The covalent union in UMPS stabilizes the domains containing the respective catalytic centers, improving its activity in multicellular organisms where concentrations tend to be 1/10th of the separate counterparts in prokaryotes.

[12] Fusion events between OPRTase and ODCase, which have led to the formation of the bifunctional enzyme UMPS, have occurred distinctly in different branches of the tree of life.

For instance, Metazoa, Amoebozoa, Plantae, and Heterolobosea have eukaryotic ODCase and OPRTase, whereas Alveolata and stramenopiles have bacterial ones.

[13] Merging both the fusion order and evolutionary origin, organisms end up having fused UMPS where one of its catalytic domains comes from bacteria and the other from eukaryotes.

[14] To determine the driving force of protein association, several experiments have been performed separating both domains and changing the linker peptide that keeps them together.

In Plasmodium falciparum, the OPRTase-OMPDCase complex increases the kinetic and thermal stability when compared to monofunctional enzymes.

[15] In H. sapiens, even though separate and fused domains have a similar activity, the former have a higher sensitivity to conditions promoting monomer dissociation.

[14] In Leishmania donovani, separate OPRTase does not have detectable activity possibly due to lower thermal stability or lack of its linker peptide.

Transition state analyses have used isotopic effects and quantum calculations to reveal a completely dissociated dianionic orotate structure, a ribocation, and a nucleophilic pyrophosphate molecule.

Nonetheless, this is unexpected, since most N-ribosyltransferases involve protonated and neutral leaving groups, whereas deprotonated orotate is not a good one in the cationic transition state.

[21] There are two possibilities as far as the loop movement is concerned: It could move in a rigid manner or it could come from a disordered structure that acquires order.

Nonetheless, crystallographic analyses and the lack of S. cerevisiae enzyme affinity to substrate analogues where the carboxylate groups is replaced by a cationic substituent have shown some evidence against this theory.

Proton donor absence near O4 or O2 in crystallographic structures is evidence against it along with the ylide generation exclusion as a limiting step in 15N experiments.

The strain has a pleiotropic phenotype including reduced viability and fertility, slow growth, and radiation sensitivity.

[25] UMPS and its two separate domains, ODCase and OPRTase, have been shown to be essential to viability in parasites from the Chromoalveolata taxon such as L. donovani or P.

Abdo et al. (2010) performed reactions on 2-ethoxiethanselenic acid using electron-rich aromatic substrates to produce (2-ethoxiethyl)seleno ethers.