The steroid nucleus (core structure) is called gonane (cyclopentanoperhydrophenanthrene).[9] It is typically composed of seventeen carbon atoms, bonded in four fused rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxy group at position three and a skeleton derived from cholestane.[1]: 1785f [10] Steroids can also be more radically modified, such as by changes to the ring structure, for example, cutting one of the rings. Cutting Ring B produces secosteroids one of which is vitamin D3.
Space-filling representation
Ball-and-stick representation
5α-dihydroprogesterone (5α-DHP), a steroid. The shape of the four rings of most steroids is illustrated (carbon atoms in black, oxygens in red and hydrogens in grey). The nonpolar "slab" of hydrocarbon in the middle (grey, black) and the polar groups at opposing ends (red) are common features of natural steroids. 5α-DHP is an endogenous steroid hormone and a biosynthetic intermediate.
^Lednicer D (2011). Steroid Chemistry at a Glance. Hoboken: Wiley. ISBN978-0-470-66084-3.
^Rhen T, Cidlowski JA (October 2005). "Antiinflammatory action of glucocorticoids--new mechanisms for old drugs". The New England Journal of Medicine. 353 (16): 1711–1723. doi:10.1056/NEJMra050541. PMID16236742. S2CID5744727.
^"Lanosterol biosynthesis". Recommendations on Biochemical & Organic Nomenclature, Symbols & Terminology. International Union Of Biochemistry And Molecular Biology. Archived from the original on 8 March 2011. Retrieved 28 November 2006.
^Arago F, Gay-Lussac JL (1816). Annales de chimie et de physique (Annals of Chemistry and Physics) (in French). Chez Crochard. p. 346. "Je nommerai cholesterine, de χολη, bile, et στερεος, solide, la substance cristallisée des calculs biliares humains, ... " (I will name cholesterine – from χολη (bile) and στερεος (solid) – the crystalized substance from human gallstones ... )
^Also available in print at Hill RA, Makin HL, Kirk DN, Murphy GM (1991). Dictionary of Steroids. London, GBR: Chapman and Hall. pp. xxx–lix. ISBN978-0-412-27060-4. Retrieved 20 June 2015.