PAICS, which stands for Phosphoribosylaminoimidazole Carboxylase and Phosphoribosylaminoimidazolesuccinocarboxamide Synthase, is a bifunctional enzyme that plays a critical role in the de novo purine biosynthesis pathway. Purines are essential components of nucleic acids (DNA and RNA) and are involved in various cellular processes, including the synthesis of ATP and GTP, which are critical for energy metabolism and cellular function.

PAICS has two distinct enzymatic activities within the de novo purine biosynthesis pathway:

Phosphoribosylaminoimidazole Carboxylase (PAICS-AIRcarboxylase) Activity: This part of the enzyme catalyzes the conversion of 5-aminoimidazole ribonucleotide (AIR) to 5-carboxyaminoimidazole ribonucleotide (CAIR). This step is a critical early stage in the purine biosynthesis pathway and helps form the purine ring structure.

Phosphoribosylaminoimidazolesuccinocarboxamide Synthase (PAICS-SAICAR synthase) Activity: In the second part of its activity, PAICS converts CAIR to phosphoribosylaminoimidazolesuccinocarboxamide (SAICAR). This is another essential step in purine biosynthesis, as it continues the formation of the purine ring.

The de novo purine biosynthesis pathway is essential for the synthesis of purines from simple precursors. These purines are then incorporated into nucleotides and, ultimately, into DNA and RNA. The pathway is crucial for cell growth, DNA replication, and other vital cellular processes.

Dysregulation of the de novo purine biosynthesis pathway, including abnormalities in enzymes like PAICS, can lead to various disorders and diseases. Understanding the functions of enzymes like PAICS is significant in the context of both normal cellular function and disease states, particularly those related to purine metabolism.