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Roles for cysteine residues in the regulatory CXXC motif of human mitochondrial branched chain aminotransferase enzyme

Conway, Myra E.; Poole, Leslie B.; Hutson, Susan M.


Myra Conway
Occasional Associate Lecturer - CHSS - DAS

Leslie B. Poole

Susan M. Hutson


The redox-active dithiol/disulfide C315-Xaa-Xaa-C318 center has been implicated in the regulation of the human mitochondrial branched chain aminotransferase isozyme (hBCATm) [Conway, M. E., Yennawar, N., Wallin, R., Poole, L. B., and Hutson, S. M. (2002) Biochemistry 41, 9070-9078]. To explore further the mechanistic details of this CXXC center, mutants of the Cys residues at positions 315 and 318 of hBCATm were individually and in combination converted to alanine or serine by site-directed mutagenesis (C315A, C315S, C318A, C318S, C315/318A, and C315/318S). The effects of these mutations on cofactor absorbance, secondary structures, steady-state kinetics, and sensitivity toward hydrogen peroxide (H 2 O 2 ) treatment were examined. Neither the UV-visible spectroscopic studies nor the circular dichroism data showed any major perturbations in the structure of the mutants. Kinetic analyses of the CXXC mutant proteins indicated primarily a modest reduction in k cat with no significant change in K m . The largest effect on the steady-state kinetics was observed with the C315 single mutants, in which substitution of the thiol group resulted in a reduced k cat (to 26-33% of that of wild-type hBCATm). Moreover, the C315 single mutants lost their sensitivity to oxidation by H 2 O 2 . The kinetic parameters of the C318 mutants were largely unaffected by the substitutions, and as with wild-type hBCATm, reaction of the C318A mutant protein with H 2 O 2 resulted in the complete loss of activity. In the case of oxidized C318A, this loss was largely irreversible on incubation with dithiothreitol. Mass spectrometry and dimedone modification results revealed overoxidation of the thiol group at position 315 to sulfonic acid occurring via a sulfenic acid intermediate in the H 2 O 2 -treated C318A enzyme. Thus, C315 appears to be the sensor for redox regulation of BCAT activity, whereas C318 acts as the "resolving cysteine", allowing for reversible formation of a disulfide bond.


Conway, M. E., Poole, L. B., & Hutson, S. M. (2004). Roles for cysteine residues in the regulatory CXXC motif of human mitochondrial branched chain aminotransferase enzyme. Biochemistry, 43(23), 7356-7364.

Journal Article Type Article
Publication Date Jun 15, 2004
Journal Biochemistry
Print ISSN 0006-2960
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
Volume 43
Issue 23
Pages 7356-7364
Keywords aminotransferase, CXCC motif
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