Arresting the Catalytic Arginine in Chlorite Dismutases: Impact on Heme Coordination, Thermal Stability, and Catalysis

Chlorite dismutases (Clds) are heme b-containing oxidoreductases that can decompose chlorite to chloride and molecular oxygen. They are divided in two clades that differ in oligomerization, subunit architecture, and the hydrogen-bonding network of the distal catalytic arginine, which is proposed to switch between two conformations during turnover. To understand the impact of the conformational dynamics of this basic amino acid on heme coordination, structure, and catalysis, Cld from Cyanothece sp. PCC7425 was used as a model enzyme. As typical for a Glade 2 Cld, its distal arginine 127 is hydrogen-bonded to glutamine 74. The latter has been exchanged with either glutamate (Q74E) to arrest R127 in a salt bridge or valine (Q74V) that mirrors the setting in Glade 1 Clds. 'We present the X-ray crystal structures of Q74V and Q74E and demonstrate the pH-induced changes in the environment and coordination of the heme iron by ultraviolet-visible, circular dichroism, and electron paramagnetic resonance spectroscopies as well as differential scanning calorimetry. The conformational dynamics of R127 is shown to have a significant role in heme coordination during the alkaline transition and in the thermal stability of the heme cavity, whereas its impact on the catalytic efficiency of chlorite degradation is relatively small. The findings are discussed with respect to (i) the flexible loop connecting the N-terminal and C-terminal ferredoxin-like domains, which differs in Glade 1 and Glade 2 Clds and carries Q74 in Glade 2 proteins, and (ii) the proposed role(s) of the arginine in catalysis.

Automated summary

Chlorite dismutases (Clds) are oxidoreductases that can decompose chlorite to chloride and molecular oxygen. They are divided into two clades based on differences in oligomerization, subunit architecture, and hydrogen-bonding networks. The conformational dynamics of the distal catalytic arginine have a significant impact on heme coordination, structure, and catalysis, with a minor effect on the catalytic efficiency of chlorite degradation.