br Introduction In mammals there are ten known isoforms of
Introduction In mammals there are ten known isoforms of the enzyme diacylglycerol kinase (DGK), as well as gene splice variants. Enzymes of this family all have a homologous active site but differ widely in their molecular mass, mechanisms of activation and biological function (Shulga et al., 2011a). Among these isoforms, only the epsilon isoform of diacylglycerol kinase (DGKϵ) has a particularly hydrophobic segment that is predicted to be a transmembrane helix. DGKϵ has several unique features among all the isoforms of DGK (unless otherwise stated, DGK refers to human isoforms, although the sequence is highly conserved among all multicellular eukaryotes). DGKϵ is the only isoform believed to be permanently attached to a membrane. It is the smallest isoform in terms of molecular mass and it is the only form known to exhibit acyl chain specificity of the lipid substrate, diacylglycerol (DAG). DGKϵ shows high specificity for 1-stearoyl-2-arachidonoyl glycerol (SAG), the species of DAG with the same acyl chain composition found in the lipid intermediates of the phosphatidylinositol Miltefosine calculator (PI-cycle) (D’Souza and Epand, 2014). All DGK isoforms have two or three C1 domains, but the epsilon isoform is the only one that does not have any other recognizable folding domain.
Membrane binding of DGKϵ
Comparison of DGK from bacteria and from mammals Bacteria express two forms of DGK; DgkA and DgkB. DgkA is an integral membrane protein comprised of three transmembrane alpha helices. It is capable of phosphorylating DAG and other lipids such as ceramide (Van Horn et al., 2009, Van Horn and Sanders, 2012). DgkA is amongst the smallest known kinases, and is a homotrimer that is structurally distinct from other kinases. The sequence, membrane topology and substrate specificity of DgkA is unique among DGKs from all species. There are two families of DgkA, those from Gram positive bacteria and those from Gram negative bacteria. The DgkA from Gram positive bacteria is primarily an undecaprenol kinase, while DgkA from Gram negative bacteria is a DAG-kinase (Jerga et al., 2007). DgkA from the Gram negative bacteria, Escherichia coli, also catalyzes the transfer of phosphate from ATP not only to lipid species, but also to molecules of water and glycerol (Ullrich et al., 2011, Prodeus et al., 2013). A study using 31P NMR to detect ATP hydrolysis and DAG phosphorylation was used to investigate ATPase activity of human DGKϵ and determine if water competes with DAG as an acceptor of the γ phosphate of ATP as is seen with DgkA (Prodeus et al., 2013). The results indicate that with human DGKϵ, water does not compete with DAG for the γ phosphate of ATP. Essentially all of the γ phosphate of ATP is transferred to the lipid substrate and there is no detectable production of inorganic phosphate resulting from ATPase activity (Prodeus et al., 2013). This work suggests that in the evolution of DGK from bacteria to mammals, the phosphoryl transfer reaction became more efficient and specific with the exclusion of water and glycerol. Furthermore, the work revealed that the substrate specificity of human DGKϵ is not a consequence of different degrees of ATP hydrolysis in the presence of different species of DAG (Prodeus et al., 2013). Since mammalian DGKϵ shows minimal ATPase activity, and is specific for DAG species with acyl chain compositions that are also predominant in PI species (stearoyl and arachidonoyl), it is believed to be committed to PI re-synthesis and acyl chain enrichment (D\'Souza and Epand, 2014). A second bacterial DGK is the water soluble DgkB found in gram-positive bacteria. DgkB forms a tight homodimer, and each monomer is believed to be capable of independently supporting substrate binding and catalysis (Miller et al., 2008). DgkB shares 15–18% sequence identity with the human DGKs and its catalytic site is conserved in all mammalian DGKs. In comparison, the DGK found in yeast is less closely related to mammalian DGKs, and uses CTP as the phosphate donor as opposed to ATP (Han et al., 2008). Another distinction between DGK expressed in multicellular organisms versus bacteria and yeast is that multicellular organisms express several DGK isoforms to fulfill unique biological functions specific to multicellular organisms, such as brain function and immune response. To date, ten mammalian DGK isoforms have been identified, which can be grouped by common structural elements into five subfamilies. Like the DGKs found in other multicellular eukaryotes, all of the mammalian isoforms have at least two cysteine-rich, C1 domains and a catalytic domain (Shulga et al., 2011a). Of the ten mammalian isoforms of DGK, only DGKϵ has the unique property of being specific for DAG species with particular acyl chains. NCBI\'s BLAST reveals that human DGKϵ is homologous to forms from a wide variety of multicellular eukaryotes; the top 100 hits consist entirely of DGKϵ from various multicellular eukaryotes. Unsurprisingly, human DGKϵ is nearly 100% identical to a variety of primate species.