ABSTRACT
T h e enzyme s activatin g th e amin o acid s an d formin g aminoacyl - tRNA s ar e
called aminoacyl-tRNA synthetases an d are localized in the cytosol . Thes e enzyme s
catalyze tw o distinguishabl e reactions . I n the firs t reaction , w h i c h i s analogou s
to fatt y aci d activatio n (Chapte r 19), th e amin o acid s ar e converte d t o transien t
enzyme-bound aminoacy l adenylate s i n w h i c h th e carboxy l group o f th e amin o
acid i s l inke d t o th e 5'-phosphat e grou p o f A M P by a n anhydrid e bond (Figur e
12.1). I n a second reaction , the aminoacy l moiety i s transferred to eithe r the 2' - o r
3'-hydroxyl o f th e ribos e uni t a t the 3 ' terminu s o f th e t R N A t o for m aminoacyl -
t R N A . Becaus e subsequen t acy l migratio n ca n readil y occur , som e equilibriu m
mixture o f th e 2 ' an d 3 ' isomer s i s found . W h e n th e amin o aci d is transferre d t o
the growin g polypeptid e chain , however , i t mus t leav e th e 3'-hydroxy l group .
Because th e fre e energ y o f hydrolysi s of th e este r bon d i n t R N A i s simila r to tha t
of th e termina l phosphat e grou p o f ATP , th e reactio n proceed s wi t h littl e fre e
energy change . Th e overal l reactio n i s drive n b y th e subsequen t hydrolysi s o f
pyrophosphate. T h u s , tw o high-energ y phosphat e bond s ar e ultimately use d fo r
each aminoacyl - tRN A formed, makin g the overal l reaction highly exergoni c an d
virtually irreversible.
