Orotidine 5 -monophosphate decarboxylase decarboxylation

The process of substitution undertaken on carboxylic acids and the derivatives of carboxylic acids (anhydrides, acid halides, esters, amides, and nitriles) generally involves a series of replacement processes. Thus, individually, substitution may involve replacement of (a) the proton attached to oxygen of the -OH group (i.e., ionization of the acid) (b) the hydroxyl (-OH) portion of the carboxylic acid (or derivative) (e.g., esterification) (c) the carbonyl oxygen and the hydroxyl (-OH) (e.g., orthoester formation, vide infra) (d) the entire carboxylic acid functionality (e.g., the Hunsdiecker reaction, already discussed Scheme 9.101) and the decarboxylation of orotic acid (as orotidine monophosphate) to uracil (as uridine monophosphate)—catalyzed by the enzyme orotidine monophosphate decarboxylase (Scheme 9.115) or (e) the protons (if any) on the carbon to which the carboxylic acid functional group is attached (e.g., the Dieckman cycUzation, already discussed earlier, c Equation 9.91). Indeed, processes already discussed (i.e., reduction and oxidation) have also accomplished some of these ends. Some additional substitutions for the carboxylic acid group itself are presented in Table 9.6, while other substitutions for derivatives of carboxylic acids are shown in Tables 9.7-9.10 and discussed subsequently. 

Miller and Wolfenden6 compared the rates of decarboxylation of the substrate of orotidine-5 -monophosphate decarboxylase (OMPDC) in quantitative detail, on and off the enzyme. They showed that the apparent unimolecular rate constant of decarboxylation of the substrate when bound to the enzyme is about 1015 times greater than the decarboxylation process in the absence of the enzyme. Further studies confirm that the enzyme-promoted reaction does not involve additional intermediates or covalent alterations of the substrate. The reaction consists of carbon dioxide being formed and the resulting carbanion becoming protonated. Since thermodynamic barriers are not altered by catalysis, the energy of the carbanion must be a component that reflects the energy of the environment in which it is created, one in which the carbanion that is formed is selectively stabilized. 

The thermal decarboxylation of 1,3-dimethylorotic acid 56 was studied as a model for uncatalysed decarboxylation of orotidine 5 -monophosphate to form uridine 5 -monophosphate.97 This reaction catalysed by the enzyme oritidine 5 -monophosphate decarboxylase is the essential step in nucleic acid biosynthesis.98 Samples of 1,3-dimethylorotic acid were heated at 190°C to conversion over 90% and unreacted substrate was recovered after conversion into the methyl ester. 

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... 

This article summarizes the mechanistic, crystallographic, and computational evidence for the mechanism of decarboxylation of OMP by the family of orotidine 5 -monophosphate decarboxylase enzymes, and offers a critical evaluation of the various mechanisms based upon this evidence. 

Stanton CL et al (2007) QM/MM metadynamics study of the direct decarboxylation mechanism for orotidine-5-monophosphate decarboxylase using two different QM regions acceleration too small to explain rate of enzyme catalysis. J Phys Chem B 111 12573-12581... 

Reaction of aspartic acid (14) with carbamoyl phosphoric acid (17) in the presence of the allosteric enzyme aspartate carbamoyltransferase (aspartate transcar-bamoylase) gives N-carbamoyl aspartic acid (18), which is cyclised to L-dihy-droorotic acid (19) by dihydroorotase. Oxidation of L-dihydroorotic acid by flavoprotein, orotate reductase gives orotic acid (20), which reacts with 5-phosphori-bosy 1-1-pyrophosphate (PRPP) in the presence of orotate phosphoribosyl transferase to form orotidine 5 -monophosphate (OMP, 21). Decarboxylation of OMP by orotid-ine 5 -phosphate decarboxylase yields uridine 5 -monophosphate (UMP, 22), which acts as precursor for the cytidine nucleotides (CTP) (Chart 6). 

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. 

The final steps of pyrimidine biosynthesis novo which are catalyzed by two sequential enzymes, orotate phosphoribosyltransfer-ase (OPRT) and orotidylic decarboxylase (ODC), involve the PP-ribose P dependent conversion of orotic acid to orotidine-5 -monophosphate (OMP) followed by decarboxylation at the 7 position to form uridine 5 -monophosphate (UMP) (Fig. 1). UMP is then utilized further in the synthesis of nucleic acids and co-enzymes. Defects at this site in this metabolic pathway are important for they can result in "pyrimidine starvation" from depletion of the intracellular pool of pyrimidine nucleotides. In man the rare genetic disease, orotic aciduria, involves a deficiency of both OPRT and ODC (Type 1) (Smith, Sullivan and Huguley, 1961) or, less commonly, only ODC (Type II) (Fox, 0 Sullivan and Firken, 1969). 

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