o forms , ▁Gr x 2 a ▁and ▁Gr x 2 b , ▁respectively , ▁differ ▁in ▁the ▁first ▁ex on , ▁with ▁the ▁ex on ▁ 1 ▁in ▁Gr x 2 b ▁located ▁up stream ▁of ▁that ▁in ▁Gr x 2 a . ▁Gr x 2 c ▁is ▁derived ▁from ▁alternative ▁sp lic ing ▁of ▁the ▁Gr x 2 b ▁trans cript ▁with ▁a ▁shorter ▁ex on ▁ 1 ▁than ▁that ▁of ▁Gr x 2 b . ▁ ▁Prote in ▁▁ ▁As ▁a ▁GR X ▁family ▁protein , ▁Gr x 2 ▁has ▁an ▁N - term inal ▁th ior ed ox in ▁domain , ▁possess ing ▁a ▁ 3 7 CS Y C 4 0 ▁active ▁site ▁mot if ▁with ▁a ▁ser ine ▁resid ue ▁replacing ▁the ▁conser ved ▁pro line ▁resid ue . ▁ ▁This ▁am ino ▁acid ▁substitution ▁allows ▁the ▁main ▁chain ▁of ▁Gr x 2 ▁to ▁be ▁more ▁flexible , ▁prom oting ▁coord ination ▁of ▁the ▁iron - s ulf ur ▁cluster ▁and ▁facil it ating ▁deg l ut ath ion yl ation ▁by ▁en hanced ▁gl ut ath ione - binding . ▁The ▁c yst e ine ▁pair ▁( C ys 2 8 , ▁C ys 1 1 3 ) ▁falls ▁outside ▁of ▁the ▁active ▁site , ▁and ▁it ▁is ▁completely ▁conser ved ▁in ▁Gr x 2 ▁prote ins ▁but ▁not ▁found ▁in ▁some ▁other ▁GR X ▁family ▁prote ins ▁( i . e . ▁Gr x 1 ▁and ▁Gr x 5 ). ▁A ▁dis ulf ide ▁bond