The catalytic power of magnesium chelatase: a benchmark for the AAA (+) ATPases

In the first committed reaction of chlorophyll biosynthesis, magnesium chelatase couples ATP hydrolysis to the thermodynamically unfavorable Mg(2+) insertion into protoporphyrin IX (ΔG°′ of circa 25–33 kJ·mol(−1)). We explored the thermodynamic constraints on magnesium chelatase and demonstrate the effect of nucleotide hydrolysis on both the reaction kinetics and thermodynamics. The enzyme produces a significant rate enhancement (k (cat)/k (uncat) of 400 × 10(6) m) and a catalytic rate enhancement, [Formula: see text] , of 30 × 10(15) m (−1), increasing to 300 × 10(15) m (−1) with the activator protein Gun4. This is the first demonstration of the thermodynamic benefit of ATP hydrolysis in the AAA (+) family.