= Solution
\b[<GDP>–<GTP> exchange resets translation factors after <GTP> hydrolysis; it does not phosphorylate bound <GDP> back into <GTP>.] A <guanine nucleotide exchange factor> lowers nucleotide affinity, allowing bound <GDP> to leave. The cytosolic nucleotide pool then supplies <GTP>. The GTP-bound and GDP-bound conformations bind different partners, while the ribosome and associated factors control when hydrolysis occurs. This couples repeated factor use to the ordered steps of <biological translation>.
During bacterial elongation, <EF-Tu> binds <GTP> and an <aminoacyl-tRNA>, delivering it to the ribosomal A site. Correct <codon>–<anticodon> recognition promotes rearrangements and <GTPase> activation. Hydrolysis and phosphate release permit EF-Tu departure and tRNA accommodation; rejection before and after hydrolysis contributes to <kinetic proofreading>. The GDP-bound factor is poorly suited to another delivery cycle until <EF-Ts> accelerates nucleotide exchange. Structural analysis of their complex showed disruption of the magnesium-binding environment, explaining reduced nucleotide affinity rather than a transfer of phosphate onto bound <GDP>.<The structure of the Escherichia coli EF-Tu.EF-Ts complex at 2.5 A resolution by T. Kawashima and others>[Kawashima and colleagues, 1996, abstract and nucleotide-binding-site structural analysis]{ref} The analogous eukaryotic delivery factor <eEF1A> is recycled by the <eEF1B> exchange machinery.
After peptide-bond formation, <EF-G> uses <GTP> in promoting bacterial <ribosomal translocation>; <eEF2> performs the corresponding eukaryotic role. <GDP> release and <GTP> binding again regenerate active factor, but EF-G does not depend on EF-Ts for recycling in the way EF-Tu does: its nucleotide exchange is sufficiently rapid under physiological conditions. The contrast matters because “every translational GTPase requires its own exchange factor” would be incorrect. Different factors employ a common nucleotide switch with different exchange kinetics and regulatory partners.
Exchange also regulates initiation. <eIF2>–<GTP> carries initiating <methionyl-tRNA> in a ternary complex. Start-site recognition is coupled to its GTPase cycle and release in the <GDP> form; <eIF2B> supplies exchange for another round. Stress-induced <phosphorylation> of eIF2's alpha subunit inhibits productive exchange by eIF2B and lowers ternary-complex availability. Overall initiation falls, while particular messages with regulatory <upstream open reading frames>, including ATF4, can be translated more efficiently. Experiments manipulating those upstream reading frames establish how changing initiation-factor availability changes the choice of initiation site.<Reinitiation involving upstream ORFs regulates ATF4 mRNA translation in mammalian cells by Krishna M. Vattem and Ronald C. Wek>[Vattem and Wek, 2004, results on upstream reading frames and ATF4 reinitiation]{ref} This is regulation of the rate of regeneration, not an alteration of the genetic code.
In bacterial termination, the <RF3> GTPase assists recycling of class-I release factors; hydrolysis of peptidyl-tRNA itself is catalysed by the release factor, not directly by <GTP> cleavage. Likewise, peptide-bond formation draws on the activated ester linkage of charged tRNA, whose preparation consumed <ATP>. Approximately two elongation <GTP> molecules are used per incorporated residue, one for delivery and one for translocation, apart from initiation, termination and exceptional events. Nonhydrolysable <GTP> analogues, exchange-factor depletion and nucleotide-release measurements distinguish binding, hydrolysis and exchange. \b[<GDP>–<GTP> exchange supplies reusable, regulatable molecular switches throughout translation.]
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