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Divergence in the de novo purine biosynthetic pathway. Humans convert AIR to CAIR using the enzyme AIR carboxylase with CO2 as the one-carbon source (top). Microbes convert AIR to N5-CAIR using the enzyme N5-CAIR synthetase and HCO3− as the one-carbon source along with ATP. The conversion of N5-CAIR to CAIR is conducted by the enzyme N5-CAIR mutase (bottom).
