Decarboxylase pyridoxal phosphate-dependent

Kamath AV, GL Vaaler, EE Snell (1991) Pyridoxal phosphate-dependent histidine decarboxylases. Cloning, sequencing, and expression of genes from Klebsiella planticola and Enterobacter aerogenes and properties of the overexpressed enzymes. J Biol Chem 266 9432-9437. 

Scheme 18.45 Postulated inhibition mechanism of pyridoxal phosphate-dependent decarboxylases by a-allenic a-amino acids.

Most people have heard of antihistamines, even if they have little concept of the nature of histamine. Histamine is the decarboxylation product from histidine, and is formed from the amino acid by the action of the enzyme histidine decarboxylase. The mechanism of this pyridoxal phosphate-dependent reaction will be studied in more detail later (see Section 15.7). 

This pyridoxal-phosphate-dependent enzyme [EC 4.1.1.29], also referred to as sulfinoalanine decarboxylase, catalyzes the conversion of 3-sulfino-L-alanine to... 

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). 

Non-pyridoxal Phosphate Dependent. Figure 2 depicts the postulated mechanism for a non-pyridoxal phosphate catal) zed decarboxylation of histidine to histamine involving a pyruvoyl residue instead of pyridoxal -5 - phosphate (20). Histidine decarboxylases from Lactobacillus 30a and a Micrococcus sp. have been shown to contain a covalently bound pyruvoyl residue on the active site. The pyruvoyl group is covalently bound to the amino group of a phenylalanine residue on the enzyme, and is derived from a serine residue (21) of an inactive proenzyme (22). The pyruvoyl residue acts in a manner similar to pyridoxal phosphate in the decarboxylation reaction. 

Figure 1. Pyridoxal Phosphate Dependent Decarboxylase Mechanism...

Due to the absence of a hydrogen atom on the a-carbon, the a-fluoroalkyl amino acids (except, of course, the fluoroalanines, vide supra) cannot undergo an elimination of HR Consequently, they are more stable than fluoroalanines and other jS-fluoro amino acids previously described. On the other hand, similar to proteogenic amino acids, jS-fluoro amino acids and a-fluoroalkyl amino acids are generally substrates of pyridoxal phosphate depending on enzymes such as racemases and decarboxylases. When an amino acid is a substrate of such enzymes, the enzyme induces the development of a negative charge on the a-carbon, which can initiate a /(-elimination process. This reaction affords an electrophilic species (Michael acceptor type), which is able to add a nucleophilic residue of the enzyme. This notion of mechanism-based inhibitor is detailed in Chapter 7. 

Tissues of the mammalian central nervous system contain a pyridoxal phosphate-dependent glutamate decarboxylase that catalyzes conversion of Glu to y-aminobutyrate (GABA), an inhibitory synaptic transmitter. GABA is degraded by trans-imination with a-oxoglutarate as the acceptor to yield succinic semialdehyde, which then is oxidized to succinate by an NAD-linked dehydrogenase. 

Finally, the decarboxylation of amino acids catalyzed by several pyridoxal phosphate-dependent enzymes has been shown to proceed by a retention of configuration at the Ca atom144. The stereochemical course of the decarboxylation of 5-hydroxy tryptophan to 5-hydroxytryptamine (serotonin) catalyzed by the pyridoxal phosphate-dependent aromatic L-amino acid decarboxylase (equation 15) exemplifies such studies145. 

The pyridoxal phosphate-dependent ornithine decarboxylase converts S-ornithine to 1,4-diaminobutane (equation 23). It was reported that (R)-hex-5-yne- 1,4-diamine is an irreversible inhibitor of the enzyme and evidence was presented in favor of the proposition that the enzyme performs stereoselective proton abstraction of the pro (R)... 

Table 9.2 Amines Formed by Pyridoxal Phosphate-Dependent Decarboxylases...

Some pyridoxal phosphate-dependent enzymes are normally fuUy saturated with cofactor and show the same activity on assay in vitro whether additional pyridoxal phosphate is present in the incubation medium or not. Examples of this class of enzymes include liver cysteine sulfinate decarboxylase (which is involved in the synthesis of taurine from cysteine Section 14.5.1) and the brain and liver glutamate and aspartate aminotransferases. 

