Steroids are one subclass of a broad group of lipids known as isoprenoids or terpenes. Steroids are a group of molecules derived from or having a structural relationship to cholesterol (Figure 19.17).
All 27 carbons in cholesterol can be traced to a two-carbon precursor - acetate.
Cholesterol biosynthesis springs from a six-carbon intermediate called mevalonate (Figure 19.18). Mevalonate arises, in turn, from linkage of two acetyl-CoAs in the mitochondrion to form acetaoacetyl-CoA (4 carbons), followed by addition of another acetyl group from a third acetyl-CoA to give 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). This latter compound is reduced by HMG-CoA reductase in the endoplasmic reticulum, using two NADPHs, with coincident loss of CoASH.
HMG-CoA reductase is the major regulatory enzyme in cholesterol biosynthesis. HMG-CoA reductase is controlled hormonally by insulin and glucagon and transcription and translation of the enzyme can be suppressed by the presence of cholesterol in cells.
Classes of molecules made from cholesterol include bile acids (Figure 19.23) and steroid hormones (Figure 19.24).