Abstract
Natural rubber (NR), poly(cis-1,4-isoprene), is used in an industrial scale for more than 100 years. Most of the NR-derived materials are released to the environment as waste or by abrasion of small particles from our tires. Furthermore, compounds with isoprene units in their molecular structures are part of many biomolecules such as terpenoids and carotenoids. Therefore, it is not surprising that NR-degrading bacteria are widespread in nature. NR has one carbon-carbon double bond per isoprene unit and this functional group is the primary target of NR-cleaving enzymes, so-called rubber oxygenases. Rubber oxygenases are secreted by rubber-degrading bacteria to initiate the break-down of the polymer and to use the generated cleavage products as a carbon source. Three main types of rubber oxygenases have been described so far. One is rubber oxygenase RoxA that was first isolated from Xanthomonas sp. 35Y but was later also identified in other Gram-negative rubber-degrading species. The second type of rubber oxygenase is the latex clearing protein (Lcp) that has been regularly found in Gram-positive rubber degraders. Recently, a third type of rubber oxygenase (RoxB) with distant relationship to RoxAs was identified in Gram-negative bacteria. All rubber oxygenases described so far are haem-containing enzymes and oxidatively cleave polyisoprene to low molecular weight oligoisoprenoids with terminal CHO and CO–CH3 functions between a variable number of intact isoprene units, depending on the type of rubber oxygenase. This contribution summarises the properties of RoxAs, RoxBs and Lcps.
Highlights
Natural rubber (NR) is produced mainly by plants and is the characteristic and main component of rubber latex particles
The amino acid sequences of these rubber oxygenase B (RoxB) proteins include two typical binding motifs (CxxCH) for covalent attachment of c-type haem groups, a conserved F317 homologue and a MauG motif as present in all currently known rubber oxygenase A (RoxA) (Table 1) (Birke et al 2013). These findings suggest that the characterised and predicted RoxB proteins represent a separate subgroup of RoxA homologues
ODTD, the C15 oligoisoprenoid main cleavage product of RoxA-mediated rubber degradation, was formed only in minor amounts by RoxB35Y or RoxBNS21. These results indicated that RoxBs cleave rubber in an endotype reaction
Summary
Natural rubber (NR) is produced mainly by plants and is the characteristic and main component of rubber latex particles. The amino acid sequences of these RoxB proteins include two typical binding motifs (CxxCH) for covalent attachment of c-type haem groups, a conserved F317 homologue and a MauG motif as present in all currently known RoxAs (Table 1) (Birke et al 2013).
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