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 (kcat /kuncat of 400 × 10(6) m) and a catalytic rate enhancement, kcat/KmDIXK0.5Mgkuncat, 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.