Arginine purine synthesis

Aspartic acid and arginines are important substrates for the biosynthesis of purine bases. They are also glycosylation sites in proteins. These reasons have been at the origin of the synthesis of their mono and difluoro analogues. 

IMP is the key intermediate of purine nucleotide biosynthesis. IMP can react along two pathways that yield either GMP or AMP. Oxidation of the 2 position makes xanthine monophosphate, which is transamidated to GMP. Alternatively, the a-amino group of aspartate can replace the ring oxygen of IMP to make AMP. (Note again how this reaction is similar to the synthesis of arginine fromcitrulline.)... 

What about the other enzymes in the urea cycle Ornithine transcarbamoylase is homologous to aspartate transcarbamoylase and the structures of their catalytic subunits are quite similar (Figure 23.18). Thus, two consecutive steps in the pyrimidine biosynthetic pathway were adapted for urea synthesis. The next step in the urea cycle is the addition of aspartate to citrulline to form argininosuccinate, and the subsequent step is the removal of fumarate. These two steps together accomplish the net addition of an amino group to citrulline to form arginine. Remarkably, these steps are analogous to two consecutive steps in the purine biosynthetic pathway (Section 25.2 3). 

D. Valine is an essential amino acid and is not synthesized in the human. Glydne reacts with sucdnyl CoA in the first step of heme synthesis and with arginine in the first step of creatine synthesis. The entire glycine molecule is incorporated into the growing purine ring. 

Aspartate has many fates, too. For example, its nitrogen is used in the biosynthesis of arginine and urea. Similar reactions are involved in purine nucleotide synthesis. The entire aspartate molecule is used in pyrimidine nucleotide biosynthesis. In plants and bacteria, aspartate is a precursor to three other amino acids (i.e., methionine,threonine, and isoleucine) via its conversion to homoserine (see here). Homoserine then leads in separate pathways to methionine and threonine. Threonine, in turn, can be converted to isoleucine. In bacteria, aspartic / -semialdehyde is a precursor to lysine. 

The presence of similar structural groups in their molecules led to much speculation and experimental work to demonstrate whether histidine could serve as a precursor for the synthesis of arginine, creatine, the purines, and the pyrimidines. These deductions have all been shown to be erroneous in the course of time. 

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