Pyridine nucleotides biosynthesis

Mann, D.F. and R.U. Byermm Activation of the de novo pathway for pyridine nucleotide biosynthesis prior to ricinine biosynthesis in castor beans Plant Physiol. 53 (1974) 603-609. 

As shown in the biosynthesis of granaticin, a hydride shift occurs intramolecularly. This process is mediated by an enzyme-bond pyridine nucleotide. A concerted abstraction of H-4 as a hydride in la and a C-5 deprotonation in 2a leads to the 4,5-enol ether 3a. The reduced form of the pyridine nucleotide transfers the hydride to C-6, simultaneously releasing a hydroxide to give 4a. Final tautomerization yields the dTDP-4-keto-6-deoxy-sugar in v-xylo configuration 4a. In other enzymes of the oxidoreductase type, the active site may show a different configuration. Thus, the intermediate 3a can be protonated from above at C-5 to yield the l-arabino isomer of 4a [2]. The stereochemistry of this mechanism was demonstrated by double labelling (cf. l-4b series), and as a net result proved a suprafacial 4—>6 hydride shift. 

Apart from hydrogen evolution, the electrons of reduced ferredoxin can take alternative routes leading to biosynthesis. In anaerobic bacteria, reduced ferredoxin can be used directly for the reduction of pyridine nucleotides (Tagawa and Arnon (99) Valentine, Brill and Wolfe (107) Fredericks and Stadtman (44)) for the reduction of hydroxyla-mine to ammonia (Valentine, Mortenson, Mower, Jackson, and Wolfe (109) for COa fixation in the reductive carboxylation of acetyl-CoA to pyruvate (Bachofen, Buchanan, and Arnon (13) Raeburn and Rabino-witz (83) Andrews and Morris (3) Stern (98)) for the reduction of sulfite to sulfide (Akagi (1)) and, in the presence of ATP, it can be used for the reduction of N2 to NH3 (Mortenson (72,73) D Eustachio and Hardy (40)). The role of ferredoxin in these reactions as well as in the oxidative degradative reactions discussed above is summarized in Fig. 10. 

Figure 6 Biosynthesis of pyridine nucleotides. A) in animals and yeasts ...

The pyridine nucleotide coenzymes, NAD and NADP, provide the reducing equivalents necessary for energy transduction and biosynthesis in living cells. The reduced coenzymes are generated by the transfer of a hydride anion from a wide variety of substrates and catalyzed by an equally wide variety of dehydrogenases, the half reaction being... 

The biosynthesis, function, and degradation of the pyridine nucleotides have been reviewed by Singer and Kearney (812a). 

It was originally postulated that the methyl groups at C-4 were removed as COj -a suggestion that has proved to be correct. These groups are hydroxylated by a mixed-function oxidase which is NAD(P)H and Oj dependent. First, the 4a-methyl is attacked, yielding the 4 -hydroxymethyl-4 -methyl sterol. This reaction is catalyzed by a methyl sterol oxidase which has been solubilized and partially purified in Gaylor s laboratory [108]. The same enzyme preparation will, with reduced pyridine nucleotide and dioxygen, oxidize the C-30 carbon to a carboxylic acid. The 4a-methyl-4/8-hydroxymethyl-5a-cholestan-3j8-ol is not a substrate for sterol biosynthesis while its epimer is [5]. The detailed mechanisms for the enzymatic removal of C-30 and C-31 are not fully understood. The initial reaction yields a 4a-hydroxy-methyl sterol by inference however, neither the isolation nor the enzymatic formation of a 4a-hydroxymethyl sterol has been demonstrated in animal tissues. This may well result from the fact that the hydroxylation reaction is the slow step in the demethylation process [5]. 

A. G. Moat J. W. Foster, Biosynthesis and Salvage Pathways of Pyridine Nucleotides. In Pyridine Nucleotide Coenzymes - Part B P. R. Dolphin, 0. Avramovich, Eds. John Wiley Sons New York, 1987 pp 1-20. 

Like the acyl-CoA desaturases (Chapter 7), the 1 -alkyl desaturase exhibits the typical requirements of a microsomal mixed-function oxidase. Molecular oxygen, a reduced pyridine nucleotide, cytochrome b, cytochrome reductase, and a terminal desaturase protein that is sensitive to cyanide are all required. The precise reaction mechanism responsible for the biosynthesis of ethanolamine plasmalogens is unknown, but it is clear from an investigation with a tritiated fatty alcohol that only the 15 and 25 (erythro)-labeled hydrogens are lost during the formation of the alk-l -enyl moiety of ethanolamine plasmalogens. 

Ricinine and ethionine were found to inhibit ricinine biosynthesis with shunting of radioactivity from [6- C]quinolinic acid through the pyridine nucleotide cycle into N-methylnicotinic acid (30) and N-methylnicotinamide (31) (c/. Scheme 6), and quinolinic acid consumption was reduced. 

