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Catalytic mechanism of C-C hydrolase MhpC from Escherichia coli: Kinetic analysis of His263 and Ser110 site-directed mutants

Record type: Article

C–C hydrolase MhpC (2-hydroxy-6-keto-nona-1,9-dioic acid 5,6-hydrolase) from Escherichia coli catalyses the hydrolytic C–C cleavage of the meta-ring fission product on the phenylpropionic acid catabolic pathway. The crystal structure of E. coli MhpC has revealed a number of active-site amino acid residues that may participate in catalysis. Site-directed mutants of His263, Ser110, His114, and Ser40 have been analysed using steady-state and stopped-flow kinetics. Mutants H263A, S110A and S110G show 104-fold reduced catalytic efficiency, but still retain catalytic activity for C–C cleavage. Two distinct steps are observed by stopped-flow UV/Vis spectrophotometry, corresponding to ketonisation and C–C cleavage: H263A exhibits very slow ketonisation and C–C cleavage, whereas S110A and S110G exhibit fast ketonisation, an intermediate phase, and slow C–C cleavage. H114A shows only twofold-reduced catalytic efficiency, ruling out a catalytic role, but shows a fivefold-reduced KM for the natural substrate, and an ability to process an aryl-containing substrate, implying a role for His114 in positioning of the substrate. S40A shows only twofold-reduced catalytic efficiency, but shows a very fast (500 s?1) interconversion of dienol (317 nm) to dienolate (394 nm) forms of the substrate, indicating that the enzyme accepts the dienol form of the substrate. These data imply that His263 is responsible for both ketonisation of the substrate and for deprotonation of water for C–C cleavage, a novel catalytic role in a serine hydrolase. Ser110 has an important but non-essential role in catalysis, which appears not to be to act as a nucleophile. A catalytic mechanism is proposed involving stabilisation of reactive intermediates and activation of a nucleophilic water molecule by Ser110.

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Citation

Li, C., Montgomery, M.G., Mohammed, F., Li, J.J., Wood, S.P. and Bugg, T.D.H. (2005) Catalytic mechanism of C-C hydrolase MhpC from Escherichia coli: Kinetic analysis of His263 and Ser110 site-directed mutants Journal of Molecular Biology, 346, (1), pp. 241-251. (doi:10.1016/j.jmb.2004.11.032).

More information

Submitted date: 24 September 2004
Published date: 1 February 2005
Keywords: C–C hydrolase, ??-hydrolase, serine catalytic triad, general base mechanism
Organisations: Biological Sciences

Identifiers

Local EPrints ID: 35610
URI: http://eprints.soton.ac.uk/id/eprint/35610
ISSN: 0022-2836
PURE UUID: 826d2026-f20c-4225-9e42-d1225433a3c4

Catalogue record

Date deposited: 22 May 2006
Last modified: 17 Jul 2017 15:47

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Contributors

Author: C. Li
Author: M.G. Montgomery
Author: F. Mohammed
Author: J.J. Li
Author: S.P. Wood
Author: T.D.H. Bugg

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