Introduction to Steroids, Cholesterol and Ergosterol

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LEARNING OBJECTIVES

In this article, author has explained the structure, functions and properties of steroids; cholesterol and ergosterol.

The compounds which contain cyclic steroid nucleus cyclopentanoperhydrophenanthrene are called steroids. The cyclopentane ring is attached to the phenanthrene nucleus.

Image showing structure of steroid nucleus

They are various steroids of biological importance. Cholesterol, bile acids, sex hormones, vitamin D, adrenocortical hormones, sitosterols, alkaloids, and cardiac glycosides are some steroids. The steroid is known as sterol if the hydroxyl group is present.

Cholesterol

Cholesterol is the most abundant sterol found in animals. It is found in all cells and found in cell membranes and lipoproteins. It was first isolated from the bile. The word cholesterol means “solid alcohol from bile”. Cholesterol is found in the structure of the cell membrane. Due to this property, cholesterol determines the permeability of the cell membrane. Cholesterol is abundantly found in the membrane of sub-cellular organelles.

Structure of cholesterol

Cholesterol is a 27-carbon compound. The OH group is present at C3. The double bond is present between C5 and C6.  An 8-carbon aliphatic chain is present at C17. Cholesterol has a total of 5 methyl groups. Due to the presence of the OH group, cholesterol is a weak amphiphilic.

Image showing the structure of cholesterol

In the OH group of C3, esterification occurs with fatty acid. As a result of this esterification, cholesteryl ester is formed.

Properties of cholesterol

  • Cholesterol is a yellowish crystalline solid.
  • It is insoluble in water.
  • Soluble in organic solvents such as chloroform, benzene, ether, etc.
  • The molecular formula of cholesterol is C27H46O.
  • The molecular mass of cholesterol is 386.664g/mol.
  • The melting point of cholesterol is 148 to 1500C.
  • The density of cholesterol is 1.052g/cm3.

Functions of cholesterol

  • Cholesterol is an important structural component of the cell membrane. It determines the fluidity of the membrane.
  • Due to the high dielectric constant, cholesterol is a poor conductor of heat and electricity. It is present in nervous tissues. It provides the insulating cover for the transmission of electrical impulses in nervous tissues.
  • Cholesterol is a component of bile salt. Bile salt is needed for the breakdown of fats in the digestive tract so they can easily absorb into the body.
  • It helps in the digestion of fat-soluble vitamins A, D, E, and K.
  • Cholesterol acts as a precursor for the production of sex hormones like estrogen and testosterone. Other hormones particularly adrenal gland hormones also require cholesterol for production.
  • The body uses cholesterol to produce cortisol. Cortisol helps the body to respond to stress.
  • Cholesterol also acts precursor for the production of vitamin D. Vitamin D is necessary for bone health.
  • Besides these, cholesterol has also an important role in brain synapses. Brain synapses are a point of contact between nerve cells and the brain.
  • Cholesterol also plays role in the immune system. The cell membrane of T cells (a type of white blood cell) would not maintain without cholesterol which can cause the rupturing of the cells.

Types of cholesterol

There are two types of cholesterol;

Low-density lipoprotein cholesterol

LDL contains a high amount of lipids. It is also considered “bad cholesterol”. LDL causes fatty build-ups in the arteries. It can cause narrowing of arteries and increase the risk of heart attack, stroke, and peripheral artery disease.

High-density lipoprotein cholesterol

HDL contains fewer amounts of fats. It is referred to as “good cholesterol”. HDL carries LDL away from the arteries back to the liver. Here LDL broke down and passed from the body. HDL does not eliminate LDL.

Ergosterol

Ergosterol is found in plants. It is a structural component of the cell membranes of fungi and protozoa. It maintains the cell membrane integrity of fungi. Ergosterol serves the same function in fungi as cholesterol serves in animals. It is also known as “provitamin D2”.

Structure of ergosterol

Ergosterol is a phytosterol consisting of ergostane. The double bond is present between C5and C6, C7, and C8 and between C22 and C23. The OH group is present at C3.

Image showing the structure of ergosterol

Properties of ergosterol

  • Ergosterol is a white crystalline solid.
  • The molecular formula of ergosterol is C28H44O.
  • Molar mass of ergosterol is 396.65g/mol.
  • The melting point of ergosterol is 1600C.
  • The boiling point of ergosterol is 2500C.

Functions of ergosterol

  • Ergosterol is a primary component of the cell membrane of fungi.
  • Ergosterols regulate membrane fluidity and plasma membrane biogenesis.
  • Ergosterol homeostasis is critical for fungal cells.
  • Ergosterol is an important precursor of vitamin D. When ergosterol is exposed to light, the ring B of sterol opens and is converted to ergocalciferol. This compound maintains the activity of vitamin D.
  • Ergosterol is a useful target for antifungal drugs. For example, allylamines, polyenes, and azoles target ergosterol.