group. Inosine can then be deribosylated (removed from ribose) by another enzyme called purine nucleoside phosphorylase (PNP), converting it to hypoxanthine. Mechanism of catalysis The proposed mechanism for ADA-catalyzed deamination is stereospecific addition-elimination via tetrahedral intermediate. By either mechanism, Zn2+ as a strong electrophile activates a water molecule, which is deprotonated by the basic Asp295 to form the attacking hydroxide. His238 orients the water molecule and stabilizes the charge of the attacking hydroxide. Glu217 is protonated to donate a proton to N1 of the substrate. The reaction is stereospecific due to the location of the zinc, Asp295, and His238 residues, which all face the B-side of the purine ring of the substrate. Competitive inhibition has been observed for ADA, where the product inosine acts at the competitive inhibitor to enzymatic activity. Function ADA is considered one of the key enzymes of purine metabolism. The enzyme has been found in bacteria, plants, invertebrates, vertebrates, and mammals, with high conservation of amino acid sequence. The high degree of amino acid sequence conservation suggests the crucial nature of ADA in the purine salvage pathway. Primarily, ADA in humans is involved in the development and maintenance of the immune system. However, ADA association has also been observed with epithelial cell differentiation, neurotransmission, and gestation maintenance. It has also been proposed that ADA, in addition to adenosine breakdown, stimulates release of excitatory amino acids and is necessary to the coupling of A1 adenosine receptors and heterotrimeric G proteins. Adenosine deaminase deficiency leads to pulmonary fibrosis, suggesting that chronic exposure to high levels of adenosine can exacerbate inflammation responses rather than suppressing them. It has also been recognized that adenosine deaminase protein and activity is upregulated in mouse hearts that overexpress HIF-1α, which in part explains the attenuated levels of adenosine in HIF-1α expressing hearts during ischemic stress. Pathology Some mutations in the gene for adenosine deaminase cause it not to be expressed. The resulting deficiency is one cause of severe combined immunodeficiency (SCID), particularly of autosomal recessive inheritance. Deficient levels of ADA have also been associated with pulmonary inflammation, thymic cell death, and defective T-cell receptor signaling. Conversely, mutations causing this enzyme to be overexpressed are one cause of hemolytic anemia. There is some evidence that a different allele (ADA2) may lead to autism. Elevated levels of ADA has also been associated with AIDS. Isoforms There are 2 isoforms of ADA: ADA1 and ADA2. ADA1