The characterization of ancient Methanococcales malate dehydrogenases reveals that strong thermal stability prevents unfolding under intense γ-irradiation.

Malate dehydrogenases (MalDH) (EC.1.1.1.37), which are involved in the conversion of oxaloacetate to pyruvate in the tricarboxylic acid cycle, are a relevant model for the study of enzyme evolution and adaptation. Likewise, a recent study showed that Methanococcales, a major lineage of Archaea, is a good model to study the molecular processes of proteome thermoadaptation in prokaryotes. Here, we use ancestral sequence reconstruction and paleoenzymology to characterize both ancient and extant MalDHs. We observe a good correlation between inferred optimal growth temperatures (OGTs) and experimental optimal temperatures for activity (A-Topt). In particular, we show that the MalDH present in the ancestor of Methanococcales was hyperthermostable and had an A-Topt of 80°C, consistent with a hyperthermophilic lifestyle. This ancestor gave rise to two lineages with different thermal constraints, one remaining hyperthermophilic while the other underwent several independent adaptations to colder environments. Surprisingly, the enzymes of the first lineage have retained a thermoresistant behavior (i.e., strong thermostability and high A-Topt), whereas the ancestor of the second lineage shows a strong thermostability, but a reduced A-Topt. Using mutants, we mimic the adaptation trajectory towards mesophily and show that it is possible to significantly reduce the A-Topt without altering the thermostability of the enzyme by introducing a few mutations. Finally, we reveal an unexpected link between thermostability and the ability to resist γ-irradiation-induced unfolding.