Characterization of a dimeric Arginine kinase in the protozoan, Phytophthora sojae, suggests an early origin of Phosphagen kinase dimers

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The Phosphagen kinase (PK) enzyme family plays a critical role in energy homeostasis by catalyzing the reversible transfer of a high‐energy phosphoryl group from ATP to a guanadino‐containing acceptor molecule. Creatine kinase (CK), found in vertebrates and some invertebrates, exists as a dimer. Arginine kinase (AK), found in invertebrates, protozoa, and some bacteria, usually exists as a monomer, but several dimeric AKs have been characterized. The evolutionary relationship between these two forms of AK and the CK is unclear. Dimeric AKs described previously in invertebrates were proposed to have derived from CK, however multiple separate occurrences of this ‘back evolution’ make this scenario problematic. Using sequence motifs known through previous work to be important for dimerization in PKs, we identified an AK in a protozoan, the oomycete Phytophthora sojae, that we predicted would be dimeric. This putative dimeric AK has been cloned and expressed and shown to exhibit AK activity. More importantly, preliminary size‐exclusion chromatography data indicate that it is likely a dimeric enzyme. To expand this characterization, isothermal titration calorimetry will be used to determine whether the enzyme exhibits subunit cooperativity. These data imply that dimeric AKs were present much earlier than previously thought and suggest a different evolutionary history for PK enzymes than previously proposed.