Insights into pyrrolysine function from structures of a trimethylamine methyltransferase and its corrinoid protein complex

Abstract

The 22nd genetically encoded amino acid, pyrrolysine, plays a unique role in the key step in the growth of methanogens on mono-, di-, and tri-methylamines by activating the methyl group of these substrates for transfer to a corrinoid cofactor. Previous crystal structures of the Methanosarcina barkeri monomethylamine methyltransferase elucidated the structure of pyrrolysine and provide insight into its role in monomethylamine activation. Herein, we report the second structure of a pyrrolysine-containing protein, the M. barkeri trimethylamine methyltransferase MttB, and its structure bound to sulfite, a substrate analog of trimethylamine. We also report the structure of MttB in complex with its cognate corrinoid protein MttC, which specifically receives the methyl group from the pyrrolysine-activated trimethylamine substrate during methanogenesis. Together these structures provide key insights into the role of pyrrolysine in methyl group transfer from trimethylamine to the corrinoid cofactor in MttC.

Asgard archaea modulate potential methanogenesis substrates in wetland soil

“…S11) belong to a previously uncharacterized group of methyltransferases, similar to those found in Njordarchaeales, Helarchaeales, Odinarchaeia and TACK members, including Brockarchaeia and Thermoproteota. In methanogens that encode mttB , this gene has an amber codon encoding the amino acid pyrrolysine in the active site ( 40 , 41 ). The archaea from this study do not encode pyrrolysine, suggesting Freyarchaeia and Atabeyarchaeia encode a non-pyrrolysine MttB homolog, likely a quaternary amine (QA) dependent methyltransferase ( 42 ).…”

Evolution of Pyrrolysyl-tRNA Synthetase: From Methanogenesis to Genetic Code Expansion

“…41 The remarkable substrate tolerance (promiscuity) of the wild-type enzyme, in activating substrates with different amino acid side chains and several carboxylic acids not containing any amino group, 42 supports the view that the PylRS is an evolutionary very old enzyme. Given that this aminoacyl tRNA synthetase (aaRS) is exclusively employed for incorporating Pyl into three genes (mttB, mtbB, and mtmB) at a single site, 43 it appears that there has been no significant evolutionary pressure to optimize this enzyme for efficiency. Moreover, because the considerable size of Pyl compared to canonical amino acids (cAAs) and its distinct nature from most metabolites, the evolution of highly specific chemical interactions for exclusive recognition of Pyl has been unnecessary.…”