▁▁ ▁Class ▁III ▁en zym es ▁( GS III ) ▁have , ▁currently , ▁only ▁been ▁found ▁in ▁B act ero ides ▁frag ilis ▁and ▁in ▁But y riv ib rio ▁fib ris ol vens . ▁It ▁is ▁a ▁double - ring ed ▁d ode c amer ▁of ▁identical ▁ch ains . ▁It ▁is ▁much ▁larger ▁( about ▁ 7 0 0 ▁am ino ▁ac ids ) ▁than ▁the ▁G SI ▁( 4 5 0 ▁to ▁ 4 7 0 ▁am ino ▁ac ids ) ▁or ▁G S II ▁( 3 5 0 ▁to ▁ 4 2 0 ▁am ino ▁ac ids ) ▁en zym es . ▁ ▁While ▁the ▁three ▁classes ▁of ▁G S s ▁are ▁clearly ▁struct ur ally ▁related , ▁the ▁sequence ▁similar ities ▁are ▁not ▁so ▁extensive . ▁ ▁Reg ulation ▁and ▁in hib ition ▁ ▁Reg ulation ▁of ▁G S ▁only ▁occurs ▁in ▁pro k ary otes . ▁G S ▁is ▁subject ▁to ▁revers ible ▁c oval ent ▁modification . ▁T yr 3 9 7 ▁of ▁all ▁ 1 2 ▁sub un its ▁can ▁under go ▁ad eny ly lation ▁or ▁de aden y ly lation ▁by ▁ad eny ly l ▁transfer ase ▁( AT ), ▁a ▁b if unction al ▁regul atory ▁en zym e . ▁A den y ly lation ▁is ▁a ▁post - trans l ational ▁modification ▁involving ▁the ▁c oval ent ▁attachment ▁of ▁A MP ▁to ▁a ▁protein ▁side ▁chain . ▁Each ▁ad eny ly lation ▁requires ▁an ▁ATP ▁and ▁complete ▁in hib ition