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The 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) cycle in thaumarchaeota contributes significantly to global organic carbon fixation as the most energetically efficient aerobic carbon fixation pathway. The thaumarchaeal 3-Hydroxypropionyl-CoA Synthetase (ADP-forming; Nmar₁309) is crucial to this efficiency, utilizing ATP to ADP catalysis. This first reported structure of Nmar₁309 reveals a homodimer with a unique domain organization and a distinct linker between subdomains 4 and 1. This structure includes the bound substrates 3HP, non-hydrolyzable ATP (ADPNP), and a phosphate which suggest an intermediate state mimicking the non-covalent interaction between 3-hydroxypropionyl-phosphate and the active site histidine prior to reaction with Coenzyme-A. Conformational differences were observed between the two chains of the homodimer, likely influenced by the binding of a single ADPNP molecule in one chain. Phylogenetic analysis suggests that while 4HB synthetases may have evolved earlier in the evolutionary timeline, 3HP synthetases in Thaumarchaeota may have occurred after the Great Oxygenation Event. These structural data provide further characterization of the 3HP/4HB cycle and, in conjunction with the structure of 4-hydroxybutyryl-CoA synthetase, Nmar₀206, provide baseline structures of the key ADP-forming Acyl-CoA synthetases within this pathway.
Johnson et al. (Tue,) studied this question.