Steroids and Other Lipids

The Steroid Backbone

Steroids are defined by a fused ring system consisting of three six-membered rings and one five-membered ring. This tetracyclic structure, known as the cyclopentanoperhydrophenanthrene nucleus, is rigid and planar. Unlike the flexible chains of fatty acids, this rigidity allows steroids to pack tightly within membranes, influencing fluidity and permeability. The basic structure is derived from cholesterol, which serves as the precursor for all other steroid hormones, including sex hormones and corticosteroids. The position and type of functional groups attached to this core determine the specific biological activity of each steroid molecule.

Cholesterol and Membrane Fluidity

Cholesterol is the most abundant steroid in animal cell membranes. It possesses a hydroxyl group at the C3 position, making it amphipathic. The hydroxyl group interacts with the polar head groups of phospholipids, while the rigid ring system and hydrophobic tail align with the fatty acid chains. This orientation allows cholesterol to modulate membrane fluidity. At high temperatures, cholesterol restricts the movement of phospholipid tails, reducing fluidity. At low temperatures, it prevents the tight packing of phospholipids, maintaining fluidity. This dual role ensures that membranes remain functional across a range of physiological temperatures.

The interaction between cholesterol and phospholipids is driven by van der Waals forces and hydrophobic effects. Mixed with saturated lipids, cholesterol produces the liquid-ordered phase: the chains are more ordered than in a fluid (liquid-disordered) membrane, yet the lipids can still diffuse sideways.

Steroid Hormones and Signalling

Steroid hormones are lipid-soluble signalling molecules that cross the plasma membrane and bind to intracellular receptors. Unlike peptide hormones, which bind to surface receptors, steroid hormones act directly on gene expression. The receptor-hormone complex translocates to the nucleus, where it binds to specific DNA sequences called hormone response elements. This binding regulates the transcription of target genes, leading to changes in protein synthesis. Examples include cortisol, which regulates metabolism and stress response, and oestrogen, which controls reproductive development.

The synthesis of steroid hormones occurs in the endoplasmic reticulum and mitochondria. The first committed step, in mitochondria, is the conversion of cholesterol to pregnenolone by CYP11A1 (P450scc); the delivery of cholesterol to that enzyme by the StAR protein is the main rate-limiting control point. This step is tightly regulated to ensure that hormone production matches physiological demand. The specificity of steroid hormones arises from the precise arrangement of functional groups on the steroid nucleus, which allows for high-affinity binding to specific receptors.

Waxes and Sphingolipids

Waxes are esters of long-chain fatty acids and long-chain alcohols. They are highly hydrophobic and serve as protective coatings in plants and animals. For example, beeswax provides a waterproof barrier for honeycomb cells, while cuticular wax in plants reduces water loss. The long hydrocarbon chains of waxes are saturated or monounsaturated, contributing to their stability and resistance to oxidation.

Sphingolipids are a class of lipids based on the amino alcohol sphingosine rather than glycerol. They include ceramides, sphingomyelins, and glycosphingolipids. Sphingomyelins are phospholipids that contain a phosphocholine head group, similar to glycerophospholipids. Glycosphingolipids, such as gangliosides, contain carbohydrate groups and are abundant in the nervous system. These lipids play crucial roles in cell recognition, signalling, and the formation of lipid rafts, which are microdomains enriched in cholesterol and sphingolipids.