Decarboxylation base-dependent decarboxylations

In the case of decarboxylations, the negative charge produced in the decarboxylation step may be neutralized by some distal positive charge. This occurs in the case of Schiff base-dependent decarboxylations, thiamin pyrophosphate-dependent decarboxylations, and in a few other cases. Such systems are often referred to as electron sinks. ... 

For diagnostic purposes, these assays are applied most commonly to freshly isolated peripheral blood lymphocytes, fresh or frozen skeletal muscle (usually quadriceps), or primary cultures of skin fibroblasts. The procedure is based on the TPP/NAD+/CoASH-dependent decarboxylation of l-14C-pyruvate, with residual activity of total PDC, El, E2, and E3 expressed individually as nanomoles product/minute incubation at 37°C/milligram total cell protein. Preincubation of cells with DCA (which inhibits PDK) or fluoride (which inhibits PDP) can be... 

The C4 cycle can be viewed as an ATP-dependent C02 pump that delivers C02 from the mesophyll cells to the bundle-sheath cells, thereby suppressing photorespiration (Hatch and Osmond, 1976). The development of the C4 syndrome has resulted in considerable modifications of inter- and intracellular transport processes. Perhaps the most striking development with regard to the formation of assimilates is that sucrose and starch formation are not only compartmented within cells, but in C4 plants also may be largely compartmented between mesophyll and bundle-sheath cells. This has been achieved together with a profound alteration of the Benson-Calvin cycle function, in that 3PGA reduction is shared between the bundle-sheath and mesophyll chloroplasts in all the C4 subtypes. Moreover, since C4 plants are polyphyletic in origin, several different metabolic and structural answers have arisen in response to the same problem of how to concentrate C02. C4 plants have three distinct mechanisms based on decarboxylation by NADP+-malic enzyme, by NAD+-malic enzyme, or by phosphoenolpy-ruvate (PEP) carboxykinase in the bundle-sheath (Hatch and Osmond, 1976). 

The pH dependence of the kinetics of histidine decarboxylase (127) demonstrates that the histidine is zwitterionic when it binds to the enzyme. The extra proton on nitrogen must, of course, be removed before the Schiff base is formed. The carboxylate of Glu-197 at the active site may accept this proton. In turn, this same group may then be responsible for proton donation to the Schiff base following decarboxylation. This is consistent with the occurrence of retention of configuration in the overall replacement of-C02 by -H (128) and with studies of enzymes altered at Glu-197 (129). When Glu-197 is replaced by Asp, the protonation that follows decarboxylation occasionally occurs on the pyruvate side, thus giving rise to decarboxylation-dependent transamination (129). 

The proposed stereochemistry represented in Figure 8 relies on the fact that most PLP-dependent decarboxylations occur with retention and with the assumption that it is the same base that abstracts the C-2 proton and reprotonates the final intermediate. This last point is consistent with the observation that when L-Ala was incubated alone with the enzyme in [ H2]0, a slow exchange was observed (with a rate constant 10 times less than the catalytic constant) and with complete retention of configuration. The overall picture represented in Figure 8 implies that the first condensation step occurs with inversion. 

Some methods to measure PLP by HPLC are complementary to enzymatic determinations of PLP. An often used enzymatic method (47) for assaying plasma PLP is based on the coenzyme dependent decarboxylation of tyrosine catalyzed by L-tyrosine decarboxylase (EC 4.1.1.25). By using a well-resolved apo-enzyme preparation, it was shown that the reaction rate is directly proportional to the amount of PLP added to the reaction mixture. Conventionally, the reaction is monitored by CO2 liberation from L-tyrosine- Ci the liberated " C02 is trapped in a potassium hydroxide solution and subsequently quantitated by liquid scintillation counting. 

The preparation of ketones and ester from (3-dicarbonyl enolates has largely been supplanted by procedures based on selective enolate formation. These procedures permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of keto ester intermediates. The development of conditions for stoichiometric formation of both kinetically and thermodynamically controlled enolates has permitted the extensive use of enolate alkylation reactions in multistep synthesis of complex molecules. One aspect of the alkylation reaction that is crucial in many cases is the stereoselectivity. The alkylation has a stereoelectronic preference for approach of the electrophile perpendicular to the plane of the enolate, because the tt electrons are involved in bond formation. A major factor in determining the stereoselectivity of ketone enolate alkylations is the difference in steric hindrance on the two faces of the enolate. The electrophile approaches from the less hindered of the two faces and the degree of stereoselectivity depends on the steric differentiation. Numerous examples of such effects have been observed.51 In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a small preference for... 

