Carboxylic decarboxylation

Figure 39. Repetitive Alternating Carboxylation-Decarboxylation Cycle (Adapted from Ref. [444]).

Note that the C02 added to pyruvate in the pyruvate carboxylase step is the same molecule that is lost in the PEP carboxykinase reaction (Fig. 14-17). This carboxylation-decarboxylation sequence represents a way of activating pyruvate, in that the decarboxylation of oxaloacetate facilitates PEP formation. In Chapter 21 we shall see how a similar carboxylation-decarboxylation sequence is used to activate acetyl-CoA for fatty acid biosynthesis (see Fig. 21-1). 

Oxidative cleavage by means of electrochemically generated cation-radicals is also possible thus benzyl ethers may be cleaved and carboxylates decarboxylated using cation-radicals of brominated triphenylamines in acetonitrile containing a weak base.34 35 Such as indirect reaction makes it... 

Fatty acid chains are taken apart two carbon atoms at a time by (3 oxidation. Biosynthesis of fatty acids reverses this process by using the two-carbon acetyl unit of acetyl-CoA as a starting material. The coupling of ATP cleavage to this process by a carboxylation-decarboxylation sequence, the role of acyl carrier protein (Section H,4), and the use of NADPH as a reductant (Section I) have been discussed and are summarized in Fig. 17-12, which gives the complete sequence of... 

A TRIAX 550 spectrometer attached to an Andor -90°C cooled CCD detector was used for all spectroscopic measurements. Ar laser lines at 488.0 nm and 514.3 nm were used. Reactions were monitored by time-resolved Raman spectroscopy, sequentially setup for two of three separate regions of interest within the spectral range. This enabled, for example, collecting information about the carboxylation/decarboxylation and hydration/dehydration processes by monitoring the various CO and CH vibration modes. This technique provided spectra in each region only after the collection of the spectra in other regions, and hence not favorable for faster kinetics. However, inclusion of OH and H2 peaks gave a reasonably quantitative estimate on the extent of the hydrothermal reaction and valuable information for mass balance calculations (see further details in the experimental results for each system)... 

The geometry of the transition state for the carboxylation/decarboxylation step is an important aspect of mechanism. The anion intermediate [Eq. (2)] is usually a conjugated anion (often an enolate) in which the negative charge lies above and below a planar atomic framework. Attacking or departing CO2 will approach from above or below the plane of the conjugated system (Scheme I), rather than from within the plane. The distinction between the two faces of the planar system can usually be made based on the stereochemistry of the carboxylated substrate or product. 

As we noted above, the transition state for a carboxylation/decarboxylation step ordinarily has a geometry in which the one-carbon fragment is above the plane of the conjugated system (cf. Scheme I). Enzymic control of this conformation can give rise to a modest increase in rate [one to two orders of magnitude... 

Interestingly, although mechanistic details vary, it is likely that a metal-chelated enolate of pyruvate is an intermediate in all these reactions and that all transition states for the carboxylation/decarboxylation step are similar. 

There are a number of variations of this pattern which have been developed. The a-aminocarbonyl compounds are usually generated in situ by nitrosation/reduction <93T2185>. For some systems it is preferrable to carry out the reduction by catalytic hydrogenation <85JOC5598>. Aminomalonate esters can be used when the desired product is a pyrrole-2-carboxylate. Decarboxylation occurs during the course of the reaction so that the aminomalonate is a glycine equivalent (Equation (47)) <87JOC3986>. 

Several mechanisms have been suggested for the thermal decomposition reaction, such as a bimolecular reaction, carboxylation-decarboxylation, and the formation of an intermediate phenyl ion. Of these, the most acceptable is the last (Ratusky and Sorm, 1959). 

De novo synthesis of fatty acids requires the combined action of acetyl-CoA carboxylase and fatty acid synthetase. The acetyl-CoA carboxylases contain biotin carboxyl carrier protein (BCCP), biotin carboxylase ahd carboxyltransferase. The reaction proceeds in two steps. Firstly, the biotin moiety of BCCP is carboxylated. Secondly, the carboxyl group is transferred to the acceptor acetyl-CoA. This latter reaction proceeds in a concerted fashion (Mildvan et aL, 1966). The mechanism of the carboxylation/decarboxylation reaction has been recently probed using the model compound N-1 -methoxy carbonylbiotin methyl... 

Furanones from 2-acetylenealcohols and carboxylic acid chlorides via carboxylation-decarboxylation... 

Formation of symmetrical dimers by the electrolysis of carboxylates (decarboxylative dimerization). The coupling of two distinct carboxylates yields unsymmetrical products ... 

The water soluble vitamin, biotin, acts as a cofactor for a set of enzymes that catalyze carboxylation, decarboxylation or transcarboxylation reactions (Moss and Lane, 1971). In plants we have characterized four biotin-containing enzymes each of which catalyze reactions required in lipid metabolic processes. These enzymes are the homomeric and heteromeric isozymes of acetyl-CoA carboxylase (ACCase), methylcrotonyl-CoA carboxylase (MCCase) and geranoyl-CoA carboxylase (GCCase). The studies of these biotin-containing enzymes has led to an interest in biotin biosynthesis, and we have cloned the gene coding for biotin synthase. 

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