XL2

XL2 (PF3D7_1001600) is a serine hydrolase superfamily enzyme that acts as a lysophospholipase in Plasmodium falciparum, catalyzing hydrolysis of host serum lysophosphatidylcholine (LPC) to liberate free fatty acids and choline for parasite scavenging during asexual intraerythrocytic development (PMID:38349879). Unlike its paralog XLH4, which is retained within the parasite, XL2 is exported to the host erythrocyte compartment (PMID:38349879). XL2 functions redundantly within a small set of parasite-encoded lysophospholipases: loss of XL2 alone has little effect on LPC hydrolysis or fatty acid scavenging, but combined loss of XL2 together with XLH4 strongly reduces fatty acid scavenging from LPC, drives hyperproduction of phosphatidylcholine, sensitizes parasites to LPC toxicity, and abolishes proliferation in human serum (PMID:38349879). Additional loss of a third lysophospholipase, PARE, abolishes the parasite's ability to complete a replication cycle when LPC is the sole exogenous fatty acid source and impairs both choline and fatty acid scavenging, establishing XL2 as one of three parasite enzymes collectively essential for LPC catabolism, with host erythrocyte enzymes excluded as a physiologically relevant source of this activity (PMID:41043577). No mechanistic detail beyond this lysophospholipase role in LPC scavenging has been characterized in the available corpus.

1. 2024 High

   It was unknown which parasite enzymes liberate fatty acids from host LPC; this work established XL2 as a dominant, erythrocyte-exported lysophospholipase acting redundantly with XLH4 to enable LPC-derived fatty acid scavenging required for growth in serum.

   Evidence Competitive activity-based profiling, single-to-quadruple knockout lines, in situ LPC hydrolysis assays, inhibitor profiling, and export/localization analysis in P. falciparum

   PMID:37131712 PMID:38349879

   Open questions at the time

   • XL2's individual catalytic contribution is masked by redundancy and was not resolved in isolation
   • No structural model or defined active-site characterization of XL2 was provided
   • The mechanism and machinery of XL2 export to the erythrocyte were not defined
2. 2025 High

   It was unclear whether XL2 and XLH4 fully accounted for LPC catabolism; adding a third enzyme, PARE, showed that XL2 alone is dispensable but is one of three enzymes collectively essential for LPC-derived fatty acid and choline scavenging, and ruled out host erythrocyte enzymes as a relevant source.

   Evidence Triple-knockout (XL2/XLH4/PARE) and PARE knockdown lines with fatty acid/choline scavenging and replication-cycle assays in defined LPC media in P. falciparum

   PMID:41043577

   Open questions at the time

   • The relative quantitative contribution of XL2 versus XLH4 and PARE to total LPC hydrolysis was not partitioned
   • Whether XL2 has substrate specificity distinct from XLH4/PARE beyond LPC was not tested

• The structural basis of XL2 catalysis, its precise erythrocyte destination and export pathway, and its non-redundant contribution to LPC catabolism remain undefined.
• No active-site or structural characterization of XL2
• Export route and final erythrocyte localization not resolved
• No substrate range beyond LPC established for XL2 specifically