GTP cyclohydrolase IIa

{{infobox enzyme

| Name = GTP cyclohydrolase IIa

| EC_number = 3.5.4.29

| CAS_number =

| GO_code = 0043740

| image =

| width =

| caption =

}}

In enzymology, a GTP cyclohydrolase IIa ({{EC number|3.5.4.29}}) is an enzyme that catalyzes the chemical reaction

:GTP + 3 H₂O $\backslash rightleftharpoons$ 2-amino-5-formylamino-6-(5-phosphoribosylamino)pyrimidin-4(3H)-one + 2 phosphate

Thus, the two substrates of this enzyme are GTP and H₂O, whereas its two products are 2-amino-5-formylamino-6-(5-phosphoribosylamino)pyrimidin-4(3H)-one and phosphate.

This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amidines. The systematic name of this enzyme class is GTP 8,9-hydrolase (phosphate-forming).