Ribose/phosphate reaction protein

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 ... 

RNA carries out its enzymic roles with many fewer chemical resources than its protein counterparts it has a ribose-phosphate backbone (Chap. 3), with four rather similar heterocyclic nucleobases, plus attendant water molecules and metal ions. They carry out the water-elimination condensation reactions in splicing certain classes of introns (Chap. 7) and in the site-specific cleavage and ligation of small replicating RNA circles in some species of organism. 

Phosphorylation of poly(ADP-ribose) synthetase by protein kinase C. As shown in Fig. 5, the addition of purified poly(ADP-ribose) synthetase to a reaction mixture for protein kinase C in place of phosphate-accepting substrate markedly stimulated the incorporation of P from jj Q... 

Fig. 5. Phosphorylation of poly(ADP-ribose) synthetase by purified rat brain protein kinase C. Enzyme reaction was performed principally as described by Kikkawa et al. (20) with l-oleoyl-2-acetyl-rac-glycerol (OAG) and phosphatidyl serine as activator. A The reaction was carried out with (O ) or without ( ) C-kinase activator (Ca /OAG/phosphatidyl-serine) the indicated concentration of poly(ADP-ribose) s mthetase was added to the reaction nuxture in place of phosphate-accepting protein. B Time course of the reaction.

Measurements of the crude specific activity (mmoles of product synthesized per minute per mg of protein in the supernatant after a 50,000 x g centrifugation) of the two isomerases in E. coli indicated that the conversion of D-ribulose-5-phosphate to D-ri-bose-5-phosphate was approximately 20- to 30-fold greater than the conversion of D-ribulose-5-phosphate to D-arabinose-5-phosphate. This rate of reaction strongly pulls the reaction substrate to D-ribose-5-phosphate, since the isomerase reaction at equilibrium strongly favors the formation of the aldo-sugar over the key intermediate D-ribulose-5-phosphate. 

The nicotinamide nucleotide coenzymes function as electron carriers in a wide variety of redox reactions. In addition, NAD is the precursor of adenine dinucleotide phosphate (ADP)-ribose for ADP-ribosylation and poly(ADP-ribosylation) of proteins and cADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). They act as second messengers and stimulate increases in intracellular calcium concentrations. 

Structurally, NADP differs from NAD only by a phosphate group esterified at the 2 C of the adenosine ribose, a difference which is reflected in the enzymatic roles NAD-dependent dehydrogenases are mostly involved in catabolic reactions, while NADP-specific enzymes are usually confined to biosynthetic pathways (1). The marked specificities displayed by dehydrogenases towards NAD and NADP have provided attractive model systems to understand the process of molecular recognition by protein engineering. 

The metabolism of D-ribose is relatively simple because of its easy link to reactions of the pentose phosphate cycle. D-Ribose is phosphorylated by ribulokinase, an enzyme that is again widely distributed among various organisms except those that are parasites depending on the metabolism of their host a total of 40 proteins are listed in COG0524. 

D-Fructose 6-phosphate and ammonia interact to produce hexosamine when they are added to a fraction of pig-kidney protein together with catalytic amounts of A -acetyl-n-glucosamine 6-phosphate. Neither L-glutamine nor L-asparagine can replace ammonium sulfate as a nitrogen source in this system, and D-fructose 6-phosphate cannot be replaced by D-fructose, n-xylose, n-glucose, or D-ribose. The presence of N-acetyl-D-glucosamine is also essential for this reaction. 

It has now been found that the ADP-ribose moiety of nicotinamide adenine dinucleotide is also transferred onto some pro-teins. " When histone serves as an acceptor, several ADP-ribose units are transferred in succession, so that a short chain of oligo-(ADP-ribose), linked covalently to the protein, is formed. In another reaction, transferase II, a soluble enzyme involved in protein synthesis in mammalian cells, acts as an acceptor of a single ADP-ribose unit in the presence of diphtheria toxin. - Treatment of the product with venom pyrophosphatase releases adenosine 5 -monophosphate, but the D-ribose 5-phosphate portion still remains attached to the protein it is, therefore, assumed that the linkage involves C-1 of D-ribose. The transferase II that carries the ADP-ribose unit is completely inactive, but it can be reactivated by incubating with nicotinamide and diphtheria toxin. Under these conditions, the reaction is reversed, generating free transferase II protein and nicotinamide adenine dinucleotide. Thus, diphtheria toxin was shown to have a very specific transglycosylase activity the mechanism of this reaction has been studied in detail. ... 

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