Metal Oxides Catalyze Formation of Dehydroamino Acids by Phosphate β-Elimination Under Physiological Conditions

The dehydroamino acids (DHAAs) dehydroalanine and dehydrobutyrine are formed in proteins posttranslationally from serine and threonine, respectively. They contain an electrophilic alkene, which reacts readily with nucleophiles by Michael addition, giving rise to a variety of chemical modifications as well as protein crosslinks. Recent work has shown DHAAs and their modifications to be highly prevalent in protein aggregates isolated from brains of Alzheimer's Disease (AD) patients. As there is no human enzyme known to catalyze the formation of DHAAs, this raised the question of how they are produced. We show here that metal oxides, particularly the iron oxides goethite and hematite, efficiently catalyze the formation of DHAAs in model phosphopeptides in vitro under physiologically relevant conditions. Iron oxides are known to accumulate in the human brain with age, and this deposition is enhanced in neurodegenerative disorders like AD. The DHAA formation by heterogeneous catalysis reported here provides a previously unknown mechanism linking iron deposition to protein crosslinking in AD and potentially other neurodegenerative diseases where iron accumulation and protein aggregation co-occur.