Diphthine—ammonia ligase

Diphthine—ammonia ligase is an enzyme that plays a crucial role in the biosynthesis of diphthamide, a unique post-translationally modified histidine residue found in eukaryotic translation elongation factor 2 (eEF-2). This modification is essential for the activity of eEF-2, which is involved in the translocation step of protein synthesis. The enzyme catalyzes the ATP-dependent addition of an ammonia molecule to diphthine, a precursor of diphthamide, resulting in the formation of diphthamide. This reaction is a key step in the diphthamide biosynthesis pathway, which is notable for its complexity and specificity.

Function

Diphthine—ammonia ligase functions in the final step of diphthamide biosynthesis. Diphthamide is critical for the protection of eEF-2 from inactivation by diphtheria toxin and related ADP-ribosylating toxins. These toxins target eEF-2 in susceptible cells, inhibiting protein synthesis and leading to cell death. The presence of diphthamide in eEF-2 is therefore essential for cellular resistance to these toxins.

Mechanism

The enzyme catalyzes the reaction in a two-step process. First, it activates the carboxylate group of diphthine using ATP, forming an acyl-phosphate intermediate. Subsequently, it mediates the nucleophilic attack of ammonia on this intermediate, resulting in the formation of diphthamide and the release of AMP and phosphate. This reaction is highly specific to diphthine and does not occur with other histidine residues in proteins.

Clinical Significance

Mutations in the gene encoding Diphthine—ammonia ligase have been linked to a rare genetic disorder characterized by developmental delays, short stature, and increased susceptibility to infections. This condition highlights the importance of diphthamide in normal cellular function and the potential consequences of its disruption.

Genetic and Molecular Biology

The gene for Diphthine—ammonia ligase is conserved across many eukaryotic species, underscoring the essential role of diphthamide in cellular biology. Studies on the structure and function of this enzyme contribute to our understanding of protein synthesis and the mechanisms by which cells defend against certain bacterial toxins.

See Also

• Eukaryotic translation elongation factor 2
• Diphtheria toxin
• Protein synthesis
• Post-translational modification