Acetoacetic acid decarboxylase

Nucleophilic groups on enzymes participate in a variety of other types of reactions in addition to hydrolytic reactions. An example is acetoacetic acid decarboxylase, which catalyzes the reaction... 

In acetoacetic acid decarboxylase, the positive charge of a protonated Schiff base intermediate pulls electrons from a nearby carbon-carbon bond, thereby releasing C02. 

Another example where mechanism and model have been developed is that for the decarboxylation of acetoacetic acid here no coenzyme is required, and the chemistry involves the enzyme itself. The mechanism for the enzymic decarboxylation with crystalline decarboxylase from Clostridium acetobutylicum has been worked out in some detail it is presented below (20, 21). The initial work, carried out in the author s laboratory by G. Hamilton (22) and I. Fridovich (23, 24) proved that the essential intermediate is a ketimine much of the subsequent development of the enzymic system resulted from the researches of W. Tagaki (25). 

Whatever the explanation, the stereochemistry of the acetoacetate decarboxylase reaction is controlled by factors not common to the other /3-keto acid decarboxylases. 

Biosynthesis catabolism Lys is formed in plants and bacteria from meso-2,6- diaminopimelic acid by diaminopimelate decarboxylase (EC 4.1.1.20). The catabolism proceeds through eleven enzymatic steps to acetoacetic acid (acetyl-CoA). L. is a precursor of the cadaverines. Because of its two amino groups it has a cross-linking function in polypeptides such as collagen and elastin, see also 5-hydroxylysine. L. is used as a fodder additive. 

BOROHYDRIDE REDUCTION ACETAZOLAMIDE Acetic acid, autoprotolysis constant, AUTOPROTOLYSIS ACETOACETATE DECARBOXYLASE Acetoacetate decarboxylase reduction, BOROHYDRIDE REDUCTION ACETOLACTATE SYNTHASE Acetone,... 

Decarboxylation. When the amino acid is aspartate, the second compound in equation 2.44 is analogous to the Schiff base in the acetoacetate decarboxylase reaction and may readily decarboxylate ... 

Aldolases such as fructose-1,6-bisphosphate aldolase (FBP-aldolase), a crucial enzyme in glycolysis, catalyze the formation of carbon-carbon bonds, a critical process for the synthesis of complex biological molecules. FBP-aldolase catalyzes the reversible condensation of dihydroxyacetone phosphate (DHAP) and glyceralde-hyde-3-phosphate (G3P) to form fructose-1,6-bisphosphate. There are two classes of aldolases the first, such as the mammalian FBP-aldolase, uses an active-site lysine to form a Schiff base, whereas the second class features an active-site zinc ion to perform the same reaction. Acetoacetate decarboxylase, an example of the second class, catalyzes the decarboxylation of /3-keto acids. A lysine residue is required for good activity of the enzyme the -amine of lysine activates the substrate carbonyl group by forming a Schiff base. 

Kresge, A.J. (1997) Electrostatic effects on acid dissociation constants. Importance of lysine 116 in determining the p /<, of lysine 115 in the active site of acetoacetate decarboxylase. Chemtracts, 10, 27. 

Primary amine catalysis (usually involving a lysine residue) has been recognised to play an important role in various enzyme-catalysed reactions. Examples are the conversion of acetoacetate to acetone catalysed by acetoacetate decarboxylase, the condensation of two molecules of S-aminolevulinic acid catalysed by -aminolevulinic deshydratase during the biosynthesis of porphyrins, and the reversible aldol condensation of dihydroxy-acetone phosphate with glyceraldehyde which in the presence of aldolase yields fructose-1-phosphate (64) (For reviews see, for example, Snell and Di Mari,... 

The actual pKa value of an active site catalytic group will be influenced by the particular microenvironment of the active site, which could raise or lower the pKa. For example, the enzyme acetoacetate decarboxylase contains an active site lysine residue that forms an imine link with its substrate its pKa value was found to be 5.9, which is much less than the expected value of 9. Adjacent to this residue in the active site is a second lysine residue, which in protonated form destabflizes the protonated amine and, therefore, reduces the pKa. Conversely, aspartic acid or glutamic acid residues that are positioned in hydrophobic active sites can have increased pKa values near 7 because the anionic form of the side chain is destabilized. 

Acetoacetate decarboxylase D-amino acid oxidase Carbonic anhydrase Catalase... 

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. 

The decarboxylation of acetoacetate is acid catalyzed (20). Metals do not catalyze the spontaneous decarboxylation of acetoacetate, presumably because the substrate and the product acetone enol are poor ligands for the metal (27). Primary amines catalyze the decarboxylation of acetoacetate by a Schiff base mechanism (Scheme VII), and this provides the best model for acetoacetate decarboxylase (94). 

The decarboxylation of y8-keto acids by both primary amines and by acetoacetate decarboxylase has been studied by the method of carbon isotope effects [90,91]. A typical isotope effect for the amine-catalyzed reaction is 1.03 for the cyanomethylamine-catalyzed decarboxylation of acetoacetate at pH 5.0. This demonstrates that the carbon-carbon bond is being cleaved in the rate-determining decarboxylation step. The comparable carbon isotope effect for the enzymatic decarboxylation is = 1.018 and is pH-independent over the range pH... 

Acetoacetate Metabolism. An active deacylase in liver is responsible for the formation of free acetoacetate from its CoA derivative. The j8-hydroxybutyric dehydrogenase mentioned above and a decarboxylase are capable of converting acetoacetate into the other ketone bodies, /3-hydroxybutyrate, and acetone. liver does not contain a mechanism for activating acetoacetate. Heart muscle has been found to contain a specific thiophorase that forms acetoacetyl CoA at the expense of suc-cinyl CoA. Acetoacetate is thus used by peripheral tissues by activation through transfer, then reaction with either the enzymes of fatty acid synthesis or jS-ketothiolase and the enzymes that use acetyl CoA. 

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