INTRINSIC ACTIVITY AND STABILITY OF BIFUNCTIONAL HUMAN UMP SYNTHASE AND ITS 2 SEPARATE CATALYTIC DOMAINS, OROTATE PHOSPHORIBOSYLTRANSFERASEE AND OROTIDINE-5'-PHOSPHATE DECARBOXYLASE
Mj. Yablonski et al., INTRINSIC ACTIVITY AND STABILITY OF BIFUNCTIONAL HUMAN UMP SYNTHASE AND ITS 2 SEPARATE CATALYTIC DOMAINS, OROTATE PHOSPHORIBOSYLTRANSFERASEE AND OROTIDINE-5'-PHOSPHATE DECARBOXYLASE, The Journal of biological chemistry, 271(18), 1996, pp. 10704-10708
Human UMP synthase is a bifunctional protein containing two separate c
atalytic domains, orotate phosphoribosyltransferase (EC 2.4.2.10) and
orotidine-5'-phosphate decarboxylase (EC 4.1.1.23), These studies addr
ess the question of why the last two reactions in pyrimidine nucleotid
e synthesis are catalyzed by a bi-functional enzyme in mammalian cells
, but by two separate enzymes in microorganisms, From existing data on
subunit associations of the respective enzymes and calculations showi
ng the molar concentration of enzyme to be far lower in mammalian cell
s than in microorganisms, we hypothesize that the covalent union in UM
P synthase stabilizes the domains containing the respective catalytic
centers, Evidence supporting this hypothesis comes from studies of sta
bility of enzyme activity in vitro, at physiological concentrations, o
f UMP synthase, the two isolated catalytic domains prepared by site-di
rected mutagenesis of UMP synthase, and the yeast ODCase, The two engi
neered domains have activities very similar to the native UMP synthase
, but unlike the bifunctional protein, the domains are quite unstable
under conditions promoting the dissociated monomer.