proceeds by two processes, exogenous and endogenous. Hydrolyzable polymers are readily depolymerized endogenously by hydrolytic enzymes to yield momoner units that are metabolized by central metabolic

Fig. 16.1 Chemical structures of xenobiotic polymers discussed in this article

pathways. Generally, esterases/lipases contribute to the hydrolysis of synthetic polyesters. PHA- and PLA depolymerases are also categorized as polyester-degrading enzymes, but their enantioselectivities and substrate specificities indicate that there are three categories of polyester-degrading enzymes, general lipases, PHA depolymerases, and PLA-degrading enzymes (Tokiwa and Calabia 2004), although some cross these boundaries. PHA- and PLA-degrading microorganisms have different specific depolymerases for PHA and PLA, respectively.

Table 16.1 Biodegradability of xenobiotic polymers and oligomers by enzymes and microorganisms

Chemical structure

Biodegradable molecular size

Degradation mechanism

Relevant enzymes/ microorganism

A. Polyethers Polyethylene glycol (PEG)

Polypropylene glycol (PPG) Polytetramethylene glycol (PTMG) Polyputylene oxide (PBO)

B. Vinyl polymers PEwax

Polyvinyl alcohol (PVA)

Polyacrylate (PAA)

Polyisoprene

Butadiene

Styrene

Acrylonitril

C. Polyamides Polyasparatate (PAS) Nylon

D. Polyesters Polycaprolactone (PCL)

Polylactic acid (PLA)

MW: 3,000-4,000 Dimer-octamer MW: 2,000

Tetramer-decamer

Dimer

Trimer

MW: ca. 20,000 Dimer-hexamer

Up to a few hundreds of thousands

Exogenous

Exogenous Exogenous Exogenous

Exogenous Endogenous

Exogenous

Endogenous/exogenous Endogenous/exogenous

Endogenous

Endogenous

Dehydrogenases; aerobic bacteria (anaerobic bacteria) Dehydrogenase: bacteria Dehydrogenase: bacteria Dehydrogenase; bacteria

Bacteria and fungi Oxidase/dehydrogenase and hydrolase: bacteria and fungi Bacteria Bacteria Baceria Bacteria Fungus

Hydrolases; bacteria Hydrolases; bacteria

Lipases and cutinases: bacteria and fungi Proteases and lipases (cutinases): actinomycetes and Bacillus

(continued)

Chemical structure

Biodegradable molecular size

Degradation mechanism

Relevant enzymes/ microorganism

Aliphatic

Endogenous

Lipases and cutianases: bacteria

and fungi

Aromatic

Endogenous

Cutinases

Aliphatic- co- aromatic

Endogenous

Hydrolase; actinomycetes and

Bacillus-rslatsd species

E. Polyurethane

Ether type

?

?

Ester type

Endogenous

Lipases: bacteria and fungi

Thus the biodegradability of a given polymer is not uniform, and its evaluation requires substantial insight, even if the basic chemical structures and underlying mechanisms are the same.

On the other hand, some synthetic polymers are oxidized repeatedly and are cleaved exogenously by one terminal monomer unit that is assimilated as a carbon source into the central metabolic pathway. Typical examples of exogenous depo-lymerization are the oxidative degradation of polyethers, PAA, and probably polyolefins by a series of oxidative steps for polyethers, and b-oxidation-like processes for PAA and polyolefins. PVA is degraded by a combination of primary oxidation and hydrolysis of oxidized PVA, as described below. Except for PVA, high molecular weight carbon-backbone polymers are recalcitrant to biodegradation, although their oligomers, including polyethylene wax (PEwax), isoprene oligomers, styrene oligomers, and triacrylonitrile, are biodegradable (Kawai 1995). Hetero-backbone polymers are more susceptible to biodegradation than carbon-backbone ones. Examples of the former are polyethers, such as PEG, most polyesters, including PHA and PLA, and polyamides, such as PAS (polyaspartate).

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