Ribose/phosphate reaction

The mode of ribose frs entation was fiirther investigated by TLC analysis of the less volatile quinoxaline products from the ribose/phosphate reaction. 

In theory, periodate oxidation could have given a clear-cut answer as to the composition of the isomeric mixture of deoxy ribose phosphates. The 4-phosphate (73), devoid of vicinal diol groups, should be resistant to periodate the 3-phosphate (74) should reduce one and only one molar equivalent of the oxidant and yield one molar equivalent of both formaldehyde and the phosphorylated dialdehyde (75), whereas the 5-phosphate (76) could be expected to reduce one molar equivalent of periodate relatively rapidly, followed by a slower overoxidation reaction owing to the oxidation of malonaldehyde, formed as a result of the glycol cleavage. 

One obvious way in which to attach a nitroxide group to B12 is to simple alkylate Cob(I)aiamin with a suitable nitroxide derivative. This would result in having the nitroxide covalently bound to the corrinoid at the upper axial coordination position of the cobalt. Such a procedure is outlined in Fig. 19. In this reaction 4-bromoacetamido 2,2,6,6-tetra-methylpiperidine-N-oxyl is used to alkylate Cob(I)alamin. This results in a Co(III)-nitroxalkylcobalamin. The corresponding cobinamide can then be produced by hydrolyzing the ribose-phosphate linkage (119). 

NMN is basically half of the NAD+ molecule nicotinamide ribose phosphate. NADP+ is NAD+ bearing a phosphate group at C3 of the ribose group attached to the adenine. The redox chemistry is the same in all three forms of the coenzymes. NAD+ is the form most frequently employed for biochemical oxidation reactions in catabohsm and NADP+ (in its reduced form NADPH) is the form usually employed for biochemical reduction reactions in anabohsm. NMN is employed infrequently. 

The aqueous electron will react rapidly with all four of the nucleobases. Reaction of the aqueous electron with deoxyribose or ribose phosphate is 2 orders of magnitude lower, so the dominant interaction is with the bases. 

The routes involved in the formation of the various furan sulphides and disulphides involve the interaction of hydrogen sulphide with dicarbonyls, furanones and furfurals. Possible pathways are shown in Scheme 12.8. Furanthiols have been found in heated model systems containing hydrogen sulphide or cysteine with pentoses [56-58]. 2-Methyl-3-furanthiol has also been found as a major product in the reaction of 4-hydroxy-5-methyl-3(2H)-furanone with hydrogen sulphide or cysteine [21, 59]. This furanone is formed in the Maillard reaction of pentoses alternatively it has been suggested that it may be produced by the dephosphorylation and dehydration of ribose phosphate, and that this may be a route to its formation in cooked meat [21, 60]. 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

PRPP is synthesized from ribose 5-phosphate derived from the pentose phosphate pathway (see Fig. 14-21), in a reaction catalyzed by ribose phosphate pyro-phosphokinase ... 

Purines that result from the normal turnover of cellular nucleic acids can be reconverted into nucleoside triphosphates and used by the body. Thus, they are "salvaged" instead of being degraded to uric acid. PRPP is the source of the ribose-phosphate, and the reactions are catalyzed by adenine phosphoribosyltransferase, and hypoxanthine-guanine phosphoribosyltransferase (HPRT). 

During the conversion of anthranilate to tryptophan, two additional carbon atoms must be incorporated to form the indole ring. These are derived from phosphoribosyl pyrophosphate (PRPP) which is formed from ribose 5-phosphate by transfer of a pyro-phospho group from ATP.60 61 The - OH group on the anomeric carbon of the ribose phosphate displaces AMP by attack on Pp of ATP (Eq. 25-5). In many organisms the enzyme that catalyzes this reaction is fused to subunit II of anthranilate synthase.62 PRPP is also the donor of phosphoribosyl groups for biosynthesis of histidine (Fig. 25-13) and of nucleotides (Figs. 

The atoms of the pyrimidine ring are derived from carbamoyl phosphate and aspartate, as shown in Fig. 15-14. The de novo biosynthesis of pyrimidine nucleotides is shown in Fig. 15-15. The first completely formed pyrimidine ring is that of dihydroorotate. Only after oxidation to orotate is the ribose attached to produce orotidylate. The compound 5-phosphoribosyl 1-pyrophosphate (P-Rib-PP) provides the ribose phosphate. L-Glutamine is used as a substrate donating nitrogen atoms at reactions 1 and 9, catalyzed by carbamoyl phosphate synthetase II and CTP synthetase, respectively a second... 

The pyrimidine ring is assembled first and then linked to ribose phosphate to form a pyrimidine nucleotide. PRPP is the donor of the ribose phosphate moiety. The synthesis of the pyrimidine ring starts with the formation of carbamoylaspartate from carbamoyl phosphate and aspartate, a reaction catalyzed by aspartate transcarbamoylase. Dehydration, cyclization, and oxidation yield orotate, which reacts with PRPP to give orotidylate. Decarboxylation of this pyrimidine nucleotide yields UMP. CTP is then formed by the amination of UTP. 

Following an 80 min reaction, ribose/phosphate buffer mixtures were rapidly cooled to 25 C, treated with o-phenylenediamine and incubated at 50 C for 30 min. Quinoxalines and unreacted OPD were extracted and concentrated for GC/MS analysis or separation by preparative TLC. Volatile quinoxaline and methylquinoxalines were identified by Rt and MS of available standards. 2-Ethylquinoxaline was tentatively identified by its MS alone. Non-volatile quinoxalines were isolated by preparative TLC and tentatively identified through literature Rf, NMR and UV data. Addition of OPD at die start of ribose/phosphate buffer reactions produced somewhat higher yields of the same quinoxalines in a similar ratio. No evidence for quinoxaline products was found in control experiments run in bis-tris buffer. 

Like other biologically active substances we have met, flavin is found combined as a nucleotide, linked to ribose phosphate and sometimes also to adenine nucleotide, forming flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD). The flavin nucleotides, unlike NAD, are bound firmly on to proteins and cannot be removed by dialysis. Such proteins are known as flavoproteins (FP). The typical reaction in which the flavoproteins participate is the oxidation of NADH2 ... 

Fig. 6.9 Reactions of ADP ribosyl cyclase. Structures of NADP, nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose phosphate (cADPRP). ADP-ribosyl cyclase, in base ex-...

Finally, Buchtal et al. (1949) investigated whether chemical changes take place in F-actomyosin when the threads shrink. Threads of F-actomyosin and also of L-myosin were found to contain, after treatment with 2 X 10 M ATP followed by 7-12 washings, a content of phosphate, adenine and ribose three to five times greater than before. The relative amounts of the different phosphate reactions (c/. Section III, 4a) are not appreciably altered from those already present. The effect is as specific for ATP as shrinkage and contraction, but it probably has no direct connection with the fundamental process of contraction since it also occurs when actomyosin and L-myosin are in the dissolved state (c/. Section III, 5d). 

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