Pyridoxal pyrophosphate

TRA - blocks pyridoxal pyrophosphate-dependent enzymes by forming oxime with the coenzyme (OTCase) [lysine antimetabolite]... 

Pyridoxal pyrophosphate glucose" structure in reconstituted phosphorylase... 

Pyridinium triflates, 620 (2,6)-Pyridinophanes, 51 2-Pyridones, 479 Pyridoxal pyrophosphate, 114 Pyrocatechase, 429... 

The first reaction in the catabolism of most amino acids is transamination, a reaction that requires the coenzyme pyridoxal pyrophosphate. We saw that transamination replaces the amino group of the amino acid with a ketone group (Section 24.5). para-Hydroxyphenylpyruvate, the product of the transamination of tyrosine, is converted by a series of reactions to fumarate and acetyl-CoA. 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

Nucleophilic catalysis is a specific example of covalent catalysis the substrate is transiently modified by formation of a covalent bond with the catalyst to give a reactive intermediate. There are also many examples of electrophilic catalysis by covalent modification. It will be seen later that in the reactions of pyridoxal phosphate, Schiff base formation, and thiamine pyrophosphate, electrons are stabilized by delocalization. 

Effective concentration 65-72 entropy and 68-72 in general-acid-base catalysis 66 in nucleophilic catalysis 66 Elastase 26-30, 40 acylenzyme 27, 40 binding energies of subsites 356, 357 binding site 26-30 kinetic constants for peptide hydrolysis 357 specificity 27 Electrophiles 276 Electrophilic catalysis 61 metal ions 74-77 pyridoxal phosphate 79-82 Schiff bases 77-82 thiamine pyrophosphate 82-84 Electrostatic catalysis 61, 73, 74,498 Electrostatic effects on enzyme-substrate association rates 159-161... 

In addition to the larger families of preparatively useful aldolases, some less common aldolases have been evaluated lately for preparative use. A range of mechanistically distinct enzymes, which are actually categorized as transferases but which also catalyze aldol-related additions through the aid of cofactors such as pyridoxal 5-phosphate (PLP), thiamine pyrophosphate (TPP), tetrahydro-folate (THF), or coenzyme A (CoA), are becoming more frequently applied in organic synthesis. Because of their emerging importance and/or commercial availability, a selection of these enzymes and examples of their synthetic utility will be included in further separate chapters. 

Whyte MP, Landt M, Ryan LM, Mulivor RA, Henthorn PS, Fedde KN, Mahuren JD, Coburn SP. 1995. Alkaline phosphatase placental and tissue-nonspecific isoenzymes hydrolyze phosphoethanolamine, inorganic pyrophosphate, and pyridoxal 5 -phosphate. Substrate accumulation in carriers of hypophos-phatasia corrects during pregnancy. J Clin Invest 95 1440-5. 

There is a general requirement for pyridoxal-5-phosphate (24, 25, 27, 44) although not all of the activity lost on dialysis is restored by adding the cofactor. This requirement explains the inhibition by hydroxylamine and hydrazine (24, 25). The reaction is a typical pyridoxal-5-phosphate catalyzed a,/ -elimination with a mechanism similar to serine dehydrase and cysteine desulfhydrase (45). The coenzyme is probably bound as a Schiff base with an amino group of the enzyme since there is an absorption maximum at 415 nm in solutions of the purified garlic enzyme (40). The inhibition by L-cysteine is presumably caused by formation of a thiazolidine with the coenzyme (46). Added pyridoxal-5-phosphate also combines directly with the substrate. The dissociation constant for the complex is about 5 X lO M. When this is taken into account, the dissociation constant of the holoenzyme can be shown to be about 5 X 10 M (47). The higher enzyme activity in pyrophosphate buflFer than in Tris or phosphate may be explained by pyrophosphate chelation of metal ions which otherwise form tighter complexes with the substrate and coenzyme (47). This decreases the availability of added coenzyme. 

Figure 4 Biosynthesis of thiamine (vitamin ). 37, aminoimidazole ribotide 38, 2-methyl-4-amino-5-hydroxymethyl-pyrimidine phosphate 39, pyridoxal 5 -phosphate 40, histidine 41, 2-methyl-4-amino-5-hydroxymethyl-pyrimidine pyrophosphate 42, 4-methyl-5-p-hydroxyethylthiazole phosphate 43,1 -deoxy-D-xylulose 5-phosphate 44, 5-ADP-D-ribulose 45, thiamine phosphate 46, thiamine pyrophosphate.

Several coenzymes are involved in the biosynthesis of their own precursors. Thus, thiamine is the cofactor of the enzyme that converts 1-deoxy-D-xylulose 5-phosphate (43) (the precursor of thiamine pyrophosphate, pyridoxal 5 -phosphate and of iso-prenoids via the nomnevalonate pathway) into 2 C-methyl-D-erythritol 4-phosphate (90, Fig. 11). Similarly, two enzymes required for the biosynthesis of GTP, which is the precursor of tetrahydrofolate, require tetrahydrofolate derivatives as cofactors (Fig. 3). When a given coenzyme is involved in its own biosynthesis, we are faced with a hen and egg problem, namely how the biosynthesis could have evolved in the absence of the cmcially required final product. The answers to that question must remain speculative. The final product may have been formed via an alternative biosynthetic pathway that has been abandoned in later phases of evolution or that may persist in certain organisms but remains to be discovered. Alternatively, the coenzyme under study may have been accessible by a prebiotic sequence of spontaneous reactions. An interesting example in this respect is the biosynthesis of flavin coenzymes, in which several reaction steps can proceed without enzyme catalysis despite their mechanistic complexity. 

