GTP—See Guanosine triphosphate

These organisms have been used frequently in the elucidation of the biosynthetic pathway (37,38). The mechanism of riboflavin biosynthesis has formally been deduced from data derived from several experiments involving a variety of organisms (Fig. 5). Included are conversion of a purine such as guanosine triphosphate (GTP) to 6,7-dimethyl-8-D-ribityUuma2ine (16) (39), and the conversion of (16) to (1). This concept of the biochemical formation of riboflavin was verified in vitro under nonen2ymatic conditions (40) (see Microbial transformations). 

The subsequent cleavage of the thio-ester succinylCoA into succinate and coenzyme A by succinic acid-CoA ligase (succinyl CoA synthetase, succinic thiokinase) is strongly exergonic and is used to synthesize a phosphoric acid anhydride bond ( substrate level phosphorylation , see p. 124). However, it is not ATP that is produced here as is otherwise usually the case, but instead guanosine triphosphate (CTP). However, GTP can be converted into ATP by a nucleoside diphosphate kinase (not shown). 

G-Protein Coupling. The heterotrimenc guanosine triphosphate (GTP) binding proteins, known as G-proteins, are a principal family of proteins serving to couple membrane receptors of the G-protein family to ionic and biochemical processes. The G-proteins are heterotrimers made of three families of subunits, o,/3. and y, which can interact specifically with discrete regions on G-protein-coupied receptors. This includes most receptors for neurotransmitters and polypeptide hormones (see Neuroregulators), G-protein-eoupled receptors also embrace the odorant receptor family and the rhodopsin-linked visual cascade. 

Jhe synthesis of proteins, as characterized by the in vitro incorporation of amino acids into the protein component of cytoplasmic ribonu-cleoprotein, is known to require the nonparticulate portion of the cytoplasm, ATP (adenosine triphosphate) and GTP (guanosine triphosphate) (15, 23). The initial reactions involve the carboxyl activation of amino acids in the presence of amino acid-activating enzymes (aminoacyl sRNA synthetases) and ATP, to form enzyme-bound aminoacyl adenylates and the enzymatic transfer of the aminoacyl moiety from aminoacyl adenylates to soluble ribonucleic acid (sRNA) which results in the formation of specific RNA-amino acid complexes—see, for example, reviews by Hoagland (12) and Berg (1). The subsequent steps in pro-... 

Figure 4 Minimal mechanism by which aa-RNs are incorporated into nascent protein (see discussion for comments). AatRNA, aminoacyl tRNA EFTu, elongation factor GTP, guanosine triphosphate GDP, guanosine diphosphate R, ribosome T, ternary complex )aa-tRNA GTP EFTu). Redrawn from Thompson 198831...

In the nucleic acids (DNA and RNA sections 9.2.1 and 9.2.2 respectively) it is the purine or pyrimidine that is important, carrying the genetic information. However, in the link between energy-yielding metabolism and the performance of physical and chemical work, what is important is the phosphorylation of the ribose. Although most reactions are linked to adenosine triphosphate, a small number are linked to guanosine triphosphate (GTP see, for example, sections 5.7 and 9-2.3-2) or uridine triphosphate (UTP section 5.5.3). 

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