Human UMP synthase is a bifunctional protein containing two separate catalytic domains, orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine-5'-phosphate decarboxylase (EC 4.1.1.23). These studies address the question of why the last two reactions in pyrimidine nucleotide synthesis are catalyzed by a bifunctional enzyme in mammalian cells, but by two separate enzymes in microorganisms. From existing data on subunit associations of the respective enzymes and calculations showing the molar concentration of enzyme to be far lower in mammalian cells than in microorganisms, we hypothesize that the covalent union in UMP synthase stabilizes the domains containing the respective catalytic centers. Evidence supporting this hypothesis comes from studies of stability of enzyme activity in vitro, at physiological concentrations, of UMP synthase, the two isolated catalytic domains prepared by site-directed mutagenesis of UMP synthase, and the yeast ODCase. The two engineered 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.
Ferritin is a large protein, highly conserved among higher eukaryotes, which reversibly stores iron as a mineral of hydrated ferric oxide. Twenty-four polypeptides assemble to form a hollow coat with the mineral inside. Multiple steps occur in iron core formation. First, Fe2+ enters the protein. Then, several alternate paths may be followed which include oxidation at site(s) on the protein, oxidation on the core surface, and mineralization. Sequence variations occur among ferritin subunits which are classified as H or L; Fe2+ oxidation at sites on the protein appears to be H-subunit-specific or protein-specific. Other steps of ferritin core formation are likely to involve conserved sites in ferritins. Since incorporation of Fe2+ into the protein must precede any of the other steps in core formation, it may involve sites conserved among the various ferritin proteins. In this study, accessibility of Fe2+ to 1,10-phenanthroline, previously shown to be inaccessible to Fe2+ inside ferritin, was used to measure Fe2+ incorporation in two different ferritins under various conditions. Horse spleen ferritin (L/H = 10-20:1) and sheep spleen ferritin (L/H = 1:1.6) were compared. The results showed that iron incorporation measured as inaccessibility of Fe2+ to 1,10-phenanthroline increased with pH. The effect was the same for both proteins, indicating that a step in iron core formation common among ferritins was being measured. Conserved sites previously proposed for different steps in ferritin core formation are at the interfaces of pairs and trios of subunits. Dinitrophenol cross-links, which modify pairs of subunits and affect iron oxidation, had no effect on Fe2+ incorporation.(ABSTRACT TRUNCATED AT 250 WORDS)
Human uridine monophosphate (UMP) synthase, a bifunctional protein containing orotate phsophoribosyltransferase (OPRTase, EC 2.4.2.10) and orotidine 5'-monophosphate decarboxylase (ODCase, EC 4.1.1.23) activities, has been overproduced by construction and use of a recombinant baculovirus containing the cDNA for this protein. Expression of the virus in cabbage looper larvae produces a crude larval homogenate having UMP synthase enriched about 180-fold over human placental homogenates and allows larger quantities of this human protein as well as analog proteins to be prepared for structure/function studies. A vastly improved purification procedure using a monoclonal immunoaffinity column was developed. Human UMP synthase purified from larval extracts yielded a product which comigrates in SDS gel electrophoresis with UMP synthase purified from human placenta; pure proteins prepared from these two tissue sources have the same specific activities. We found that OPRTase requires Pi ions in the assay buffers for optimal OPRTase activity; BSA in the assay vessel increases to a lesser degree both OPRTase and ODCase activities. These changes in the assay are essential to observe a parallel enrichment of the two enzyme activities. The baculovirus system was used to express human UMP synthase because it usually yields a product with posttranslational modifications that reflect those of the organism that provided the cDNA. We report data to show that human UMP synthase derived from either human placenta or larval extracts both have a sequence in which the N-terminal methionine has been removed and the formerly penultimate alanine has been acetylated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.