Why do two EF-Tu molecules act in the elongation cycle of protein biosynthesis?

In the elongation cycle of bacterial protein biosynthesis, the binding of aminoacyl-tRNA (aa-tRNA) to the A-site of mRNA-programmed ribosomes is mediated by elongation factor Tu (EF-Tu) and associated with the hydrolysis of GTP. Recently, in the case of cognate aa-tRNA, the participation of two GTP molecules has been implicated in this reaction. These are likely to be involved in preventing the indiscriminate binding of aa-tRNA to the ribosomal A-site. This article integrates this unexpected finding with our current knowledge of the structure-function relationships of the macro-molecules involved in the elongation cycle.