ODCase decarboxylase

PRTase - phosphoribosyl transferase ODCase - OMP decarboxylase OMP - orotidine 5 -phosphate... 

Fig. 44 Pathways for uridylate biosynthesis. Mutants lacking enzymes PRTase or ODCase can complete a route to UMP provided by an antibody orotate decarboxylase in conjunction with the naturally occurring uracil PRTase. Decarboxylation of orotic acid [135] is thought to proceed through the transition state [136], for which the hapten [137] was developed (Smiley and Benkovic, 1994).

Orotidine S -monophosphate decarboxylase (ODCase) is a key enzyme in the biosynthesis of nucleic acids, effecting the decarboxylation of orotidine 5 -monophosphate (OMP, 1) to form uridine S -monophosphate (UMP, 2, Scheme l).1,2 The conversion of OMP to UMP is biomechanistically intriguing, because the decarboxylation appears to result, uniquely, in a carbanion (3, mechanism i, Scheme 2) that cannot delocalize into a it orbital.3,4 The uncatalyzed reaction in solution is therefore extremely unfavorable, with a AG of... 

Like purine metabolism, the polyamine biosynthetic pathway has served as another paradigm for rational therapeutic intervention in parasitic disease. Polyamine synthesis in T. brucei and Leishmania, like that in mammalian cells, is initiated by the enzyme ornithine decarboxylase (ODCase), although T. cruzi may synthesize polyamines by... 

The mechanism of the enzymatic decarboxylation of orotidine 5 -mono-phosphate (OMP) to uridine 5 -monophosphate (UMP) (see Fig. 1) is an intriguing problem for which many solutions have been offered. Even before 1995 when Wolfenden and Radzicka declared OMP decarboxylase (ODCase) to be the most proficient enzyme [1], several different mechanisms had been proposed. Since that time, other mechanisms have been advocated. Curiously, the appearance of crystal structures for various wild-type and mutant ODCases has led not to a definitive picture of catalysis, but to even more conjecture and controversy concerning the mechanism. 

Figure 3 compares the proficiencies (kcat/K]v[/kun) of ODCase, several other enzyme decarboxylases [2], and some antibody decarboxylases [3]. The proficiencies of the decarboxylase enzymes, including a variety of amino acid decarboxylases, are nearly equal. Many decarboxylases employ iminium intermediates formed by reaction of an amino acid with a cofactor such as pyruvoyl or pyridoxal, or by reaction of a -keto ester with an active-site lysine residue. These intermediates have been found to be so reactive that the... 

As noted above, many decarboxylases are known to exploit Schiff base formation in the active-site as a source of catalysis. Shostack and Jones explored this possibility in the case of ODCase [8]. They found that when the enzymatic reaction is performed in 0 water, the product does not incorporate from bulk solvent. For this reason, a covalent iminium mechanism for ODCase was abandoned, in spite of its attractive similarities to other decarboxylase mechanisms. 

Orotidine 5 -phosphate decarboxylase (ODCase, E. C. 4.1.1.23) catalyzes the decarboxylation of orotidine 5 -phosphate (OMP) to form uridine 5 -phos-phate in the sixth and final step of pyrimidine biosynthesis (Fig. 1) [1]. The discovery of ODCase in 1954 followed the identification, three years earlier, of orotic acid as the metabolic precursor of nucleic acids [2, 3]. ODCase is a distinct, monofunctional polypeptide in bacteria and fungi, whereas in mammals it combines with orotate phosphoribosyltransferase (OPRTase) to form the bifunctional enzyme UMP synthase. Human deficiencies in either OPRTase or ODCase activity result in an autosomal recessive disorder called hereditary orotic aciduria [4]. The disease is characterized by depleted levels of pyrimidine nucleotides in the blood and by the appearance of crystalline... 

OMP decarboxylase (ODCase) catalyzes the decarboxylation of OMP to UMP, a decarboxylation that must necessarily be mechanistically different from the groups of decarboxylations that occur throughout metabolism [1]. The structure of the substrate does not lend itself to decarboxylation mechanisms involving pyridoxal phosphate (typical of amino acid decarboxylases [2]), thiamine pyrophosphate (typical of a-keto acid decarboxylases [3]), or metal ions (typical of /3-keto acid decarboxylases [4]) although the presence of ions has been detected in some preparations of ODCase [5, 6], the enzyme clearly does not require for catalytic activity [7]. 

Enzymes catalyze many reactions far better than man-made catalysts. The goal of theoretical studies of enzyme mechanisms is to understand how they achieve their amazing catalytic power. In this respect, orotidine 5 -mono-phosphate decarboxylase (ODCase) is one of the most fascinating enzymes. It increases the rate of decarboxylation of its substrate orotidine 5 -mono-phosphate (OMP) (see Fig. 1) by 17 orders of magnitude. This rate acceleration is unmatched by other known enzymes [1]. Analyzing the mechanism of ODCase can therefore be an important step in understanding enzyme catalysis on a more general level. 

Abstract An energy decomposition scheme is presented to elucidate the importance of the change of protein conformation substates to the reduction of activation barrier in an enzyme-catalyzed reaction. The analysis is illustrated by the reaction of orotidine 5 -monophosphate decarboxylase (ODCase), in which the catalyzed reaction is at least 10 faster than the spontaneous reaction. Analysis reveals that the enzyme conformation is more distorted in the reactant state than in the transition state. The energy released from conformational relaxation of the protein is the main source of the rate enhancement. The proposed mechanism is consistent with results from site-directed mutagenesis where mutations remote from the reaction center affect kcat but not Kyi. 

How can an enzyme that apparently does not utilize cofactors or covalent intermediates be one of the most proficient enzymes known This is the mystery of orotidine monophosphate decarboxylase (ODCase). In this volume, experts in the field of enzyme catalysis describe their efforts to understand this puzzling enzyme. 

The anticancer nucleoside Gemcitabine as its monophosphate derivative (3) has been modified by conjugation of squalene at the N4-position. Using a combination of cryo-EM and SAXS it was shown that the squalenoyl derivative exists as a unilamellar liposome, and that the nanoassembly exhibited enhanced activity compared with Gemcitabine in a resistant cell line." Various C6-modified 2 -deoxy-2 -fluoro-dUMP derivatives have been prepared as potential inhibitors of orotidine 5 -monophosphate decarboxylase (ODCase) of these, the most potent analogue was the 6-iodo-derivative which was found to covalently inhibit ODCase. Aminoacyl-tRNAs... 

Decarboxylation reactions are common in Nature and they are involved in many pathways, including decarboxylation of keto acids, amino acid conversions, and carbohydrate synthesis. Many decarboxylases use cofactors such as metal ions, pyridoxal 5 -phosphate, biotin, and flavin, but a small subset, for example, orotidine 5 -phosphate decarboxylase (ODCase) and methyhnalonyl CoA decarboxylase do not utilize any cofactor. ODCase catalyzes the decarboxylation of orotic acid (shown in Figure 8), and it generates one of the largest rate enhancements known to be produced by any enzyme (rate of the reaction is enhanced by a factor of Several... 

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