Pyridoxal phosphate deamination

Histamine is synthesised by decarboxylation of histidine, its amino-acid precursor, by the specific enzyme histidine decarboxylase, which like glutaminic acid decarboxylase requires pyridoxal phosphate as co-factor. Histidine is a poor substrate for the L-amino-acid decarboxylase responsible for DA and NA synthesis. The synthesis of histamine in the brain can be increased by the administration of histidine, so its decarboxylase is presumably not saturated normally, but it can be inhibited by a fluoromethylhistidine. No high-affinity neuronal uptake has been demonstrated for histamine although after initial metabolism by histamine A-methyl transferase to 3-methylhistamine, it is deaminated by intraneuronal MAOb to 3-methylimidazole acetic acid (Fig. 13.4). A Ca +-dependent KCl-induced release of histamine has been demonstrated by microdialysis in the rat hypothalamus (Russell et al. 1990) but its overflow in some areas, such as the striatum, is neither increased by KCl nor reduced by tetradotoxin and probably comes from mast cells. 

Vitamin Bg is a mixture of six interrelated forms pyridoxine (or pyridoxol) (Figure 19.23), pyri-doxal, pyridoxamine, and their 5 -phosphates derivatives. Interconversion is possible between all forms. The active form of the vitamin is pyridoxal phosphate, which is a coenzyme correlated with the function of more than 60 enzymes involved in transamination, deamination, decarboxylation, or desulfuration reactions. 

Terms in bold are defined in aminotransferases 660 transaminases 660 transamination 660 pyridoxal phosphate (PLP) 660 oxidative deamination 661 l-glutamate dehydrogenase 661 glutamine synthetase 662 glutaminase 663 creatine kinase 664... 

Mortland, M. M. (1984). Deamination ofglutamic acid by pyridoxal phosphate-Cu2+-smectite catalysts. Journal of Molecular Catalysis, 27, 143-55. 

There is an important biochemical counterpart of the deamination reaction that utilizes pyridoxal phosphate, 7, as the aldehyde. Each step in the sequence is catalyzed by a specific enzyme. The a-amino group of the amino acid combines with 7 and is converted to a keto acid. The resulting pyridoxamine then reacts to form an imine with a different a-keto acid, resulting in formation of a new a-amino acid and regenerating 7. The overall process is shown in Equation 25-6 and is called transamination. It is a key part of the process whereby amino acids are metabolized. 

The reactions catalyzed by transaminases are anergonic as they do not require an input of metabolic energy. They are also freely reversible, the direction of the reaction being determined by the relative concentrations of the amino acid-keto acid pairs. Pyridoxal phosphate is not just used as the coenzyme in transamination reactions, but is also the coenzyme for several other reactions involving amino acids including decarboxylations, deaminations, racemizations and aldol cleavages. 

Oxidative deamination of alanine requires the cofactor pyridoxal phosphate and yields pyruvate as product. 

Serine and threonine are deaminated by serine dehydratase, which requires pyridoxal phosphate. Serine is converted to pyruvate, and threonine to a-ketobutyrate NH4+ is released. 

The collagen molecules formed by removal of the propeptides spontaneously assemble into fibrils. At this stage, the fibrils are still immature and lack tensile strength, which is acquired by cross-linking. The initial step in cross-link formation is the oxidative deamination of a-amino groups in certain lysyl and hydroxyly-syl residues catalyzed by lysyl oxidase. The enzyme is a copper-dependent (probably cupric) protein, and the reaction requires molecular oxygen and pyridoxal phosphate for full activity. Only native collagen fibrils function as substrates. 

The same scaffold was used to design catalysts for pyridoxal phosphate-dependent deamination of aspartic acid to form oxaloacetate, one half of the transamination reaction [8], and oxaloacetate decarboxylation [14]. Catalysis was due to binding of pyridoxal phosphate in close proximity to His residues capable of rate limiting 1,3 proton transfer. A two-residue catalytic site containing one Arg and one Lys residue was found to be the most efficient decarboxylation agent, more efficient per residue than the Benner catalyst, most likely due to a combination of efficient imine formation, pK depression and transition state stabilization. 

In addition to glutamate, a number of amino acids release their nitrogen as NH4 (see Fig. 38.5). Histidine may be directly deaminated to form NH4 and urocanate. The deaminations of serine and threonine are dehydration reactions that require pyridoxal phosphate and are catalyzed by serine dehydratase. Serine forms pyruvate, and threonine forms a-ketobutyrate. In both cases, NH4 is released. 

