Plasmodium vivax ethanolamine kinase

Eugene P. Kennedy's pioneering work on oxidative phosphorylation included elucidation of his eponymous pathway for biosynthesis of phospholipids.
Specifically, phosphotidylcholine (PC) and phosphotidylethanolamine (PE) are two common phospholipids which are key components of cell membranes.
They are produced following the Kennedy pathway: (i) the starting alcohol, namely choline or ethanolamine, is phosphorylated by action of choline kinase or ethanolamine kinase;
(ii) phosphocholine or phosphoethanolamine subsequently reacts with cytidine triphosphate (CTP);
(iii) CTP-choline or CTP-ethanolamine then reacts with a diacylglycerol to form the corresponding final phosphoglyceride.

Inhibition of biosynthesis of phospholipids has been found to be promising in development of anti-malarials.
Most of the reported work has concentrated on producing and testing inhibitors to choline kinase.
In fact, choline kinase has garnered more research efforts in general,
possibly because it was thought at one point to be the single catalyst
responsible for phosphorylating both choline and ethanolamine.
Because phosphotidylcholine can be generated in vivo by an alternative means to the Kennedy pathway - namely
by methylation of phosphotidylethanolamine, blocking the synthesis of PE may turn out to be more effective.

Although there are a number of solved structures of choline kinase, our structure of Plasmodium vivax ethanolamine kinase is first of its kind.
Despite being non-protein kinases, both choline and ethanolamine kinases,
including SGC's choline kinase alpha and beta structures
(2I7Q,
2IG7),
resemble protein kinases with a N-terminal beta lobe and C-terminal helical and beta complex.
Notable structural features of Pk-EK include:

• the canonical bilobate fold
  of a protein kinase adopted by only a few small molecule kinases (e.g. choline kinase);
• a disordered ATP-binding P-loop;
• Brenner's motif: CHCDLLSSN;
• the signature choline/ethanolamine kinase motif;
• a helix on the surface of the helical lobe that
  is not found in any of the available choline kinase structures (1NW1, 2IG7, 2I7Q);
• one of the loops that guards the
  substrate pocket is much shorter than that found in choline kinase,
  including the
  human choline kinase structure with phosphocholine bound.

Like choline kinase from many organisms, the ATP-binding loop in Pv-EK does not adopt the GXGXXG motif found in
many eukaryotic protein kinases.
Instead, it is GGITN (in both Pv-EK and

P. falciparum EK).

The widely conserved choline/ethanolamine kinase motif takes the general form of (LIV)X2ID(FWY)E(YF)X3NX3(FWY)DX6E
and is ISFIDFEYSCPMERAYDIANHFNE in Pv-EK
and P. falciparum EK,
LRLIDFEYSGYNFLSADIANFFIE in

P. falciparum choline kinase and
LRLIDFEYSGFNFLATDIANFFIE in

P. vivax choline kinase.
The first Asp of this motif is typically found at the start of the DFG triplet in protein kinases.
Its function is to coordinate Mg2+ ions in binding of the ATP-Mg complex.

Where the Ek-specific helix is situated,
a short loop is found in the human (2I7Q, 2IG7) and C. elegans choline kinase structures
which is part of their homodimer interface, interacting with a helix in the smaller lobe.
The presence of the helix in Pv-EK may prevent dimer formation,
which agrees with our observation of only monomeric protein after gel filtration.