Abstract
The CAM plasmid-coded isoenzymic diketocamphane monooxygenases induced in Pseudomonas putida ATCC 17453 (NCIMB 10007) by growth of the bacterium on the bicyclic monoterpene (rac)-camphor are notable both for their interesting history, and their strategic importance in chemoenzymatic syntheses. Originally named ‘ketolactonase—an enzyme system for cyclic lactonization’ because of its characterised mode of action, (+)-camphor-induced 2,5-diketocamphane 1,2-monooxygenase was the first example of a Baeyer-Villiger monooxygenase activity to be confirmed in vitro. Both this enzyme and the enantiocomplementary (−)-camphor-induced 3,6-diketocamphane 1,6-monooxygenase were mistakenly classified and studied as coenzyme-containing flavoproteins for nearly 40 years before being correctly recognised and reinvestigated as FMN-dependent two-component monooxygenases. As has subsequently become evident, both the nature and number of flavin reductases able to supply the requisite reduced flavin co-substrate for the monooxygenases changes progressively throughout the different phases of camphor-dependent growth. Highly purified preparations of the enantiocomplementary monooxygenases have been exploited successfully for undertaking both nucleophilic and electrophilic biooxidations generating various enantiopure lactones and sulfoxides of value as chiral synthons and auxiliaries, respectively. In this review the chequered history, current functional understanding, and scope and value as biocatalysts of the diketocamphane monooxygenases are discussed.
Highlights
The significance of the large transmissible CAM plasmid present in Pseudomonas putida ATCC17453 (P, PpG1, C1, NCIMB 10007) in coding for a number of the enzymes necessary for the catabolism of the bicyclic monoterpene camphor was first recognised from the pioneering studies undertaken by Irwin Gunsalus’ research group at the University of Illinois, commencing in the late 1960s [1,2].The discovery evolved out of a conceptually broader programme of research initiated to study carbon cycling in the biosphere which focussed on a study of the biochemical challenges posed by the degradation of alicyclic camphor to short-chain aliphatic intermediates suitable to gain entry into the central pathways of metabolism via the tricarboxylic acid (TCA) cycle
This includes two very similar copies of the gene coding for 2,5-diketocamphane 1,2-monooxygenase, the highly enantioselective ketolactonase that participates in the catabolism of (+)-camphor, the more widely distributed isomer [6]
The studies of purified preparations of both 2,5-diketocamphane 1,2-monooxygenase (E2 ) and NADH oxidase/dehydrogenase (E1 ) undertaken at the University of Illinois throughout the 1960s were conducted invariably on enzymes sourced from camphor-grown cells of P. putida ATCC 17453 in the late log-early stationary phase of growth
Summary
The significance of the large transmissible CAM plasmid present in Pseudomonas putida ATCC. The seminal study of Iwaki et al [4] confirmed for the first time that along with the requisite genes for all of the other enzymes necessary to catabolise camphor to ∆2,5 -3,4,4-trimethylpimelyl-CoA, genes coding for each of the characterised monooxygenase-catalysed steps of camphor metabolism are located exclusively within the same 40.45-kb region of the large linear 533-kb CAM plasmid (Figure 2). This includes two very similar copies (camE25-1 [orf 4], camE25-2 [orf 22]) of the gene coding for 2,5-diketocamphane 1,2-monooxygenase, the highly enantioselective ketolactonase that participates in the catabolism of (+)-camphor, the more widely distributed isomer [6]. Iwaki et al [4], with the permission from American Society for Microbiology: licence number 4487631509592
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