Amino acid metabolism requires the participation of three important cofactors. Pyridoxal phosphate is the quintessential coenzyme of amino acid metabolism (see Chapter 38). All amino acid reactions requiring pyridoxal phosphate occur with the amino group of the amino acid covalently bound to the aldehyde carbon of the coenzyme (Fig. 39.3). The pyridoxal phosphate then pulls electrons away from the bonds around the a-carbon. The result is transamination, deamination, decarboxylation, P-elimination, racemization, and -elimination, depending on which enzyme and amino acid are involved. 

In further consideration of the biosynthesis of the piperidine alkaloids the question of the significance of the incorporation of cadaverine must be answered. Accordingly further research has been directed to this point and it has been shown that cadaverine is a normal component of S. acre, that it is a specific precursor of sedamine (20), and that it is formed from lysine at the same time as sedamine. It follows then that any scheme for the biosynthesis of the piperidine alkaloids which does not accommodate cadaverine as a normal component is unrealistic An eminently reasonable hypothesis which fits all the evidence is shown in Scheme 1 it was anticipated in last year s Report. For those alkaloids derived from lysine without the intervention of a symmetrical intermediate, cadaverine formed by decarboxylation of lysine must remain enzyme-bound and therefore unsymmetrical. Exogenous cadaverine enters the pathway at this point by absorption on to the enzyme to give (29). In order to explain the incorporation of lysine into some alkaloids by way of a symmetrization step it is necessary only to postulate equilibration of bound with unbound cadaverine. The proposal that pyridoxal phosphate is involved in this pathway is more than mechanistically attractive, for L-lysinedecarboxylase (EC 4.1.1.18, L-lysine carboxy-lyase) and diamine oxidase [EC 1.4.3.6, diamine oxygen oxidoreductase (deaminating)], the two enzymes whose participation in the conversion of lysine into A -piperideine (30) is likely, both require pyridoxal phosphate as a co-factor. 

The enzyme serine dehydratase employs pyridoxal phosphate (PEP) in the direct deamination of serine. In this reaction, the a-carbon of serine undergoes a two-electron oxidation through a-elimination. Show how PEP participates in the process by writing a mechanism for serine dehydration and deamination. 

Pyridoxal (B ) Growth retardation Pyridoxal phosphate is a cofactor for transaminations and deaminations... 

Mammalian tissues contain enzymes that catalyze the nonoxidative deamination of serine, threonine, and homoserine. Since the postulated reaction mechanism involves a dehydration before the deamination, these enzymes are called dehydrases. L-Serine, L-threonine, and L-homoserine dehydrases have been partially purified and all are specific for the L-amino acid. Serine and threonine dehydrases require pyridoxal phosphate, ATP, and glutathione for activity. Pyridoxal phosphate requires the homoserine enzyme, but the need for ATP and glutathione has not been demonstrated. The reaction is likely to involve the formation of a Schiff base. The homoserine dehydrase has been... 

Pyridoxal phosphate is required for deamination, transamination, and decarboxylation of amino acids and also is involved in D-amino oxidation and in race-mization of the d- to the L-amino acid. Pyridoxal phosphate seems also to be required for diamine oxidase... 

One of the pathways to propanoyl-CoA is from catabolism of the amino acid threonine (Chapter 12). Thus, threonine (threonine dehydratase, EC 4.3.1.19, cofactor pyridoxal phosphate) undergoes deamination to give 2-oxobutanoate (a-ketobutyrate) as shown below. Then, 2-oxobutanoate (a-ketobutyrate) undergoes decarboxylation (perhaps as shown in Scheme 11.30) with formation of propanoyl dihydro-lipoamide in a (cofactor) thiamine diphosphate mediated step. Finally, as in Scheme 11.31, propanoyl-CoA is formed. An alternative pathway uses aferrodoxin to effect the decarboxylation of 2-oxobutanoate (a-ketobutyrate) Ferredoxins are small proteins containing iron and sulfur atoms in iron-sulfur clusters. 

Fig. 2a. Proposed mechanism of dehydration and deamination of serine by pyridoxal phosphate. ...

The dehydration nd deamination may be effected through a Schiff s base formation with pyridoxal phosphate. Such a mechanism has been postulated by Metzler and Snell SS9). It will be noted in the discussion below that many of the dehydrases studied require pyridoxal-5-phosphate. Replacement of the hydroxyl-hydrogen atom of the substrate prevents deamination. 

The preparation in a cell-free form of a D-serine dehydrase from E. coli which requires pyridoxal phosphate has been described by Metzler and Snell 218). This enzyme is readily separated from the L-serine (and threonine) dehydrase of Wood and Gunsalus 216). Unlike the latter enzyme, the D-serine dehydrase does not require AMP or glutathione. DL-Threonine was slowly deaminated by the system. 

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