Branching of pathways is relevant in several cases. Thus, intermediates of the porphyrin biosynthetic pathway serve as precursors for chlorophyll (17, Fig. 2) and for the corrinoid ring systems of vitamin B12 (20, Fig. 2) (17). 1-Deoxy-D-xylulose 5-phosphate (43) serves as an intermediate for the biosynthesis of pyridoxal 5 -phosphate (39, Fig. 5), for the terpenoid precursor IPP (86) via the nonmevalonate pathway (Fig. 11), and for the thiazole moiety of thiamine pyrophosphate (46, Fig. 4). 7,8-Dihydroneopterin triphosphate (29, Fig. 3) serves as intermediate in the biosynthetic pathways of tetrahydrofolate (33) and tetrahydrobiopterin (31). The closely related compound 7,8-dihydroneopterin 2, 3 -cyclic phosphate is the precursor of the archaeal cofactor, tetrahydromethanopterin (34) (58). A common pyrimidine-type intermediate (23) serves as precursor for flavin and deazaflavin coenzymes. Various sulfur-containing coenzymes (thiamine (9), lipoic acid (7), biotin (6), Fig. 1) use a pyrosulfide protein precursor that is also used for the biosynthesis of inorganic sulfide as a precursor for iron/sulfur clusters (12). 

Figure 11 Biosynthesis of isoprenoid type cofactors. 18, Heme a 39, pyridoxal 5 -phosphate 43, 1-deoxy-D-xylulose 5-phosphate 46, thiamine pyrophosphate 83, acetyl-CoA 84, (S)-3-hydroxy-3-methylglutaryl-CoA 85, mevalonate 86, isopentenyl diphosphate (IPP) 87, dimethylallyl diphosphate (DMAPP) 88, pyruvate 89, D-glyceraldehyde 3-phosphate 90, 2C-methyl-D-erythritol 4-phosphate 91, 2C-methyl-erythritol 2,4-cyclodiphosphate 92, 1-hydroxy-2-methyl-2-( )-butenyl 4-diphosphate 93, polyprenyl diphosphate 94, cholecalciferol 95, fS-carotene 96, retinol 97, ubiquinone 98, menaquinone 99, a-tocopherol.

Another metabolic disorder that is hereditary and little known is hypophosphatasia. Hypophosphatasia is an inherited metabolic (chemical) bone disease that results from low levels of an enzyme called alkaline phosphatase (ALP). ALP is normally present in large amounts in bones and the liver. In hypophosphatasia, abnormalities in the gene that makes ALP lead to the production of inactive ALP. Subsequently, several chemicals, including phosphoethanolamine, pyridoxal 57-phosphate (a form of vitamin B ) and inorganic pyrophosphate, accumulate in the body and are found in large amounts in the blood and urine. It appears that the accumulation of inorganic pyrophosphate is the cause of the characteristic defective calcification of bones seen in infants and children (rickets) and in adults (osteomalacia). 

Figure 4-8. The structures of thiamine pyrophosphate (A), biotin (fi), pyridoxal phosphate (Q, and ascorbate (D). Arrows indicate the reactive sites. When an a-keto acid binds to thiamine pyrophosphate, the keto group attaches and the carboxyl group is released as C02.

Glyoxalate can be transaminated to glycine, reduced to glycolate, converted to a-hydroxy-/3-ketoadipate by reaction with a-ketoglutarate, or oxidized to oxalate and excreted in urine. The first three reactions require pyridoxal phosphate, NADH, and thiamine pyrophosphate, respectively. In humans, ascorbic acid (vitamin C) is a precursor of urinary oxalate (Chapter 38). Since calcium oxalate is poorly soluble in water, it can cause nephrolithiasis and nephrocalcinosis due to hyperoxaluria. 

The isopentenyl pyrophosphate and the dimethylallyl pyrophosphate precursors to the octaprenyl moiety are derived from 1-deoxy-D-xylulose-5-phosphate rather than from mevalonic acid (see also thiamin and pyridoxal sections) [183, 184]. 

Bi) is converted to thiamine pyrophosphate simply by the addition of pyrophosphate. It is involved in aldehyde group transfer. Niacin (nicotinic acid) is esterified to adenine dinucleotide and its two phosphates to form nicotinamide adenine dinucleotide. Pyridoxine (vitamin B ) is converted to either pyridoxal phosphate or pyridoxamine phosphate before complexing with enzymes. Riboflavin becomes flavin mononucleotide by obtaining one phosphate (riboflavin 5 -phosphate). If it complexes with adenine dinucleotide via a pyrophosphate ester linkage, it becomes flavin adenine dinucleotide. 

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