Molecular asymmetry in pigeonpea urease: pH inactivation studies

摘要

Pigeonpea (Cajanus cajan) urease was inactivated by incubating it in buffer of low pHs i.e., 4.8 and 4.5. The pattern of inactivation at both pHs was found to be biphasic, in which half of the activity was destroyed more rapidly than the remaining half. This distribution of active site into two categories is suggestive of site-site heterogeneity, or more specifically, the half-site reactivity of the enzyme moiety. Our pH studies on the rate of reaction showed the presence of two ionizable groups ofpK_a values 6.2 +- 0.1 and 8.8 +- 0.1, respectively (Srivastava PK & Kayastha AM, J Mol Catal B: Enz, 16 (2001) 81-89). The later group corresponds to the pK_a value of cysteine group. Here we correlate the loss of urease activity by low pH treatment isdue to the effect on essential thiol residues.