Early evolution of the creatine kinase gene family and the capacity for creatine biosynthesis and membrane transport.

The creatine kinase (CK)/phosphocreatine (PCr) energy buffering system is widespread in animal groups. Recent genomic sequencing and experimental results support the view that the capacity for creatine biosynthesis and membrane transport may have evolved quite early, perhaps coincident with CK. Conventional wisdom would suggest that CK evolved from an ancestral protein most similar to the CK homologue, arginine kinase. This early CK gene subsequently diverged into the cytoplasmic, mitochondrial and flagellar CK gene families. It is now clear that both the mitochondrial and cytoplasmic-flagellar genes were present prior to the divergence of sponges from the multi-cellular animal (metazoan) lineage, possibly as long as a billion years ago. Sponges constitute the most ancient, extant metazoan group. It is likely that the primary function of the CK-PCr system in these primitive animals was to mitigate reaction-diffusion constraints in highly polarized cells such as spermatozoa and choanocytes, the water current generating cells in sponges.