What is a Steroid?
A steroid is an organic molecule having four rings organised in a certain molecular configuration that is physiologically active. Steroids serve two main biological functions: they are key components of cell membranes that affect membrane fluidity, and they are signalling chemicals. Plants, animals, and fungi all contain hundreds of steroids. The sterols lanosterol (opisthokonts) and cycloartenol (cycloartenol) are used to make all steroids in cells (plants). The triterpene squalene is cyclized to produce lanosterol and cycloartenol.
Three six-member cyclohexane rings (rings A, B, and C in the first figure) and one five-member cyclopentane ring make up the steroid core structure (the D ring). The functional groups connected to this four-ring core, as well as the oxidation state of the rings, distinguish steroids. Sterols are steroid derivatives having a hydroxy group at position three and a cholestane-derived skeleton.
Steroids can also be altered more significantly, for example, by altering the ring structure, such as by severing one of the rings. Vitamin D3 is one of the secosteroids produced by Cutting Ring B.
Cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory medication dexamethasone are all examples.
Distribution and function of species
Steroids are found in eukaryotes such as fungus, mammals, and plants.
Steroids from fungi
Ergosterols are fungal steroids that have a role in maintaining the integrity of the fungal cellular membrane. This information is used by antifungal medications like amphotericin B and azole antifungals to destroy pathogenic fungus. Fungi can change their ergosterol content to acquire resistance to medications that target ergosterol (for example, by loss of function mutations in the enzymes ERG3 or ERG6, which causes ergosterol depletion, or mutations that decrease ergosterol content).
Ergosterol is similar to cholesterol, which is present in animal (including human) cellular membranes, and phytosterols, which are found in plant cellular membranes. Ergosterol is abundant in all mushrooms, ranging from tens to hundreds of milligrammes per 100 grammes of dry weight. In fungus, oxygen is required for ergosterol production. The vitamin D concentration in mushrooms is due to ergosterol, which is chemically transformed into provitamin D2 when exposed to ultraviolet light. Vitamin D2 is formed spontaneously from provitamin D2. However, not all fungi use ergosterol in their cellular membranes; for example, Pneumocystis jirovecii, a pathogenic fungal species, does not, which has severe clinical consequences (given the mechanism of action of many antifungal drugs).
Steroids from animals
Animal steroids comprise substances derived from vertebrates and insects, including ecdysteroids like ecdysterone (controlling moulting in some species). Steroid hormones and cholesterol are two examples of vertebrate instances; the latter is a structural component of cell membranes that helps govern the fluidity of cell membranes and is a major component of plaque (implicated in atherosclerosis). The following hormones are steroid hormones:
Sex hormones play a role in sex differences and reproduction. Androgens, estrogens, and progestogens are examples of these hormones.
Corticosteroids include most synthetic steroid medicines, as well as the glucocorticoids (which regulate many aspects of metabolism and immunological function) and mineralocorticoids (which regulate many aspects of metabolism and immune function) (which help maintain blood volume and control renal excretion of electrolytes)
Natural and manufactured anabolic steroids that interact with androgen receptors to promote muscle and bone production. The term “steroids” is commonly used to refer to anabolic steroids.
Steroids from plants
Steroid alkaloids found in the Solanaceae and Melanthiaceae families (especially the species Veratrum), cardiac glycosides, phytosterols, and brassinosteroids are all examples of plant steroids (which include several plant hormones).
Prokaryotes
Biosynthetic routes exist in prokaryotes for both the tetracyclic steroid framework (e.g. in mycobacteria), which is thought to have originated in eukaryotes, and the more prevalent pentacyclic triterpinoid hopanoid framework.