The ATPase Activity of the ChlI Subunit of Magnesium Chelatase and Formation of a Heptameric AAA+ Ring

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• 作者：James D. Reid ; C. Alistair Siebert ; Per A. Bullough ; and C. Neil Hunter
• 刊名：Biochemistry
• 出版年：2003
• 出版时间：June 10, 2003
• 年：2003
• 卷：42
• 期：22
• 页码：6912 - 6920
• 全文大小：139K
• 年卷期：v.42,no.22(June 10, 2003)
• ISSN：1520-4995

文摘

The AAA⁺ ATPase component of magnesium chelatase (ChlI) drives the insertion of Mg²⁺into protoporphyrin IX; this is the first step in chlorophyll biosynthesis. We describe the ATPase activity,nucleotide binding kinetics, and structural organization of the ChlI protein. A consistent reaction schemearises from our detailed steady state description of the ATPase activity of the ChlI subunit and fromtransient kinetic analysis of nucleotide binding. We provide the first demonstration of metal ion bindingto a specific subunit of any of the multimeric chelatases and characterize binding of Mg²⁺ to the free andMgATP^2- bound forms of ChlI. Transient kinetic studies with the fluorescent substrate analogue TNP-ATP show that there are two forms of monomeric enzyme, which have distinct magnesium bindingproperties. Additionally, we describe the self-association properties of the subunit and provide a structuralanalysis of the multimeric ring formed by this enzyme in the presence of nucleotide. This single particleanalysis demonstrates that this species has a 7-fold rotational symmetry, which is in marked contrast tomost members of the AAA⁺ family that tend to form hexamers.