▁fifth ▁step ▁are ▁catal yz ed ▁by ▁trif unction al ▁pur ine ▁b ios yn th etic ▁protein ▁ad en os ine - 3 , ▁which ▁is ▁encoded ▁by ▁the ▁G ART ▁gene . ▁ ▁Both ▁nin th ▁and ▁t enth ▁step ▁are ▁accomplished ▁by ▁a ▁single ▁protein ▁named ▁B if unction al ▁pur ine ▁b ios yn thesis ▁protein ▁P UR H , ▁encoded ▁by ▁the ▁AT IC ▁gene . ▁ ▁G MP ▁ ▁I MP ▁de h ydro gen ase ▁( I MP D H ) ▁converts ▁I MP ▁into ▁X MP ▁ ▁G MP ▁synth ase ▁converts ▁X MP ▁into ▁G MP ▁ ▁G MP ▁re duct ase ▁converts ▁G MP ▁back ▁into ▁I MP ▁ ▁A MP ▁ ▁ad en yl os u cc inate ▁synth ase ▁converts ▁I MP ▁to ▁ad en yl os u cc inate ▁ ▁ad en yl os u cc inate ▁ly ase ▁converts ▁ad en yl os u cc inate ▁into ▁A MP ▁ ▁A MP ▁de amin ase ▁converts ▁A MP ▁back ▁into ▁I MP ▁ ▁D eg rad ation ▁Pur ines ▁are ▁met abol ised ▁by ▁several ▁en zym es : ▁ ▁Gu an ine ▁ ▁A ▁nuc lease ▁fre es ▁the ▁nucle ot ide ▁ ▁A ▁nucle ot id ase ▁creates ▁gu anos ine ▁ ▁Pur ine ▁nucle os ide ▁ph osph ory l ase ▁converts ▁gu anos ine ▁to ▁gu an ine ▁ ▁Gu an ase ▁converts ▁gu an ine ▁to ▁x anth ine ▁ ▁X anth ine ▁ox