Magnesium chelatase (Mg-chelatase)

Magnesium chelatase is a multi-subunit enzyme complex essential for the biosynthesis of chlorophyll in plants, algae, and cyanobacteria. It uniquely catalyzes the ATP-dependent insertion of a magnesium ion into protoporphyrin IX, forming Mg-protoporphyrin IX, the first chlorophyll-specific intermediate in the tetrapyrrole pathway. The enzyme is composed of three core subunits—ChlI (an AAA+ ATPase), ChlD (another AAA+ ATPase), and ChlH (the catalytic and porphyrin-binding subunit). Through its activity, magnesium chelatase regulates the metabolic branchpoint between heme and chlorophyll synthesis, strongly influencing photosynthetic capacity and plant development. Control over Mg-chelatase is also central for plant responses to environmental signals, with additional regulatory proteins such as GUN4 modulating its activity and linking it to plastid signaling pathways. Magnesium chelatase dysfunction or inhibition results in halted chlorophyll biosynthesis, leading to plant bleaching and death. There are no known drugs targeting magnesium chelatase in medicine, but it serves as a valuable target in agriculture and herbicide research.

Other names

Mg-chelataseMg-protoporphyrin IX magnesium-lyase (systematic name)ChlI, ChlD, and ChlH subunits (refers to individual protein components)EC 6.6.1.1CAST no. 9074-88-8Magnesium chelatase complex

ATP-dependent catalysis of magnesium insertion: requires ATP hydrolysis by AAA+ subunits (ChlI and ChlD), with ChlH acting as the porphyrin substrate-binding and catalytically active site. The enzyme converts protoporphyrin IX and Mg2+ (with ATP and water) into Mg-protoporphyrin IX, ADP, phosphate, and H+.