Multiple isoforms of choline kinase from Caenorhabditis elegans: cloning, expression, purification, and characterization

Choline kinase is the first enzymatic step in the CDP–choline pathway for phosphatidylcholine biosynthesis. The genome of the nematode, Caenorhabditis elegans, contains seven genes that appear likely to encode choline and/or ethanolamine kinases. We cloned five and expressed four of these genes, and purified or partially purified three of the encoded enzymes. All expressed proteins had choline kinase activity; those that most closely resemble the mammalian choline kinases were the most active. CKA-2, a very active form, was purified to near homogeneity. The Km values for CKA-2 were 1.6 and 2.4 mM for choline and ATP, respectively, and kcat was 74 s-1. CKA-2 was predominantly a homodimer as assessed by glycerol gradient sedimentation and dynamic light scattering. CKB-2, which was less similar to mammalian choline kinases, had Km values for choline and ATP of 13 and 0.7 mM, and kcat was 3.8 s-1. Both of these highly purified enzymes required magnesium, had very alkaline pH optima, and were much more active with choline as substrate than with ethanolamine. These results provide a foundation for future studies on the structure and function of choline kinases, as well as studies on the genetic analysis of the function of the multiple isoforms in this organism. [Copyright &y& Elsevier]