The Asymmetric Phospholipids in Acyl Chains

Surrounded by flexible and selectively permeable membranes, phospholipids are crucial to living cells. The membranes should allow molecules to get in and out of the cells and divide into new cells. Phospholipids contain two fatty acyl chains, which are either saturated or unsaturated. Most of the phospholipids found in cell membranes are asymmetrical with respect to fatty acyl chains, and they normally contain two chains, one saturated and one unsaturated.

To explore the influence of the acyl asymmetry and polyunsaturation on the mechanical activity of dynamin, researchers utilized a comprehensive analysis. The structure of acyl chain determines whether the membranes are resistant or permissive to dynamin-mediated membrane vesiculation. There is a narrow window that allows phospholipid membranes to be highly transformable and still impermeable to small solutes. The asymmetric phospholipids that contain one unsaturated and one polyunsaturated are less leaky than the membranes that are composed of symmetrical polyunsaturated acyl chains. Besides, the asymmetrical phospholipids are easily vesiculated by dynamin.

A main factor of the polyunsaturated acyl chains is rotational freedom. Compared to the  saturated carbons surrounded by zero or one unsaturated carbon atom, the saturated carbons existing between unsaturated carbons have exceptionally higher rotational freedom. The greater the amount of unsaturation in an acyl chain, the greater the rotational freedom observed. As a result, the polyunsaturated acyl chain should allow membranes to easily adapt the conformation of membrane curvature. Meanwhile, the saturated acyl chain provide safe lipid packaging to prevent the access of small molecules.

The explanation can also be applied to the diverse lipid ratios in human health. For instance, aturated-decosahexaenoic acid (DHA) (omega-3) is more beneficial to membrane vesiculation than other saturated-polyunsaturated phospholipids, such as saturated-arachidonate (omega-6). The study found that omega-6/omega-3 ratio is important to human health and the balance of the phospholipids affect human health.

The key to a membrane’s function is its ability of vesiculation and its effect acting as a selective barrier. Such properties should be balanced to allow for both, and the balance can be trimmed through asymmetry and unsaturation in acyl chain lengths of phospholipids.

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