F. Kurosaki, DISSOCIATION OF DIMERIC 6-HYDROXYMELLEIN SYNTHASE, A POLYKETIDE BIOSYNTHETIC ENZYME IN CARROT CELL-EXTRACTS, WITH LOSS OF KETO-REDUCING ACTIVITY, Archives of biochemistry and biophysics, 321(1), 1995, pp. 239-244
6-Hydroxymellein synthase, an inducible polyketide biosynthetic enzyme
in carrot cell extracts, is composed of two identical subunits, and t
he homodimer is dissociated to monomeric peptides under high-ionic-str
ength conditions with loss of the synthase activity. Appreciable radio
activities were associated with the synthase proteins when the monomer
enzyme was incubated with the radiolabeled substrates, acetyl-coenzym
e A (CoA) and malonyl-CoA. Therefore, it appeared that the synthase do
es not lose the ability of binding the substrate even after the dissoc
iation to monomers. The monomeric synthase liberated triacetic acid la
ctone as the derailment product instead of 6-hydroxymellein from the e
nzyme-attached triketomethylene chain which is the immediate precursor
of an NADPH-dependent keto-reducing reaction involved in 6-hydroxymel
lein biosynthesis. These observations strongly suggest that the monome
ric synthase retains the ability of ketomethylene chain elongation by
the condensation of acyl-CoAs, but is lacking in an NADPH-dependent ke
to-reducing activity toward the triketide intermediate. Results obtain
ed in the present experiments imply that the catalytic domain of acyl-
CoA condensation is able to associate with that of keto reduction, pos
sibly belonging to another subunit, only in the homodimeric structure
to organize the multicatalytic reaction center. (C) 1995 Academic Pres
s, Inc.