The rates of synthesis and cataholism of some pyridoxal phosphate-dependent enzymes are altered in deficiency For example, within a few days of feeding a vitamin Be-free diet to animals, there is a fall in the activity of cysteine sulfinate decarboxylase in fiver after 2 weeks, the amount of the enzyme protein has fallen to extremely low levels. It is Ukely that these enzymes are sacrificed to release pyridoxal phosphate for other, more essential enzymes. Other enzymes show the opposite response - apparent induction of the apoenzyme in vitamin Be deficiency, presumably in an attempt to trap as much of the available pyridoxal phosphate as possible. Sato and coworkers (1996) demonstrated increased cataboUsm of apocystathionase in vitamin Be deficiency, but no decrease in the amount of immunoreactive protein in the liver, as a result of increased transcription. 

There is a great deal of evidence that deficiency of serotonin (5-hydroxytryptamine) is a factor in depressive illness, and many antidepressant drugs act to decrease its catabolism or enhance its interaction with receptors. A key enzyme involved in the synthesis of serotonin (and the catecholamines) is aromatic amino acid decarboxylase, which is pyridoxal phosphate-dependent. Therefore, it has been suggested that vitamin Be deficiency may result in reduced formation of the neurotransmitters and thus be a factor in the etiology of depression. Conversely, it has been suggested that supplements of vitamin Be may increase aromatic amino acid decarboxylase activity, and increase amine synthesis and have a mood-elevating or antidepressant effect. There is little evidence that vitamin Be deficiency affects the activity of aromatic amino acid decarboxylase. In patients with kidney failure, undergoing renal dialysis, the brain concentration of pyridoxal phosphate falls to about 50% of normal, with no effect on serotonin, catecholamines, or their metabolites (Perry etal., 1985). 

Aspartate undergoes /3-decarboxylation to /S-alanine unlike most amino acid decarboxylases, aspartate decarboxylase is not pyridoxal phosphate-dependent, but has a catalytic pyruvate residue, derived by postsynthetic modification of a serine residue (Section 9.8.1). Pantothenic acid results from the formation of a peptide bond between /3-alanine and pantoic acid. 

Zhang F, Thottananiyil M, Martin DL, and Chen CH (1999) Conformational alteration in serum albumin as a carrier for pyridoxal phosphate a distinction from pyridoxal phosphate-dependent glutamate decarboxylase. Archives of Biochemistry and Biophysics 3S4, 195-202. 

The enzymes, amino acid decarboxylases are pyridoxal phosphate- dependent enzymes. Pyridoxal phosphate forms a Schiff s base with e amino acid so as to stabilise the a-carbanion formed by the cleavage of bond between carboxyl and a-carbon atom. The physiologically active amines epinephrine, nor-epinephrine, dopamine, serotonin, y-amino butyrate and histamine are formed through decarboxylation of the corresponding precursor amino acids... 

Transamination Reactions of Other Pyridoxal Phosphate Enzymes In addition to their mean reactions, a number of pyridoxal phosphate-dependent enzymes also catalyze the half-reaction of tremstunination. Such enzymes include serine hydroxymethyltiemsfertise (Section 10.3.1.1), severed decarboxylases, and kynureninase (Section 8.3.3.2). 

Dopa is decarboxylated to 2-(3,4-dihydroxyphenyl) ethylamine (dopamine) by aromatic L-amino acid decarboxylase, a nonspecific cytosolic pyridoxal phosphate-dependent enzyme also involved in formation of other amines (e.g., 5-hydroxytryptamine). 

According to O Leary the A /A KIE (for loss of COj) varies between 1.01 and 1.03 for a variety of pyridoxal phosphate-dependent decarboxylases . Assuming (based on models) that the intrinsic KIE is ca 1,05 for the decarboxylation step these numbers suggest that decarboxylation is not totally rate-determining, rather Schiff-base interchange (transimination) and C—C scission are both participating in rate-limitation ki a s) see Scheme 13. 

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