We can now see that one or more GTP-binding proteins participate in each stage of translation. These proteins belong to the GTPase superfamily of switch proteins that cycle between a GTP-bound active form and a GDP-bound inactive form (see Figure 3-34). Hydrolysis of the bound GTP causes a conformational change in the GTPase itself and in other associated proteins that are critical to various complex molecular processes. In translation initiation, for instance, hydrolysis of eIF2·GTP to eIF2·GDP prevents further scanning of the mRNA once the start site is encountered and allows binding of the large ribosomal subunit to the small subunit (see Figure 5-24, step 6). Similarly, hydrolysis of eIF5B·GTP monitors successful association of the large and small ribosomal subunits (Figure 5-24 step 8). Recall that if the correct complex does not form, eIF5B·GTP does not hydrolyze the GTP and the complex is unstable, free to diffuse apart and try again. When the precise alignment of the subunits required for elongation occurs, eIF5B hydrolyzes the GTP to GDP, locking the correct complex in place. Energy released from the high energy β-γ bond in GTP drives the reaction in one direction. In another example, hydrolysis of EF1α·GTP to EF1α·GDP during chain elongation occurs only when the A site is occupied by a charged tRNA with an anticodon that base-pairs with the codon in that site. GTP hydrolysis causes a conformational change in EF1α that results in the release of its bound tRNA, allowing the aminoacylated 3′ end of the charged tRNA to move into the position required for peptide bond formation (see Figure 5-25, step 2). Hydrolysis of EF2·GTP to EF2·GDP leads to correct translocation of the ribosome along the mRNA (see Figure 5-25, step 4), and hydrolysis of eRF3·GTP to eRF3·GDP ensures correct termination of translation (see Figure 5-26).