5-Phosphoribose 1-pyrophosphate

The )8-(1 2)- and a-(1 5)-linked arabinose residues are incorporated into the polymer from the activated polyprenyl sugar phosphate 10, which is in turn synthesized from glucose via 5-phosphoribose pyrophosphate (pRpp) [46-48]. Elongation of the polymer chain is believed to involve a family of arabinosyltransferases (AraT s) that recognize both 10 and arabinofuranoside-based acceptors of differing structures (Fig. 5) [18,19,49,50]. In AG biosynthesis, the entire polysaccharide appears to be assembled as a polyprenol diphosphate intermediate, which is transferred to peptidoglycan prior to the addition of the mycolate esters [18]. In LAM biogenesis, the arabinan portion is believed to be synthesized as a polyprenol phosphate that is transferred to lipomannan [51]. 

The precise energy requirement depends on how PRPP (phosphoribose pyrophosphate) is regenerated if, as shown above, it is regenerated from ribose, then a further 3ATPs are required, but if the PRPP is split from inosinic acid by pyiophosphorolysis, then no further ATP is required Reactions requiring ATP ... 

Biosynthesis Phosphoribose pyrophosphate + nicotinic acid - nicotinic acid ribonucleotide. Coupling with ATP under cleavage of diphosphate furnishes de-amido-NAD. The amide nitrogen originates from glutamine. 

Most of the free purines derived from the breakdown of DNA, RNA, and nucleotides in the diet are catabolized to xanthine and then to uric acid in the gut mucosa. The AMP and GMP biosynthesized in the body can also be bmken down to free purines, such as adenine, guanine, and hypoxanthine. These purines, in contrast to those derived frcim the diet, are largely reused for the synthesis of ATP and GTP- They are first converted back to AMP or GMP in a pathway of reutiliza-lion called the purine salvage pathway. For example, adenine phosphoribosyl-transferase (PRPP) catalyzes the conversion of adenine to AMP. Here, PRPP serves as the source of the phosphoribose group. Pyrophosphate is a product of the reaction. 

Fig, 17.6 Hexose monophosphate pathway. 1, glucose-6-P-dehydrogenase 2, 6-P-gluconolactonase 3,6-P-gluconate dehydrogenase 4, phosphoribose isomerase 5, phosphoketopentose epimerase 6, transketolase 7, transaldolase TPP, thiamine pyrophosphate. 

Clotting factors (factor V, factor VIII) 5-Phosphoribose-1 -pyrophosphate synthetase... 

Transketolase catalyzes a reaction [Eq. (19)] between thiamine pyrophosphate and sedoheptulose-7-phosphate to produce thiamine pyrophosphate-glycolaldehyde and a five-carbon compound, ribose-5-phosphate (XI), labeled in carbon atoms 1, 2, and 3. Phosphoribose isomerase (Axelrod and Jang, 1954) converts this compound to ribulose-5-phosphate (I) [Eq. (20)]. 

This enzyme is quite weak and actually may not play a role in DPN synthesis. A reaction (2d) between nicotinamide and 5-phosphoribose-l-pyrophosphate (PRPP) has been detected by Preiss and Handler 118) in human erythrocytes. 

Reaction b, a transfer of pyrophosphate, occurs comparatively rarely. One example is the conversion of ribose 5-phosphate to 5-phosphoribose-l-pyrophosphate (cf. Chapt. VII-2). 

---