Recombinant Human Betaine -Homocysteine S -Methyltransferase: Discovery as a Zinc Metalloenzyme, and Regulation of Activity by Redox Status
The lack of activity in the absence of reducing agent is not due to loss of Zn at the catalytic site. When reducing agent-free preparations of 5Cys/Ala are incubated with the Zn chelator, PAR, and the absorbence of the Zn-PAR chelate is monitored, there is no change in absorbence unless methyl-methanethiosulfonate or H2O2 are added, and then the amount of Zn released from the enzyme is equivalent to the amount of 5Cys/Ala in solution. Finally, when BHMT-5Cys/Ala is in the presence or absence of reducing agent, and in the former condition the reducing agent is removed by gel filtration, the number of DTNB-modifiable residues is three and one, respectively. These data indicate that the redox effect on BHMT activity is mediated by the formation of a reversible disulfide bond between two of the three thiolates that normally chelate Zn. When oxidized, the loss of activity is likely due to the change in the electronics of the Zn-ligand interactions.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign