Phosphatidylethanolamine.

PEMT converts phosphatidylethanolamine to phosphatidylcholine by adding three methyl groups, each donated by SAM. That single reaction is one of the largest consumers of methyl groups in the liver.

Three successive SAM-dependent methylations turn the ethanolamine head group into a choline head group. Methyl donor supply sets how much of this conversion can run.

Each methylation of the ethanolamine head group releases homocysteine, which betaine remethylates back to methionine through BHMT. Betaine keeps the methyl cycle turning while PEMT is drawing on it.

Methionine synthase uses methylcobalamin and 5-MTHF to regenerate methionine from the homocysteine that PEMT methylation produces. It is the folate-dependent counterpart to the betaine route.

Cells make phosphatidylcholine either from dietary choline through the Kennedy pathway or by methylating phosphatidylethanolamine. Ample choline lowers the demand on the methylation route and spares methyl groups.

Phosphatidylethanolamine is the direct substrate PEMT converts into phosphatidylcholine, and the ratio of the two governs how membranes curve and how bilayers pack. They are two states of one lipid backbone.

Phosphatidylserine synthase 2 swaps serine for the ethanolamine head group, and decarboxylation of phosphatidylserine gives the ethanolamine lipid back. The two lipids cycle through each other with serine as the exchanged unit.

Mitochondria decarboxylate phosphatidylserine to phosphatidylethanolamine, which is the main source of the inner membrane pool. The exchange runs in both directions depending on which synthase is active.

The CDP-ethanolamine route needs CTP, made from uridine nucleotides, to activate phosphoethanolamine before it is transferred onto diacylglycerol. Uridine supply constrains how fast this lipid can be built de novo.

Ethanolamine phospholipids carry the largest share of DHA in neural and retinal membranes, holding it at the sn-2 position. The head group and the fatty acid are assembled into the same molecule.

DHA is enriched in the ethanolamine class of phospholipids rather than spread evenly across membrane lipids. Building more of the head group gives the fatty acid its usual carrier.

Most human plasmalogens are ethanolamine plasmalogens, carrying a vinyl ether chain on the same head group. They are a subclass of this lipid rather than a separate family.

Ethanolamine phospholipids carry highly unsaturated chains such as arachidonic acid and DHA, which are the chains most prone to peroxidation. Alpha-tocopherol sits in the bilayer and stops the chain reaction at those double bonds.

A polyunsaturated phospholipid raises the amount of oxidisable material in the membrane, and tocopherols are the resident chain-breaking antioxidants there. Formulators pair the two for that reason.

Converting one molecule of PE to PC consumes three methyl groups, each supplied by SAM, and SAM is made from methionine and ATP. Methionine availability therefore sits directly upstream of that conversion. This is settled pathway biochemistry, not a claim about taking the two together.

After SAM donates a methyl group it becomes homocysteine, which is remethylated to methionine either by methionine synthase using 5-MTHF or by betaine-homocysteine methyltransferase using betaine. Folate status therefore constrains how fast the methyl pool refills. The PE to PC conversion is one of the largest single consumers of SAM in the liver.

Methionine synthase transfers the methyl group from 5-MTHF to homocysteine and cannot do so without cobalamin. Without it, folate is trapped in its methyl form and the methionine and SAM pools fall. That directly limits the methylation route from PE to PC.

The Kennedy pathway runs through ethanolamine kinase, which is an ATP-dependent kinase, and CTP:phosphoethanolamine cytidylyltransferase, which handles CTP. Both depend on magnesium-nucleotide complexes as the true substrate. The relationship is general to nucleotide-dependent enzymes rather than specific to this lipid.

Phosphatidylcholine is made through the CDP-choline branch and phosphatidylethanolamine through the CDP-ethanolamine branch, using structurally analogous kinase, cytidylyltransferase and transferase steps. The two branches feed the two most abundant membrane phospholipids and are metabolically linked through PEMT and PSD. Supplying one branch does not supply the other.

Alpha-GPC raises circulating choline and free choline for the CDP-choline branch. When choline supply is adequate, the liver relies less on the PEMT route that converts PE to PC and spares SAM. The relationship is a reciprocal one between the two routes to the same product.

Krill oil delivers omega-3 fatty acids already esterified to phospholipid backbones rather than to glycerol as triglyceride, and PE is one of those backbones. That means krill oil is partly a source of the same molecular class. Anyone adding both is increasing intake of one lipid family, not two separate ones.

PE is one of the main membrane reservoirs for long chain polyunsaturates, which are moved in and out by the Lands cycle acyltransferases. Supplying EPA changes which fatty acids occupy those positions. The headgroup and the acyl chains are two separate supply questions and both matter to membrane composition.

Arachidonic acid is held esterified at the sn-2 position of PE and PC and is liberated by cytosolic phospholipase A2 when signalling calls for it. PE is the pool that is drawn on preferentially in several cell types. This describes where the fatty acid sits, not an effect of taking the two together.

Linoleic acid is desaturated and elongated to arachidonic acid, which is then esterified into membrane phospholipids including PE. Dietary fatty acid pattern therefore shapes PE acyl composition over weeks. Note this is an upstream supply relationship and says nothing about an added effect of the pair.

Crude lecithin from sunflower, soy or egg contains phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidic acid together. PE-enriched materials are made by fractionating that mixture. A lecithin product therefore already carries some PE, which matters when totalling intake across a formula.

Phosphatidic acid is dephosphorylated to diacylglycerol, which accepts the CDP-ethanolamine headgroup in the final step of the Kennedy pathway. It sits directly upstream of PE synthesis. It is also a signalling lipid in its own right, so the two are not interchangeable.

A large share of cellular PE is generated inside mitochondria by decarboxylation of phosphatidylserine, and PE together with cardiolipin shapes the curvature and cristae structure the respiratory complexes sit in. Ubiquinone shuttles electrons within that same membrane. An animal lipidomics study reported that CoQ10 supplementation shifted hepatic glycerophospholipid profiles, which is a marker-level animal finding rather than a human outcome.

In finishing pigs, selenium-enriched yeast was associated with shifts in glycerophospholipid metabolites on multi-omics analysis. That is a metabolite-profile association in livestock, not a demonstrated effect on human membrane phospholipid status. The row is included at early confidence for the mechanistic lead only.