Before an amino acid can be incorporated into a growing polypeptide, it
must first be attached to a molecule called transfer RNA, or tRNA, in a process
known as tRNA charging. The charged tRNA will then carry the activated amino
acid to the ribosome. The process begins when the enzyme aminoacyl-tRNA-synthetase,
depicted in blue, catalyzes an energy-releasing reaction between an amino
acid and a molecule called adenosine triphosphate, or ATP. In this step, ATP
loses two phosphates and becomes linked to the amino acid as adenosine monophosphate, or AMP. This initial reaction, although it
is not shown here, is necessary because it provides energy to drive
subsequent steps in the tRNA charging process. Here, the tRNA molecule is
represented by the colored wavy lines and binds to the aminoacyl-tRNA-synthetase.
An amino acid, shown as the green sphere, also binds to the enzyme, losing
its AMP molecule, depicted as the small hook. Using the energy released from
the loss of AMP, the aminoacyl-tRNA-synthetase
attaches the amino acid to the tRNA molecule and then dissociates from it.
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