Allenic amino acids belong to the classical suicide substrates for the irreversible mechanism-based inhibition of enzymes [5], Among the different types of allenic substrates used for enzyme inhibition [128, 129], the deactivation of vitamin B6 (pyr-idoxal phosphate)-dependent decarboxylases by a-allenic a-amino acids plays an important role (Scheme 18.45). In analogy with the corresponding activity of other /3,y-unsaturated amino acids [102,130], it is assumed that the allenic amino acid 139 reacts with the decarboxylase 138 to furnish the imine 140, which is transformed into a Michael acceptor of type 141 by decarboxylation or deprotonation. Subsequent attack of a suitable nucleophilic group of the active site then leads to inhibition of the decarboxylase by irreversible formation of the adduct 142 [131,132]. 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

Interpretation of KIEs on enzymatic processes (see Chapter 11) has been frequently based on the assumption that the intrinsic value of the kinetic isotope effect is known. Chemical reactions have long been used as models for catalytic events occurring in enzyme active sites and in some cases this analogy has worked quite well. One example is the decarboxylation of 4-pyridylacetic acid presented in Fig. 10.9. Depending on the solvent, either the zwitterionic or the neutral form dominates in the solution. Since the reaction rates in D20/H20 solvent mixtures are the same (see Section 11.4 for a discussion of aqueous D/H solvent isotope effects), as are the carbon KIEs for the carboxylic carbon, it is safe to assume that this is a single step reaction. The isotope effects on pKa are expected to be close to the value of 1.0014 determined for benzoic acid. This in mind, changes in the isotope effects have been attributed to changes in solvation. 

Lactobacillus delbrueckii. In 1953, Rodwell suggested that the histidine decarboxylase of Lactobacillus 30a was not dependent upon pyridoxal phosphate (11). Rodwell based his suggestion upon the fact that the organism lost its ability to decarboxylate ornithine but retained high histidine decarboxylase activity when grown in media deficient in pyridoxine. It was not until 1965 that E. E. Snell and coworkers (12) isolated the enzyme and showed that it was, indeed, free of pyridoxal phosphate. Further advances in characterization of the enzyme were made by Riley and Snell (13) and Recsei and Snell (14) who demonstrated the existence of a pyruvoyl residue and the participation of the pyruvoyl residue in histidine catalysis by forming a Schiff base intermediate in a manner similar to pyridoxal phosphate dependent enzymes. Recent studies by Hackert et al. (15) established the subunit structure of the enzyme which is similar to the subunit structure of a pyruvoyl decarboxylase of a Micrococcus species (16). 

Similarly, vesicular reactivity is dependent on bilayer fluidity and Arrhenius (or Eyring) plots for the decarboxylation of 6-NBIC show a break around Tm. " For the Kemp elimination in different bilayers, it was found that the bilayer for which kinetic data had been gathered below its was least effective as a catalyst. Ester hydrolysis has also been found to be faster above r. For the decarboxylation of 6-NBIC, the increase in catalytic efficiency was attributed to different aggregate surface dynamics based on the observation that vesicles above showed intermediate activation parameters between vesicles below and micelles. One could, of course, discuss causality here considering the fact that many of the bilayer... 

Decarboxylation of p-oxoacids. Beta-oxoacids such as oxaloacetic acid and acetoacetic acid are unstable, their decarboxylation being catalyzed by amines, metal ions, and other substances. Catalysis by amines depends upon Schiff base formation,232 while metal ions form chelates in which the metal assists in electron withdrawal to form an enolate anion.233 235... 

Below the structures of the adducts in Eq. 14-20 are those of a 2-oxo acid and a (3-ketol with arrows indicating the electron flow in decarboxylation and in the aldol cleavage. The similarities to the thiamin-dependent cleavage reaction are especially striking if one remembers that in some aldolases and decarboxylases the substrate carbonyl group is first converted to an N-proto-nated Schiff base before the bond cleavage. 

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