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Title: The metabolism and enzymology of aromatic ring degradation in nocardioform actinomycetes
Author: Cha, C. J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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Rhodococcus rhodochrous N75, a nocardioform actinomycete, metabolises methyl-substituted aromatic compounds via the modified ortho-cleavage pathway whereby a novel 4-methylmuconolactone methyl-isomerase catalyses the conversion of 4-methylmuconolactone to 3-methylmuconolactone in order to circumvent the accumulation of the 'dead-end' metabolite 4-methylmuconolactone which is found in some other species of bacteria. R. rhodochrous N75 was found to transform a range of alkyl-substituted catechols to the corresponding muconolactones since the two enzymes initiating the degradative pathway, catechol 1,2-dioxygenase and cis,cis-muconate cycloisomerase, possess broad substrate specificities. Catechol 1,2-dioxygenase was highly purified from cells of R. rhodochrous N75 grown at the expense of benzoate and p-toluate as the sole sources of carbon. A single catechol 1,2-dioxygenase was found to be induced with either grown substrate. The enzyme has an estimated Mr of 71,000 consisting of two identical subunits. Catechol 1,2-dioxygenase from R. rhodochrous N75 exhibits some unusual properties including: broad substrate specificity, extradiol cleavage activity with 4-methylcatechol and low Km values for halocatechols, suggesting that this enzyme is distinct from other known catechol and chlorocatechol 1,2-dioxygenases. cis,cis-Muconate cycloisomerase was purified to homogeneity from cells of R. rhodochrous N75 grown at the expense of benzoate and p-toluate as the sole sources of carbon. A single cycloisomerase was also found to be induced in this organisms with no isoforms being detected when R. rhodochrous N75 was grown on either benzoate and p-toluate. The enzyme is hexameric with a single subunit Mr of 40,000. cis,cis-Muconate cycloisomerase from R. rhodochrous N75 displays strict regio- and stereospecificity whereby cis,cis-muconate is cyclised to (3S)-muconolactone and 2-methyl- and 3-methyl-substituted muconates are cyclised to 2-methyl- and 4-methyl-substituted muconolactones by 1,4 - and 3.6-cyclisation respectively.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available