The Thiol Group in the Catalytic Chains of Aspartate Transcarbamoylase

AUTOR(ES)

Evans, David R.

RESUMO

The allosteric enzyme aspartate transcarbamoylase (EC 2.1.3.2) was previously shown to consist of two functionally distinct types of polypeptide chains. X-ray diffraction and chemical studies showed that there are six copies of both catalytic (C) and regulatory (R) chains, and that the intact molecular complex (C6R6) has D3 symmetry. Organomercurials react preferentially with the four thiol groups on each R chain, dissociating the molecular complex. We show that 2-chloromercuri-4-nitrophenol reacts specifically and rapidly with the single C-chain thiol, which is believed to be near the catalytic site. This reaction inactivates the enzyme in solution and does not dissociate the molecular complex. Spectrophotometric titration and mercury analysis indicates that six molecules of this mercurial are firmly bound to the enzyme (R6C6), and crystallographic studies establish that only six sites, related by D3 symmetry, are modified.

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