Human ceruloplasmin - Intramolecular electron transfer kinetics and equilibration

Pulse radiolytic reduction of disulfide bridges in ceruloplasmin yielding R SSR- radicals induces a cascade of intramolecular electron transfer (ET) pr ocesses. Based on the three-dimensional structure of ceruloplasmin identifi cation of individual kinetically active disulfide groups and type 1 (T1) co pper centers, the following is proposed. The first T1 copper(II) ion to be reduced in ceruloplasmin is the blue copper center of domain 6 (T1A) by ET from RSSR- of domain 5. The rate constant is 28 +/- 2 s(-1) at 279 K and pH 7.0, T1A is in close covalent contact with the type 3 copper pair and inde ed electron equilibration between T1A and the trinuclear copper center in t he domain 1-6 interface takes place with a rate constant of 2.9 +/- 0.6 s(- 1). The equilibrium constant is 0.17. Following reduction of T1A Cu(II), an other ET process takes place between RSSR- and T1B copper(II) of domain 4 w ith a rate constant of 3.9 +/- 0.8. No reoxidation of T1B Cu(I) could be re solved. It appears that the third T1 center (T1C of domain 2) is not partic ipating in intramolecular ET, as it seems to be in a reduced state in the r esting enzyme.