Kinetics and properties of β-ketothiolase from Clostridium pasteurianum
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Ketothiolase of Clostridium pasteurianum was purified 130-fold by ammonium sulphate fractionation and by column chromatography using DEAE-Sephadex A-50 and hydroxylapatite. Subjected to gel electrophoresis β-ketothiolase revealed two distinct bands; by isoelectric focusing two enzymes with isoelectric points at pH 4.5 and 7.6 were separated. As established by sucrose density gradient centrifugation the molecular weight of both enzymes was found to be 158000.
The condensation reaction was measured by a coupled optical test using β-hydroxybutyryl-CoA dehydrogenase as auxiliary enzyme and either acetyl-CoA or free coenzyme A plus acetyl-phosphate and phosphotransacetylase (regenerating system) or acetyl-CoA plus regenerating system as substrates. β-Ketothiolase from C. pasteurianum used only 20% of the chemically synthesized acetyl-CoA; the enzyme from Alcaligenes eutrophus H 16 used 25%. When the regenerating system was added the condensation reaction continued. The enzyme from C. pasteurianum was inactivated by free coenzyme A, while the enzyme from A. eutrophus was inhibited.
When acetyl-CoA was added as the substrate the initial velocity determination was impeded by the lack of linearity. With acetyl-CoA as the substrate the K m -value was found to be 2.5 mM acetyl-CoA. If free CoASH (or acetyl-CoA) plus regenerating system was added the K m was 0.44 mM (0.42 mM) acetyl-CoA.
The β-ketothiolase activity was measured in the direction of acetoacetyl-CoA cleavage by an optical assay following the decrease of the enol and chelate form of acetoacetyl-CoA by absorption measurement at 305 nm. The activity was maximal at 24 mM MgCl2. The apparent K m values for acetoacetyl-CoA were 0.133 mM and 0.105 mM with 0.065 and 0.016 mM CoASH, respectively. The K m -values as calculated for only the keto form of acetoacetyl-CoA were 0.0471 and 0.0372 mM, respectively. The cleavage reaction was inhibited by high acetoacetyl-CoA concentrations; the inhibition was partially relieved by CoASH. In the range of low concentrations of acetoacetyl-CoA only a slight inhibition by CoASH was observed. The K m for CoASH was found to be 0.0288 and 0.0189 mM with 0.09 and 0.045 mM acetoacetyl-CoA, respectively. High concentrations of CoASH exerted an inhibitory effect on the cleavage reaction. With respect to enzyme kinetics and sensitivity to inhibitors and metabolites the β-ketothiolases of C. pasteurianum and A. eutrophus were rather similar.
Key wordsβ-Ketothiolase Clostridium pasteurianum Alcaligenes eutrophus H 16 Isoenzymes Inhibition/Inactivation by Coenzyme A
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