Triphosphate group

In E. coli GTP cyclohydrolase catalyzes the conversion of GTP (33) into 7,8-dihydroneoptetin triphosphate (34) via a three-step sequence. Hydrolysis of the triphosphate group of (34) is achieved by a nonspecific pyrophosphatase to afford dihydroneopterin (35) (65). The free alcohol (36) is obtained by the removal of residual phosphate by an unknown phosphomonoesterase. The dihydroneoptetin undergoes a retro-aldol reaction with the elimination of a hydroxy acetaldehyde moiety. Addition of a pyrophosphate group affords hydroxymethyl-7,8-dihydroptetin pyrophosphate (37). Dihydropteroate synthase catalyzes the condensation of hydroxymethyl-7,8-dihydropteroate pyrophosphate with PABA to furnish 7,8-dihydropteroate (38). Finally, L-glutamic acid is condensed with 7,8-dihydropteroate in the presence of dihydrofolate synthetase. 

Figure 2.7 Structure of adenosine tn phosphate. The molecule consists of adenine attached to ribose (forming adenosine) which is esterified to a triphosphate group with the three phosphorus atoms designated a, p and y. The primes ( ) on the carbon atoms of ribose differentiate the numbers from those of the adenine ring.

The energy released by hydrolysis of ATP and the other nucleoside triphosphates is accounted for by the structure of the triphosphate group. The bond between the ribose and the a phosphate is an ester linkage. The a, ft and ft,y linkages are phosphoanhydrides (Fig. 8-40). Hydrolysis of the ester linkage yields about 14 kJ/mol under standard conditions, whereas hydrolysis of each anhydride bond yields about 30 kJ/mol ATP hydrolysis often plays an important thermodynamic role in biosynthesis. When coupled to a reaction with a positive free-energy change, ATP hydrolysis shifts the equilibrium of the overall process to favor product forma-... 

Cap synthesis occurs as follows. The 5 end of an RNA transcript initially contains a triphosphate group arising from the fact that a nucleotide triphosphate serves as the primer in initiating transcription. The terminal phospho group is removed by a triphosphatase leaving a diphosphate, which is then guanylated by GTP (Eq. 28-6).569 The capped transcripts are exported from the nucleus, after which additional rneth-ylation may follow.570... 

Parts of the 2569-nucleotide sequence for the RNA of phage MS2498 are shown in Fig. 29-17. The 5 end (upper left center) still bears the triphosphate group of the initiating GTP. Following a number of hairpin loops there is a ribosome-protected region, which begins with the initiation codon GUG for the A protein... 

The substance that is the immediate source of energy for many biological reactions is adenosine triphosphate (ATP). Although this is a rather large and complex molecule, the business end for the purpose of this discussion is the triphosphate group. Hydrolysis of this group can occur to give adenosine diphosphate (ADP), adenosine monophosphate (AMP), or adenosine itself ... 

Many enzymes use [Mg-NTP] complexes as substrates and effectors, where the Mg + is coordinated exclusively to the oxygens of the triphosphate group forming mono-, bi-, and tridentate complexes. In solution, Mg + can be replaced by several other metal ions in these complexes with NTPs... 

The methyl group of the methionine unit is activated hy the positive charge on the adjacent sulfur atom, which makes the molecule much more reactive than N -methyltetrahydrofolate. The synthesis of -adenosylmethionine is unusual in that the triphosphate group of ATP is split into pyrophosphate and orthophosphate the pyrophosphate is subsequently hydrolyzed to two molecules of Pj. S-Adenosylhomocysteine is formed when the methyl group of -adenosylmethionine is transferred to an acceptor. -S-Adenosylhomocysteine is then hydrolyzed to homocysteine and adenosine. 

Only ribonucleoside 5 -triphosphates participate in RNA synthesis, and the first base to be laid down in the initiation event is a triphosphate. Its 3 -OH group is the point of attachment of the subsequent nucleotide. Thus, the 5 end of a growing RNA molecule terminates with a triphosphate. In tRNAs and rRNAs, and in eukaryotic mRNAs, the triphosphate group is removed. 

ATP consists of a nitrogenous base (adenine) and a phosphate ester of the five-carbon sugar ribose (Figure 15.5). The triphosphate group attached to libose is made up of three phosphate groups bonded to one another by phosphoric anhydride bonds. When two phosphate groups react with one another, a water molecule is lost. Because water is lost, the resulting bond is called a phosphoric anhydride, or phosphoanhydride, bond. 

Adenosine triphosphate, ATP, is a nucleotide composed of adenine, the sugar ribose, and a triphosphate group. The energy released by the hydrolysis of the phosphoanhydride bond between the second and third phosphoryl groups provides the energy for most cellular work. 

The ATP nucleotide is composed of the five-carbon sugar ribose, the purine adenine, and a triphosphate group. 

Figure 15.23 Nucleic acid precursors and their linkage. A, In the cell, nucleic acids are constructed from nucleoside triphosphates, precursors of the mononucleotide units. Each one consists of an N-containing base (structure not shown), a sugar, and a triphosphate group. In RNA (fop), the sugar is ribose in DNA, it is 2 -deoxyribose (C atoms of the sugar are denoted by a number primed, e.g.,...

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