Oxaloacetic acid decarboxylase

Figure 13.8 Pathway for metabolism of citrate by Leuconostoc spp. and S. lactis subsp. diacetylactis. (1) Citrate permease, (2) citrate lyase, (3) oxaloacetic acid decarboxylase, (4) pyruvate decarboxylase, (5) a-acetolactate synthetase, (6) a-acetolactate carboxylase, (7) diacetyl synthetase, (8) diacetyl reductase, and (9) acetoin reductase.

It was reported (14) that the adaptive enzyme from Lactobacillus plantarum could decarboxylate oxaloacetic acid as well as malic acid. However, in the same organism, Nathan (30) carried this work further and showed that the oxaloacetate decarboxylase activity is not related at all to the malic acid-lactic acid transformation activity. She based this conclusion on the ability of malic and oxaloacetic acids to induce oxaloacetate decarboxylase activity as well as malic enzyme activity. In her words,... 

It may be observed that, for the cycle to revolve, a supply of oxaloacetic acid is required with which the acetyl-CoA can react. Of course, oxaloacetic acid is being constantly re-formed as each revolution of the cycle is completed, but, nonetheless, it is not surprising to find that many cells, especially in bacteria, take precautions against running out of so essential a primer by having an alternative source of oxaloacetic acid. The alternative source is provided by an enzyme, oxaloacetic decarboxylase which catalyses the direct fixing of carbon dioxide on to pyruvic acid to form oxaloacetic acid as in equation (24) ... 

In an analogous manner, a carboxyl group may also be transferred to propionyl-CoA (for the biosynthesis of branched or odd-numbered fatty acids, isoleucine synthesis, or cholesterol metabolism) or to 3-methylcrotonyl-CoA (a degradation product of leucine after addition of water, there results hydroxymethyl-glutaryl-CoA, the precursor of mevalonic acid cf. section 7.1.2). [117] Oxaloacetic acid is derived from pyruvate, which is of central importance for gluco-neogenesis. [108] In addition, biotin participates also in the transfer of carboxylic acid functions. In prokaryotes, biotin functions as a cofactor for decarboxylases (Tab. 7.6). 

Pathways 8-10 are all thermodynamically favorable and produce 1 mol of ATP. Malonyl-CoA and malonic-semialdehyde can be derived from oxaloacetate by employing novel enzymes, with CoA-dependent oxaloacetate dehydrogenase and 2-keto acid decarboxylase activity, respectively. Malate can be converted to 3-HP using a novel enzyme with malate decarboxylase activity (Figure 14.4). These enzymes do not exist in nature and, because of this, it has been proposed that malate decarboxylase activity can be created by enzyme engineering in order to increase their specificity toward oxaloacetate and ability to produce the metabolic intermediates [33]. 

Alternate fates of pyruvate Compounds other than lactate to which pyruvate can be converted ALTERNATE FATES OF PYRUVATE (p. 103) Pyruvate can be oxidatively decarboxylated by pyruvate dehydrogenase, producing acetyl CoA—a major fuel for the tricarboxylic acid cycle (TCA cycle) and the building block for fatty acid synthesis. Pyruvate can be carboxylated to oxaloacetate (a TCA cycle intermediate) by pyruvate carboxylase. Pyruvate can be reduced by microorganisms to ethanol by pyruvate decarboxylase. 

Other types of enzymes When no oxidase or dehydrogenase is available for a target analyte, other types of enzymes have been used for biospedfic recognition e.g. for citric acid detection, citrate lyase, and amperometric detection was possible by coupling to two more enzymatic reactions oxaloacetate decarboxylase and pyruvate oxidase, which convert citric add into H2O2 with the latter being monitored amperometrically with an H202 probe. For detection of acetic add, acetate kinase is used, coupled to pyruvate kinase and pyruvate oxidase [34,35]. 

Prodromidis et al. [35] Citric acid Fruits, juices and sport drinks Citrate lyase (in solution), oxaloacetate decarboxylase and pyruvate oxidase/ sandwiched between a cellulose acetate membrane and a protective polycarbonate membrane H202 probe (Pt electrode) ... 

Although the utility of transaminases has been widely examined, one such limitation is the fact that the equilibrium constant for the reaction is near unity. Therefore, a shift in this equilibrium is necessary for the reaction to be synthetically useful. A number of approaches to shift the equilibrium can be found in the literature.53 124135 Another method to shift the equilibrium is a modification of that previously described. Aspartate, when used as the amino donor, is converted into oxaloacetate (32) (Scheme 19.21). Because 32 is unstable, it decomposes to pyruvate (33) and thus favors product formation. However, because pyruvate is itself an a-keto acid, it must be removed, or it will serve as a substrate and be transaminated into alanine, which could potentially cause downstream processing problems. This is accomplished by including the alsS gene encoding for the enzyme acetolactate synthase (E.C. 4.1.3.18), which condenses two moles of pyruvate to form (S)-aceto-lactate (34). The (S)-acetolactate undergoes decarboxylation either spontaneously or by the enzyme acetolactate decarboxylase (E.C. 4.1.1.5) to the final by-product, UU-acetoin (35), which is meta-bolically inert. This process, for example, can be used for the production of both l- and d-2-aminobutyrate (36 and 37, respectively) (Scheme 19.21).8132 136 137... 

Some acetogenic bacteria, which convert CO2 to acetic acid, form pyruvate for synthesis of carbohydrates, etc., by formation of formaldehyde and conversion of the latter to glycine by reversal of the PLP and lipoic acid-dependent glycine decarboxylase, a 4-protein system. The glycine is then converted to serine, pyruvate, oxaloacetate, etc. Propose a detailed pathway for this sequence. 

The enzymes described above that convert oxaloacetate to pyruvate and CO2 appear to use metal chelation to stabilize the enolate formed by decarboxylation. Many other /3-ketoacid decarboxylases use a similar mechanism. However, there are a few decarboxylations of /3-keto acids or their functional equivalents in which no metal ion is involved. One is the case of acetoacetate decarboxylase, which functions by means of a Schiif base mechanism. A few additional examples are described below. All these cases involve particularly stable enolates. 

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