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 (ΔGº' of ca. 25-33 kJ/mol). 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 x 10(6) M) and a catalytic rate enhancement, kcat /Km (DIX) K0.5 (mg) Kuncat of 30 x 10(15) M(-1) , increasing to 300 x 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. This article is protected by copyright. All rights reserved.