Amidase that may catalyze the last step of diphthamide biosynthesis using ammonium and ATP (PubMed:23169644). Diphthamide biosynthesis consists in the conversion of an L-histidine residue in the translation elongation factor (EEF2) to diphthamide (By similarity). {ECO:0000250|UniProtKB:Q12429, ECO:0000269|PubMed:23169644}. This is the function of DPH6 (diphthamine biosynthesis 6, Ensembl gene identifier ENSG00000134146).