Zinc interactions with regulatory dimers from Escherichia coli aspartate transcarbamoylase

Zn2+ is tetrahedrally bonded to the 4 nonadjacent thiols of each regulatory chain (Mr 17,000) near r-c contacts between catalytic (c) and regulatory chains (r) in aspartate transcarbamoylase (ATCase; c6r6). This paper reports on Zn2+ interactions with r dimer in the absence of stabilizing r-c contacts. After r2 and c3 subunits were separated, -SH groups of r2 were titrated with p-(hydroxymercuri)benzenesulfonate (PMPS) at pH 7.0. The concomitant release of Zn2+ (2 equiv/r dimer) was quantitated with 4-(2-pyridylazo)resorcinol (PAR) and was a linear function of PMPS added until 8 mercaptide bonds per r2 were formed. Breakage of 1 of 4 Zn2(+)-sulfur bonds in a Zn2+ binding cluster therefore makes the other three bonds more labile. From stopped-flow measurements, the PMPS-promoted Zn2+ release from r2 or mercaptide bond formation with 10- to 20-fold excess PMPS/r2-SH at pH 7.0 was first order with an Arrhenius activation energy Ea = 10 kcal/mol and a half-time t 1/2 = 9 +/- 2 ms at 20 degrees C without inhibitory anions present. The rate of mercurial-promoted Zn2+ release from r2 is at least 77 times faster than that from intact c6r6 [Hunt, J.B., Neece, S.H., Schachman, H.K., and Ginsburg, A. (1984) J. Biol. Chem. 259, 14793]; this indicates that Zn2+ binding clusters are more accessible to attack by PMPS than are those in ATCase. The addition of a 25-fold excess of the multidentate fluorescent chelator quin-2 to r2 gave a rate of Zn2+ dissociation that was 1/210th of that observed with excess mercurial. Furthermore, the Zn(PAR)1 complex was identified as the active species in the transfer of Zn2+ from Zn(PAR)2 to aporegulatory subunits, with kappa = (8 +/- 3) x 10(5) M-1 s-1 at pH 7.0 and 15 degrees C for this second-order association reaction. Although kinetic results are dependent on the mechanisms involved, an affinity constant K'A = (1.3 +/- 0.6) x 10(12) M-1 for Zn2+ binding to r dimer at pH 7.0 and 20 degrees C in the absence and presence of 100 mM KCl could be determined spectrally by rapid equilibration with the high-affinity, sensitive metalloindicators indo-1 and quin-2. This K'A value is based on the assumptions that Zn2+ binding sites in r2 are equivalent (noninteracting) and that apo-r2 does not dissociate; if apo-r2 dissociates, K'A approximately 10(14) M-1. Within experimental error, the K'A value was independent of [indo-1]/[r2] ratios from 36 to 3 with 0.3-8 microM r2.(ABSTRACT TRUNCATED AT 400 WORDS)