Pyridoxal-5 ’-phosphate -dependent

FIGURE 14.22 Glutamate aspartate aminotransferase, an enzyme conforming to a double-displacement bisnbstrate mechanism. Glutamate aspartate aminotransferase is a pyridoxal phosphate-dependent enzyme. The pyridoxal serves as the —NH, acceptor from glntamate to form pyridoxamine. Pyridoxamine is then the amino donor to oxaloacetate to form asparate and regenerate the pyridoxal coenzyme form. (The pyridoxamine enzyme is the E form.)... 

Pyridoxal phosphate-dependent enzymes (Schiff base)... 

Write a reasonable mechanism for the 3-ketosphinganine synthase reaction, remembering that it is a pyridoxal phosphate-dependent reaction. 

The metabolism of P-hydroxy-a-amino adds involves pyridoxal phosphate-dependent enzymes, dassified as serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1) or threonine aldolases (ThrA L-threonine selective = EC 4.1.2.5, L-aHo-threonine selective = EC 4.1.2.6). Both enzymes catalyze reversible aldol-type deavage reactions yielding glycine (120) and an aldehyde (Eigure 10.45) [192]. 

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.

In addition to a-allenic a-amino acids, the corresponding allenic derivatives of y-aminobutyric acid (GABA) have also been synthesized as potential inhibitors of the pyridoxal phosphate-dependent enzyme GABA-aminotransferase (Scheme 18.49) [131,138-142]. The synthesis of y-allenyl-GABA (152) and its methylated derivatives was accomplished through Crabbe reaction [131], aza-Cope rearrangement [138] and lactam allenylation [139], whereas the fluoroallene 153 was prepared by SN2 -reduc-tion of a propargylic chloride [141]. 

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

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 enzyme [EC 2.6.1.2], also known as glutamic-pyruvic transaminase and glutamic-alanine transaminase, catalyzes the pyridoxal-phosphate-dependent reaction of alanine with 2-ketoglutarate, resulting on the production of pyruvate and glutamate. 2-Aminobutanoate will also react, albeit slowly. There is another alanine aminotransferase [EC 2.6.1.12], better known as alanine-oxo-acid aminotransferase, which catalyzes the pyridoxal-phosphate-dependent reaction of alanine and a 2-keto acid to generate pyruvate and an amino acid. See also Alanine Glyoxylate Aminotransferase... 

This enzyme [EC 2.6.1.21], also known as D-aspartate aminotransferase, D-amino acid aminotransferase, and D-amino acid transaminase, catalyzes the reversible pyridoxal-phosphate-dependent reaction of D-alanine with a-ketoglutarate to yield pyruvate and D-glutamate. The enzyme will also utilize as substrates the D-stereoisomers of leucine, aspartate, glutamate, aminobutyrate, norva-hne, and asparagine. See o-Amino Acid Aminotransferase... 

This enzyme [EC 2.6.1.18], also known as j8-alanine-pyruvate aminotransferase, catalyzes the reversible pyridoxal-phosphate-dependent reaction of /3-alanine with pyruvate to generate 3-oxopropanoate and alanine. 

This enzyme [EC 5.1.1.1] catalyzes the pyridoxal-phosphate-dependent interconversion of D-alanine with L-alanine. 

This pyridoxal phosphate-dependent enzyme [EC 2.6.1.43] catalyzes the reversible reaction of 5-amino-levulinate with pyruvate to produce 4,5-dioxopentanoate and alanine. 

This enzyme [EC 2.6.1.42], also referred to as transaminase B, catalyzes the reversible reaction of leucine with a-ketoglutarate (or, 2-oxoglutarate) to produce 4-methyl-2-oxopentanoate and glutamate. The pyridoxal-phosphate-dependent enzyme will also utilize isoleucine and valine as substrates. However, this enzyme is distinct from that of valine pyruvate aminotransferase [EC 2.6.1.66]. See also Leucine Aminotransferase... 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.8], also referred to as /3-cystathionase and cystine lyase, catalyzes the hydrolysis of cystathionine to yield homocysteine, pyruvate, and ammonia. 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.13] catalyzes the conversion of an 5-substituted cysteine (that is, RS-CH2-CH(NH3")C00 ) to RSH, ammonia, and pyruvate. See also S-Substituted Cysteine Sulfoxide Lyase... 

This pyridoxal-phosphate-dependent enzyme [EC 4.2.1.22] (also known as serine sulfhydrase, /3-thionase, and methylcysteine synthase) catalyzes the reaction of homocysteine with serine to produce cystathionine and water. 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.10] catalyzes the reaction cysteine with sulfite to produce cysteate and H2S. The enzyme can also catalyze the reaction of two cysteines (thereby producing lanthio-nine) as well as other alkyl thiols as substrates. 

This enzyme [EC 4.1.1.20] is a pyridoxal-phosphate-dependent protein that catalyzes the conversion of meso-... 

This pyridoxal-phosphate-dependent enzyme [EC 4.1.1.15] catalyzes the conversion of L-glutamate to 4-aminobutanoate and carbon dioxide. The mammalian brain enzyme also acts on L-cysteate, 3-sulfino-L-alanine, and L-aspartate. 

This pyridoxal-phosphate-dependent enzyme [EC 2.1.2.5], also known as glutamate formyltransferase, catalyzes the reaction of 5-formiminotetrahydrofolate with L-glutamate to produce tetrahydrofolate and A-formim-ino-L-glutamate. The enzyme will additionally catalyze the transfer of the formyl moiety from 5-formyltetrahy-drofolate to L-glutamate. This protein occurs in eukaryotes as a bifunctional enzyme also having a formiminote-trahydrofolate cyclodeaminase activity [EC 4.3.1.4]. 

This pyridoxal-phosphate-dependent enzyme [EC 2.6.1.4] catalyzes the reaction of glycine with a-ketoglu-tarate (or, 2-oxoglutarate) to produce glyoxylate and l-glutamate. See also GlycineiOxaloacetate Aminotransferase Glyoxylate Aminotransferase A. E. Braunstein (1973) The Enzymes, 3rd ed., 9, 379. 

This pyridoxal-phosphate-dependent enzyme [EC 3.7.1.3] catalyzes the hydrolysis of kynurenine to produce anthranilate and alanine. 3 -Hydroxykynurenine and some other (3-arylcarbonyl)alanines can also be acted upon by this enzyme. 

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