Bacteria at Work - Experimental and Theoretical Studies Reveal the Catalytic Mechanism of Ectoine Synthase

Ectoine synthase (EctC) catalyses the ultimate step of ectoine biosynthesis, a kosmotropic compound produced as compatible solute by many bacteria and some archaea or eukaryotes. EctC is an Fe²⁺-dependent homodimeric cytoplasmic protein. Using Mössbauer spectroscopy, molecular dynamics simulations and QM/MM calculations, we determined the most likely coordination number and geometry of the Fe²⁺ ion and proposed a mechanism of the EctC-catalysed reaction. Most notably, we show that apart from the three amino acids binding to the iron ion (Glu57, Tyr84 and His92), one water molecule and one hydroxide ion are required as additional ligands for the reaction to occur. They fill the first coordination sphere of the Fe²⁺-cofactor and act as critical proton donors and acceptors during the cyclization reaction.