Pyridoxine phosphate, structure

Vitamin B6. Figure 1 Structure of pyridoxin, pyridoxal, pyridoxamine, and the coenzymes pyridoxal-5 -phosphate and pyridoxamine-5Y-phosphate. 

FIGURE 10.2 Structural formula of vitamin and related compounds. 1 — pyridoxine, 2 — pyridoxal, 3 — pyridoxamine, 4 — 4-pyridoxic acid 5 — pyridoxal-5 -phosphate. 

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). 

Vitamin B6 occurs naturally in three related forms pyridoxine (6.26 the alcohol form), pyridoxal (6.27 aldehyde) and pyridoxamine (6.28 amine). All are structurally related to pyridine. The active co-enzyme form of this vitamin is pyridoxal phosphate (PLP 6.29), which is a co-factor for transaminases which catalyse the transfer of amino groups (6.29). PLP is also important for amino acid decarboxylases and functions in the metabolism of glycogen and the synthesis of sphingolipids in the nervous system. In addition, PLP is involved in the formation of niacin from tryptophan (section 6.3.3) and in the initial synthesis of haem. 

VITAMIN B (Pyridoxine). Infrequently called adermine or pyridoxol, this vitamin participates in protein, carbohydrate, and lipid metabolism. The metabolically active form of B6 is pyridoxal phosphate, the structures of which are ... 

Pyridoxal-5 -phosphate is the coenzyme form of vitamin B6, and has the structure shown in figure 10.3. The name vitamin B6 is applied to any of a group of related compounds lacking the phosphoryl group, including pyridoxal, pyridoxamine, and pyridoxine. 

Pyridoxine, pyridoxal, and pyridoxamine, which occur in foodstuffs, are collectively known as vitamin Bg. In the body, all three are converted to pyridoxal phosphate which is the coenzyme for amino-acid decarboxylase and for transaminase. The structures of the three active forms of vitamin Bg and the pyridoxal phosphate, are shown below (55). 

Fig. 4. Structures of (a) pyridoxine (vitamin Bg), (b) pyridoxal phosphate and (c) pyridoxamine phosphate.

Vitamins of group B were analysed in different forms [530]. Isopropylidene derivatives showed selectivity of the chromatographic separation which was caused by even minor structural differences. Several compounds from the pyridoxine group can be analysed after their conversion into acetates acetylation followed by GC also appeared suitable for three vitamins and 4-pyridoxic lactone. TMS derivatives were recommended for GC separation of the phosphate form of vitamins. When treated with BSTFA—pyridine (1 1) at 60°C for 15 min, biotin provides a completely silylated derivative, which was analysed on a column packed with 3% of OV-17 [531 ]. 

Hayon and co-workers have studied, by pulse radiolysis, 1-hydropyridinyl radicals ranging from those formed from simple C-acylpyridines to those from pyridoxine and pyridoxal phosphate.210-212 Structural, spectroscopic, kinetic, and thermodynamic data have been presented. Various mechanisms have been adduced which involve 35 and analogous species from quinolines,... 

A second clue to the catalytic mechanism of phosphorylase is its requirement for pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin B5, Section 8.6.1). The aldehyde group of this coenzyme forms a Schiff base with a specific lysine side chain of the enzyme (Figure 21.7). The results of structural studies indicate that the reacting... 

Vitamin B Three substances are classed under the term pyridoxine or adermine pyridoxol, pyridoxal and pyridoxamine. Pyridoxine was isolated by various study groups in 1938. Its structure was described by Folkers and Kuhn in 1939. Pyridoxal and pyridoxamine were discovered by Snell in 1942. Pyridoxal phosphate and pyridoxamine phosphate are biologically active substances. Intestinal absorption of Bg is dose-dependent and not limited. In alcoholism, a deficiency of vitamin Bg is encountered in 20—30% of cases, whereas the respective percentage is 50—70% in alcoholic cirrhosis. Vitamin Bg is an important coenzyme for transaminases, which transfer amino groups from amino adds to keto acids. In this way, biochemical pathways between the dtiic acid cycle and carbohydrate and amino acid metabolisms are created. (104)... 

Transamination reactions require the coenzyme pyridoxal-5 -phosphate (PLP), which is derived from pyridoxine (vitamin B6). PLP is also required in numerous other reactions of amino acids. Examples include racemizations, decarboxylations, and several side chain modifications. (Racemizations are reactions in which mixtures of l- and D-amino acids are formed.) The structures of the vitamin and its coenzyme form are illustrated in Figure 14.2. 

The structure of pyridoxal phosphate, the coenzyme required for all transamination reactions, and pyridoxine, vitamin Bg, the vitamin from which it is derived. 

Yeh, J.I. et al (2002) Multistate binding in pyridoxine 5 -phosphate synthase 1.96 A crystal structure in complex with 1-deoxy-D-xylulose phosphate. Biochemistry, 41 (39), 11649-11657. 

Pyridoxine, the parent substance of pyridoxal phosphate, is also known as vitamin Be (page 165). Many coenzymes contain a derivative of one or other of the B vitamins as an essential part of their structure. Coenzymes may react with a number of different enzymes which are specific for different substrates but which catalyse the same general type of reaction. 

The generic term vitamin Be refers to all 3-hydroxy-2-methylpyridine derivatives that exhibit the biological activity of pyridoxine in rats (7). Pyridoxine (3-hydroxy-4,5-bis-(hydroxymethyl)-2-methyl pyridine) should not be used as generic term synonymous with vitamin B. Two other forms of vitamin Be, pyridoxal (PL) and pyridoxamine (PM), differ from pyridoxine (PN) in the respective location of an aldehyde and amine group at the 4 position of the pyridine ring structure (Fig. 1). The 5 -phosphoric ester of pyridoxal, pyridoxal-5 -phosphate (PLP), is the metabolically active form of vitamin Be. Pyridoxamine-5 -phosphate... 

Chemistry— Vitamin B-6 is found in foods in three forms which are readily interconvertible—pyridoxine, pyridoxal, and pyridoxamine. Also vitamin B-6 is found in physiological systems in the forms of pyridoxal phosphate and pyridoxamine phosphate. The structural formulas of the three naturally occurring free forms of these compounds are given in Fig. V-30. 

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