5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxylate from 5-amino-1-(5-phospho-D-ribosyl)imidazole (N5-CAIR route): step 1/2. {ECO:0000256|HAMAP-Rule:MF_01928, ECO:0000256|RuleBase:RU361200}.