## 5 Antibiotics Affecting Elongation Factor EF-Tu  
After aminoacylation of each tRNA by its respective bacterial aaRS, all the aminoacyl-tRNA products bind to a single, highly conserved, 44-kDa protein called EF-Tu (elongation factor thermo unstable). EF-Tu is a GTPase that only binds to the aminoacyl-tRNAs in a GTP bound form and then transfers the aminoacyl-tRNAs to the A-site of the ribosome and is therefore critical to the protein translation machinery. Upon codon-anticodon recognition in the ribosome, EF-Tu undergoes a conformational change leading to the hydrolysis of the active EF-Tu\(\cdot\)GTP complex to the inactive EF-Tu\(\cdot\)GDP complex [111, 112, 113]. This results in the release of EF-Tu from the ribosome, which is then available for regeneration with GTP [114]. Inhibitors of EF-Tu are divided into two broad categories. Of these, kirromycin and enacyloxin affect the EF-Tu\(\cdot\)GTP or \(\cdot\)GDP complex [115, 116], while pulvomycin and GE2270 (also called as MDL 62,879) prevent the formation of a stable complex of aminoacyl-tRNA with EF-Tu\(\cdot\)GDP/GTP [117, 118, 119].  
Kirromycin is an antibiotic that binds to the ribosome\(\cdot\)aa-tRNA\(\cdot\)EF-Tu\(\cdot\)GDP complex. This results in inhibition of the release of the EF-Tu\(\cdot\)GDP complex from the ribosome. Failure of the release of EF-Tu from the ribosome does not affect the binding of aminoacyl-tRNA to the A-site of the ribosome, but blocks the subsequent peptide bond formation step [115, 116]. Thus, kirromycin is a potent inhibitor of the EF-Tu dependent protein translation reaction. Aurodox (also called mocimycin) is a _N_-methyl derivative of kirromycin and is produced by _Streptomyces goldiniensis_[120], and crystal structure of _Thermus thermophilus_ EF-Tu with GDP and aurodox in a 1:1:1 ratio at 2 A resolution indicates that binding of aurodox resembles the EF-Tu-GTP bound conformation [121]. Additionally this stabilization of the EF-Tu-GTP complex occurs in the absence and presence of aminoacyl-tRNA. Thus kirromycin strongly affects the conformation of EF-Tu by interfering with its allosteric function and consequently protein translation. Enacyloxin IIa is produced by _Frateuria_ sp. W-315 and is active against both Gram-positive and Gram-negative organisms [122, 123, 124]. This antibiotic affects the interaction between EF-Tu and GTP by retarding the dissociation of GTP from the complex. This results in alteration of the conformation of aa-tRNA, thereby leading to the deacylation of the aa-tRNA that is bound to the EF-Tu-GTP complex. This consequently blocks polypeptide chain formation. Enacyloxin IIa also weakly affects the binding of aa-tRNA to the A-site on the ribosome [125, 126].  
GE2270A is a thiazolly peptide antibiotic that is active against Gram-positive bacteria. Crystal structure of _E. coli_ EF-Tu-GDP-GE2270 complex has confirmed that this compound directly competes with aminoacyl-tRNA for the same binding site on EF-Tu. It also blocks the GTP to GDP conformational change in EF-Tu. A comparison of binding modes indicates that GE2270 therefore has a different binding mode compared to kirromycin and enacyloxin IIa but is limited as an antibiotic due to its low solubility [127, 128, 129]. Pulvomycin has a similar mechanism as GE2270 wherein it prevents that binding of aa-tRNA to EF-Tu [117, 118, 119]. Additionally, it prevents the binding of amino acids to the 3'-end of the tRNA. Overall, these inhibitors block protein synthesis by either directly or indirectly affecting the binding of aa-tRNA to EF-Tu and consequently its function in the ribosomal machinery.