Pyridoxal phosphate coenzyme

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

The coenzyme pyridoxal 5 -phosphate is required for the reactions in both (9) and (11). The sum of (10) and (11) gives (9), so that tryptophan synthetase is indeed a multienzyme complex catalyzing a sequence of reactions. The intermediate indole cannot be detected when the overall reaction is carried out, although the native enzyme will catalyze the partial reactions [(10) and (11)] 50 to 100 times more efficiently than the isolated subunits.86 88... 

Figure 3-23 (A) Stereoscopic a-carbon plot of the cystolic aspartate aminotransferase dimer viewed down its dyad symmetry axis. Bold lines are used for one subunit (subunit 1) and dashed lines for subunit 2. The coenzyme pyridoxal 5 -phosphate (Fig. 3-24) is seen most clearly in subunit 2 (center left). (B) Thirteen sections, spaced 0.1 nm apart, of the 2-methylaspartate difference electron density map superimposed on the a-carbon plot shown in (A). The map is contoured in increments of 2a (the zero level omitted), where a = root mean square density of the entire difference map. Positive difference density is shown as solid contours and negative difference density as dashed contours. The alternating series of negative and positive difference density features in the small domain of subunit 1 (lower right) show that the binding of L-2-methylaspartate between the two domains of this subunit induces a right-to-left movement of the small domain. (Continues)...

Figure 12-5 (A) Stereoscopic view of the structure of the catalytic site of phosphorylase b in the inhibited T-state with the inhibitor nojirimycin tetra-zole bound into the active site. Inorganic phosphate (P ) as well as the coenzyme pyridoxal 5 -phosphate (PLP) are also shown. (B) Details of interactions of the inhibitor, P , and PLP with the protein and with water molecules (small circles). This is a weak-binding state but the P has displaced the negatively charged side chain carboxylate of Asp 283 (visible at the lower right in A).

The coenzyme, pyridoxal 5 -phosphate (PLP), forms an aldimine linkage with the -amino group of Lys-258 via its aldehyde group. PLP also interacts with Asp-222, Tyr-225 and Asn-194 by forming hydrogen bonds thus it maintains proper orientation at the active site. The enzyme is in the open conformation. 

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. 

Most amino acids lose their nitrogen atom by a transamination reaction in which the -NH2 group of the amino acid changes places with the keto group of ct-ketoglutarate. The products are a new a-keto acid plus glutamate. The overall process occurs in two parts, is catalyzed by aminotransferase enzymes, and involves participation of the coenzyme pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin UJ. Different aminotransferases differ in their specificity for amino acids, but the mechanism remains the same. 

Vitamin Bg a family of molecules having vitamin Be activity these include pyridoxal, pyridoxine, and pyridoxamine precursors to the coenzyme pyridoxal phosphate. 

Glutamate can then participate in the formation of other amino acids via the process called transamination. Transamination is the exchange of the amino group from an amino acid to a keto acid, and provides the most common process for the introduction of nitrogen into amino acids, and for the removal of nitrogen from them. The reaction is catalysed by a transaminase enzyme, and the coenzyme pyridoxal phosphate (PLP) is required. 

Figure 51. Diagram of the lupinine, sparteine, lupanine and cytisine synthesis pathway. Abbreviations PLP = coenzyme pyridoxal phosphate C = cleavage of C4 unit.

Vitamin B deficiency is a rare condition, but it is prevalent in persons with chronic alcoholism due to low dietary Intake and impaired conversion ofpyridoxine to the active coenzyme pyridoxal phosphate. 

It is involved as a coenzyme (pyridoxal phosphate) in metabolism of tryptophan, in several metabolic transformations of amino acids including transamination, decarboxylation and racemization. 

An early step in the catabolism of amino acids is the separation of the amino group from the carbon skeleton. In most cases, the amino group is transferred to a-ketoglutarate to form glutamate. This transamination reaction requires the coenzyme pyridoxal phosphate. 

