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

Abstract

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, ${\mathit{k}}_{\mathbf{cat}}/K_{\mathrm{m}}^{\mathbf{DIX}}{K}_{0.5}^{\mathrm{Mg}}{\mathit{k}}_{\mathbf{uncat}}$, 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.