Structural Insights into Arginine Kinase and Phosphagen Kinase Homologs: Mechanisms of Catalysis, Regulation, and Evolution

Phosphagen kinases are vital for energy buffering and ATP regeneration in cells with high or fluctuating energy demands. Phosphagens are small, high-energy phosphate-storage molecules, such as arginine phosphate or creatine phosphate, that serve as immediate phosphate donors for rapid ATP production. Among them, arginine kinase plays a central role in invertebrates, while creatine kinase is predominant in vertebrates. This review presents a comprehensive structural analysis of arginine kinases and their homologs across diverse species, ranging from invertebrates like Daphnia magna, Scylla paramamosain, and Limulus polyphemus to the bacterial kinase McsB from Staphylococcus aureus. High-resolution crystal and cryo-EM structures reveal a common two-domain architecture and shed light on substrate-induced conformational changes, domain cooperativity, and catalytic mechanisms. Mutational studies highlight conserved residues such as His284 and their impact on enzyme dynamics. Importantly, the structure of bacterial arginine kinase-like kinases, such as McsB, unveils regulatory mechanisms mediated by activators like McsA. This structural diversity and functional specificity underscore the evolutionary adaptability of phosphagen kinases and their relevance as potential drug targets or diagnostic markers.