Pyridoxal phosphat

Coenzymes effecting transfer of groups. Examples of this class are adenosine triphosphate (ATP), biotin, coenzyme A and pyridoxal phosphate. 

Coenzymes effecting isomerization. Pyridoxal phosphate also falls into this class,... 

An example of a biologically important aide hyde is pyridoxal phosphate which is the active form of vitamin Bg and a coenzyme for many of the reac tions of a ammo acids In these reactions the ammo acid binds to the coenzyme by reacting with it to form an imine of the kind shown in the equation Re actions then take place at the ammo acid portion of the imine modifying the ammo acid In the last step enzyme catalyzed hydrolysis cleaves the imme to pyridoxal and the modified ammo acid... 

Hydrolases represent a significant class of therapeutic enzymes [Enzyme Commission (EC) 3.1—3.11] (14) (Table 1). Another group of enzymes with pharmacological uses has budt-ia cofactors, eg, in the form of pyridoxal phosphate, flavin nucleotides, or zinc (15). The synthases, and other multisubstrate enzymes that require high energy phosphates, are seldom available for use as dmgs because the required co-substrates are either absent from the extracellular space or are present ia prohibitively low coaceatratioas. 

Fig. 2. Biosynthetic pathway for epinephrine, norepinephrine, and dopamine. The enzymes cataly2ing the reaction are (1) tyrosine hydroxylase (TH), tetrahydrobiopterin and O2 are also involved (2) dopa decarboxylase (DDC) with pyridoxal phosphate (3) dopamine-P-oxidase (DBH) with ascorbate, O2 in the adrenal medulla, brain, and peripheral nerves and (4) phenethanolamine A/-methyltransferase (PNMT) with. Cadenosylmethionine in the adrenal...

Carbonic anhydrase Pyridoxal phosphate (PLP) Amino groups Aspartate aminotransferase... 

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.)... 

Muscle glycogen phosphorylase is a dimer of two identical subunits (842 residues, 97.44 kD). Each subunit contains a pyridoxal phosphate cofactor, covalently linked as a Schiff base to Lys °. Each subunit contains an active site (at the center of the subunit) and an allosteric effector site near the subunit interface (Eigure 15.15). In addition, a regulatory phosphorylation site is located at Ser on each subunit. A glycogen-binding site on each subunit facilitates prior association of glycogen phosphorylase with its substrate and also exerts regulatory control on the enzymatic reaction. 

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

The versatile chemistry of pyridoxal phosphate offers a rich learning experience for the student of mechanistic chemistry. William Jencks, in his classic text. Catalysis in Chemistry and Enzymology, writes ... 

It has been said that God created an organism especially adapted to help the biologist find an answer to every question about the physiology of living systems if this is so it must be concluded that pyridoxal phosphate was created to provide satisfaction and enlightenment to those enzymologists and chemists who enjoy pushing electrons, for no other coenzyme is involved in such a wide variety of reactions, in both enzyme and model systems, which can be reasonably interpreted in terms of the chemical properties of the coenzyme. Most of... 

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

Pyridoxal phosphate, a close relative of vitamin B6, is involved in a large number of metabolic reactions. TeJl the hybridization, and predict the bond angles for each nonterminal atom. 

The amino acid methionine is biosynthesized by a multistep roule that includes reaction of an inline of pyridoxal phosphate (PLP) to give an unsaturated imine. which then reacts with cysteine. What kinds of reactions are occurring in the two steps ... 

A heterocycle is a cyclic compound that contains atoms of two or more elements in its ring, usually carbon along with nitrogen, oxygen, or sulfur. Heterocyclic amines are particularly common, and many have important biological properties. Pyridoxal phosphate, a coenzyme sildenafil (Viagra),... 

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. 

The mechanism of the first part of transamination is shown in Figure 29.14. The process begins with reaction between the a-amino acid and pyridoxal phosphate, which is covalently bonded to the aminotransferase by an iminc linkage between the side-chain -NTI2 group of a lysine residue and the PLP aldehyde group. Deprotonation/reprotonation of the PLP-amino acid imine in steps 2 and 3 effects tautomerization of the imine C=N bond, and hydrolysis of the tautomerized imine in step 4 gives an -keto acid plus pyridoxamine... 

Under optimal conditions (pH = 8.0,67 g T1 L-aspartic add, 30°C, 1 1 ratio of enzyme activities) after addition of pyridoxal phosphate, 76 g l 1 L-phenylalanine could be produced within 72 hours (92% conversion). This illustrates how simple biochemical manipulation can increase productivity dramatically. 

Ap4A, diadenosine tetraphosphate BBG, Brilliant blue green BzATP, 2 - 3 -0-(4-benzoyl-benzoyl)-ATP cAMP, cyclic AMP CCPA, chlorocyclopentyl adenosine CPA, cyclopentyl adenosine CTP, cytosine triphosphate DPCPX, 8-cyclopentyl-1,3-dipnopylxanthine IP3, inosine triphosphate lpsl, diinosine penta phosphate a,p-meATP, a,p-methylene ATP p.y-meATP, p.y-meihylene ATP 2-MeSADP, 2-methylthio ADP 2-MeSAMP, 2-methylthio AMP 2-MeSATP, 2-methylthio ATP NECA, 5 -W-ethylcarboxamido adenosine PPADS, pyridoxal-phosphate-6-azophenyl-2, 4 -disulfonic acid PLC, phospholipase C RB2, reactive blue 2 TNP-ATP, 2, 3 -0-(2,4,6-trinitrophenyl) ATP. 

In general, pyridoxamine and pyridoxin are more stable than pyridoxal. All vitamers are relatively heat-stable in acid media, but heat labile in alkaline media. All forms of vitamin B6 are destroyed by UV light in both neutral and alkaline solution. The majority of vitamin B6 in the human body is stored in the form of pyridoxal phosphate in the muscle, bound to glycogen phos-phorylase. 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

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