Vitamin B6 is a collective term for pyridoxine, pyridoxal, and pyridox amine, all derivatives of pyridine. They differ only in the nature of the functional group attached to the ring (Figure 28.10). Pyridoxine occurs primarily in plants, whereas pyridoxal and pyridoxamine are found in foods obtained from animals. All three compounds can serve as precur sors of the biologically active coenzyme, pyridoxal phosphate. Pyridoxal phosphate functions as a coenzyme for a large number of enzymes, par ticularly those that catalyze reactions involving amino acids. 

The proton NMR spectrum of the coenzyme pyridoxal phosphate in 2HzO is shown in Fig. 3-24 as obtained with a 60-MHz spectrometer. Four things can be measured from such a spectrum (1) the intensity (area under the band). In a proton NMR spectrum, areas are usually proportional to the numbers of equivalent protons giving rise to absorption bands ... 

Other enzymes in the aconitase family include isopropylmalate isomerase and homoaconitase enzymes functioning in the chain elongation pathways to leucine and lysine, both of which are pictured in Fig. 17-18.90 There are also iron-sulfur dehydratases, some of which may function by a mechanism similar to that of aconitase. Among these are the two fumarate hydratases, fumarases A and B, which are formed in place of fumarase C by cells of E. coli growing anaerobically.9192 Also related may be bacterial L-serine and L-threonine dehydratases. These function without the coenzyme pyridoxal phosphate (Chapter 14) but contain iron-sulfur centers.93-95 A lactyl-CoA... 

The vitamin biotin is formed in nature (left) by condensation of L-alanine with pimeloyl-CoAto form 8-amino-7-oxononanoate (AON). This compound is seen at the upper left of the center structure joined as a Schiff base with the coenzyme pyridoxal phosphate (PLP). This is a product complex of the enzyme AON synthase (see Webster et ah, Biochemistry 39,516-528,2000) Courtesy of D. Alexeev,... 

The principles of the above reactions form the basis of a series of important metabolic interconversions involving the coenzyme pyridoxal phosphate (structure 2.41). This condenses with amino acids to form a Schiff base (structure 2.42). The pyridine ring in the Schiff base acts as an electron sink which very effectively stabilizes a negative charge. 

Pyridoxine (B6) Precursor of the coenzyme pyridoxal phosphate. Deficiency causes dermatitis in... 

Fig. 5). This copper-containing enzyme requires the coenzyme pyridoxal phosphate, derived from vitamin B6 (see Topic M2), for activity. The aldehyde group on allysine then reacts spontaneously with either the side-chain amino group of Lys or with other allysine residues on other polypeptide chains to form covalent interchain bonds. 

This reaction is catalyzed by the enzyme ALA synthase (Fig. 2d) which requires the coenzyme pyridoxal phosphate (see Topic M2) and is located in the mitochondria of eukaryotes. This committed step in the pathway is subject to regulation. The synthesis of ALA synthase is feedback-inhibited by heme. 

View of an aj /3 pair. The a subunit (blue) is shown with the eight strands of the central barrel (white) surrounding the bound substrate analog (indole propanol phosphate) at the active center. Residues 57-59 ( disordered loop ) and 177-191 ( Flexible loop ) are shown in darker blue. The /3 subunit N-domain (yellow) and C-domain (orange) are shown surrounding the coenzyme pyridoxal phosphate at the active site. The central /3-sheet strands of each domain are also shown in white. Note the region of the C-domain which does not have a well-defined secondary structure and which is in contact with the a subunit. 

Tire difluoromethyl analog of the natural substrate ornithine acts as a mechanism-based inhibitor of ornithine decarboxylase (OD). The final stage ofthe process is formation of a covalently bound complex with the enzyme and the coenzyme pyridoxal phosphate (PLP) [10] (Scheme 4.36). 

All of the more than fifty transaminases that have been discovered require the coenzyme pyridoxal phosphate. This coenzyme is derived from vitamin (p)rri-doxine. Figure 22.9). 

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