These results strongly indicate a dependency between ricinine biosynthesis and the pyridine nucleotide cycle. The similarity in incorporation of members of this cycle into ricinine" is explained by allowing each member to be diverted directly into a pathway leading to ricinine this explanation covers the observation that excess exogenous NAD increases the incorporation of quinolinic acid into ricinine" since this precursor can simply be shunted along one of these diversions if the formation of NAD is blocked by its presence in large excess. 

Thus, the reduced form of cytochrome P-450 functions as the oxygenactivating biocatalyst of a wide variety of mixed-function oxidations by vertebrate tissues effecting biosynthesis and catabolism of steroid hormones, bile acid formation, and the metabolism of drugs and other xeno-biotics (16). Since reduced pyridine nucleotides do not react directly with hemoproteins, the hydroxylase systems must include components that mediate the electron transport from TPNH to cytochrome P-450. There also must be distinctive diflFerences in composition causing the substrate specificity of the oxygenations. 

Physiology biosynthesis Like the tropane alkaloids, T. a. are formed in the roots and transported to the aboveground parts for storage by the plant s phloem system. In some sorts of tobacco plants a part of the nicotine is demethylated to nomicotine during transport to the shoot. Nomicotine and anabasine are often the main alkaloids in the so-called nicotine-poor tobacco plant types. The T. a. are formed biogenetically from nicotinic acid, made available via the pyridine nucleotide cycle (see nicotinamide), and a pyrrolidine or piperidine building block (figure). In the case of nicotine, like for the tropane alkaloids, Al-methylpyr-roline is an intermediate, in the biosynthesis of anabasine the intermediate is a piperidine derived from the amino acid lysine (see piperidine alkaloids). 

In the second step of phytoene formation, inorganic pyrophosphate is expelled and a proton is eliminated. Pyridine nucleotide cofactors are not involved, as is the case in triterpene biosynthesis (Fig. 26.3) (Poulter, 1990). 

Ribose phosphates phosphorylated derivatives of ribose. Ribose is phosphorylated in position 5 by the action of ribokinase (EC 2.7.1.15) and ATP ribose 5-phosphate is also produced in the Pentose phosphate cycle (see), and in the Calvin c cle (see) of photosynthesis. Phosphoribomutase cat yses the interconversion of ribose 5-phospbate and ribose 1-phosphate, and the cosubstrate of this reaction is ribose l,5-f>isphosphate. 5-Phosphoribosyl 1-pyrophos-phate donates a ribose 5-phosphate moiety in the de novo biosynthesis of purine and pyrimidine nucleotides (see Purine biosynthesis. Pyrimidine biosynthesis), in the Salvage pathway (see) of purine and pyrimidine utilization, in the biosynthesis of L-Histi-dine (see) and L-Tryptophan (see) and in the conversion of nicotinic acid into nicotinic acid ribotide (see Pyridine nucleotide cycle). Ribose 1-phosphate can also take part in nucleotide synthesis (see Salvage pathway). 

The exquisite sensitivity of resting lymphocytes to DNA damage and poly(ADP-ribose) polymerase activation may be due, in part, to a decreased capacity of the cells to synthesize NAD. Human lymphocytes possess an intact pyridine nucleotide cycle (10). The cells synthesize NAD from either nicotinamide or nicotinic acid, and they release nicotinamide as a by-product of ADP-ribosylation reactions. NMN pyrophosphorylase is the rate-limiting enzyme in NAD biosynthesis from nicotinamide, and the content of this enzyme in unstimulated human lymphocytes is quite low (9). We have examined the rate of NAD turnover in resting lymphocytes in order to quantitate the contribution of ADP-ribosylation to the overall consumption of NAD (10). In addition, we present here preliminary results of in vitro biochemical studies performed on malignant lymphocytes from patients with chronic lymphocytic leukemia (CLL), who were treated with 2-chlorodeoxyadenosine. 

Deoxyaldoses, e.g., D-digitoxose and D-cymarose (Fig. 29) may be formed by direct reduction at the level of nucleotide diphosphates by pyridine nucleotide-dependent dehydrogenases (C 2.1) in analogy to the biosynthesis of D-2-deoxy-ribose (Fig. 28) from D-ribose. 

Some of the bound keto groups may be reduced by pyridine nucleotide-dependent dehydrogenases (C 2.1, see the formation of 6-methylsalicylic acid, D 3.3.1), and some of the activated CHg-groups may be alkylated by S-adenosyl-L-meth-ionine (C 3.3, see the biosynthesis of tetracyclines, D 3.3.7), or may be substituted by dimethylaUyl pyrophosphate (D 6), see the formulas of lupulone and humulone, the bitter principles of hop cones used in brewing beer (F 1). 

In the tricarboxylic acid cycle acetyl CoA is degraded to COg and reduced pyridine nucleotides (NADH and NADPH, D 16.2). In addition it supplies intermediates for the biosynthesis of the amino acids L-glutamic acid (D 17) and L-aspartic acid (D 16). The carbon chains withdrawn from the cycle by amino acid biosynthesis are replaced by degradation of isocitric acid to succinic acid and glyoxylic acid and the formation of malic acid from the latter. The tri-carboxyhc acid cycle short-circuited by these reactions is known as the glyo-xyhc acid cycle. 

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