Biosynthesis of coenzyme M and the catabolism of halogenated aromatic compounds
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Methanogens, members of the domain Archaea, are unique in their ability to reduce carbon substrates to methane. Coenzyme M (CoM) is required in all methanogenic pathways. The biosynthesis of this coenzyme has been well studied in Class I Methanogens, but in Class II Methanogens, such as Methanosarcina acetivorans, little is known. The first step in the biosynthetic pathway might be catalyzed by cysteate synthase (CS), which converts phosphoserine to cysteate by the addition of sulfite. The 46 kDa enzyme was successfully purified from inclusion bodies and characterized. The identity of the product was confirmed by liquid chromatography-mass spectrometry (LC-MS) results as well as by derivatization of the reaction product coupled with high pressure liquid chromatography (HPLC) analysis. Kinetic analysis showed that the enzyme has a K [subscript m] of 0.43 mM for its substrate, phosphoserine, and a K [subscript m] of 0.05 mM for its required nucleophile, sulfite. Four compounds were found to be inhibitors and IC₅₀ values were determined. The results show that CS carries out a new reaction and narrows the gap in our knowledge of Class II Methanogen CoM biosynthesis. In the second part of this dissertation, five enzymes in a newly discovered but poorly characterized pathway for the degradation of halogenated aromatic compounds in Leptothrix cholodnii SP-6 were examined. The pathway reportedly culminates in the production of 2-chloroacetaldehyde, a well-known alkylating agent. In order to determine if 2-chloroacetaldehyde is produced and how the organism survives in its presence, the pathway intermediates are being identified. To this end, 4-oxalocrotonate tautomerase (4-OT), 4-oxalocrotonate decarboxylase (4-OD), vinylpyruvate hydratase (VPH), pyruvate aldolase (PA) and acetaldehyde dehydrogenase (AAD) were cloned, expressed and characterized. 4-OT was found to process the 5-(chloro)-2-hydroxymuconate, but only when the equilibrium was shifted by the addition of 4-OD and VPH. Steady state kinetic analysis showed that while there is a slight decrease in K [subscript m] for the halogenated substrate when compared to the non-halogenated substrate, indicating a difference in binding. There is also a 30-fold decrease in the turnover number, indicating a preference for the non-halogenated substrate. The identity of the product, 5-(chloro)-2-oxo-4-hydroxypentanoate, was verified by ¹H NMR spectroscopy. A stereochemical analysis was also carried out.