Cholesterol, bile acids, and steroid hormones

Sterol metabolism

In humans cholesterol and cholesterol derivatives (steroid hormones, bile acids) are essential metabolites for the physiological integration of the body of the endocrine system, regulation of cell membrane processes, and adsorption of dietary lipids. Cholesterol (C00187; systematic name Cholest-5-en-3beta-ol) and its fatty acyl esters (C02530) are important structural components of membranes. Cholesterol also serves as precursor for the synthesis of steroid hormones, vitamin D, (C05441; vitamin D2)  and bile salts, which are important in fat absorption from the small intestine into the blood circulatory system and secretion of liver waste products through feces (e.g. bilirubin, the end product of heme degradation which gives feces its dark coloration). As for glucose, the body balance of cholesterol is under control of the liver which regulates the turnover rate of cholesterol levels via lipoprotein particles.

Cholesterol synthesis
(KEGG pathway MAP00100)

While dietary cholesterol uptake in humans is limited to about 0.5g/day or less, all physiological requirments for cholesterol are supplied by the liver through de novo synthesis of cholesterol from acetyl-CoA. Cholesterol, a C27 compound is synthesized in a pathway that can be split in three parts:

1. Synthesis of mevalonate, a reduced C6 compound from 3 Acetyl-CoA units
2. Activation of mevalonate to isopentenyl-PP (isoprene unit), a C5 precursor which is used to elongate a lipid chain to squalene, a C30 intermediate
3. Cyclycation and demethylation of squalene by monooxygenases to the cyclic C27 cholesterol end product

The first part of cholesterol synthesis is located in the cytoplasmic compartment in a process reminiscent of ketone body formation in the mitochondrion. Two acetyl-CoA units are combined to acetoacetyl-CoA (C00332) by a thiolase (EC; Acetyl-CoA C-acetyltransferase) reaction.  The acetoacetyl-CoA is further combined by hydroxymethylglutaryl-CoA synthase (EC with a third acetyl-CoA unit to form the six carbon (C6) intermediate beta-hydroxy-beta-methylglutaryl-CoA (C))356; HMG-CoA).  HMG-CoA is transported to the endoplasmatic reticulum membrane where it is reduced to mevalonate (C6 compound; C00418) by HMG-CoA reductase (E.C. using 2 molecules of NADPH as reductant. HMG-CoA reductase is an integral membrane protein of the ER membrane.

HMG-CoA formation is the key control in sterol synthesis and the synthase is under control of cholesterol levels to adjust cholesterol levels according to serum cholesterol levels (LDL). If cholesterol uptake is low, liver and small intestine will synthesize cholesterol accordingly. If HMG-CoA is not needed because of high levels of dietary cholesterol, it is degraded quickly to acetoacetate (C00164) and acetyl-CoA by HMG-CoA lyase (EC in the mitochondrial matrix compartment contributing to ketone body formation.

Mevalonate is the immediate sterol synthesis precursor leading to the formation of the cholesterol ring structure. Mevalonate is decarboxylated to form the C5 intermediate isoprene or isopentenyl diphosphate (C00129). Isoprene is a key intermediate for different pathways including biosynthesis and degradation of glycoproteins (see dolichol-PP; MAP00510), terpenoids (plants; MAP00900), and synthesis of coenzyme Q (ubiquinone), vitamin K, and carotenoids (beta-carotene; retinol biosynthesis MAP00830).

Here, isoprene is used to form the C30 compound squalene (from 6 isoprene units). In a first isomerization reaction (a) isopentenyl-PP is in equilibrium with dimethyl-PP. One unit each condense (b) to form the C10 compound geranyl-PP. A third C5 unit, isopentenyl-PP is added to form the C15 intermediate farnesyl-PP, two of which condense to form squalene plus pyrophosphate. This reaction is driven by a reducing equivalent NADPH.

C5 + C5 = C10 (2x)
C10 + C5 = C15 (2x)
C15 + C15 = C30

Squalene is cyclized to cholesterol, as is implied by the figure above showing the coiled conformation of this linear aromatic molecule. The cyclization reaction pathway is shown below. In three steps, squalene is reduced in the presence of molecular oxygen and NADPH to squalene epoxide which has a different C=C bond distribution priming the molecule for carbon ring fusion. The latter is the substrate for cyclase forming Lanosterol. Lanosterol is subsequently reduced and three methyl units removed (demethylation) forming the end product cholesterol. Cholesterol is ubiquitous in eukaryotes but absent from most prokaryotes. Sterol metabolism is strictly dependent on molecular oxygen.

Regulation of sterol synthesis

HMG-CoA reductase is under negative feed back control by the sterol synthesis end products cholesterol and bile salts. The important indicator of HMG-CoA reductase is the blood cholesterol concentration which is mostly determined by the presence of low density lipoprotein particle (LDL).  The reductase is also under hormonal control through a cAMP mediated second messenger pathway. The phosphorylated reductase is inactive. The enzyme is inactivated by hydroxymethylglutaryl-CoA reductase (NADPH)-kinase (EC and reactivated by hydroxymethylglutaryl-CoA reductase (NADPH)-phosphatase (EC

Cures for high cholesterol levels?

In addition, the reductase is targeted as a control enzyme by cholesterol lowering drugs such as compactin (6-demethyl-mevinolin) which functions as an antagonist, i.e., a competitive inhibitor for the substrate HMG-CoA. Compactin has also been implicated in some forms of anticancer treatments through mechanisms of cell growth inhibition. Another inhibitor is pravastatin (C01844) for treatment of atherosclerosis by lowering blood cholesterol levels. [Note: agood inhibitor of an enzyme is called an antagonist if it binds to the same site on the protein as the substrate (active site). The inhibitor competes for the substrate, but no catalytic reaction will occur. Ideally, the inhibitor will mimic the transition state of the enzyme substrate complex. The side-chain of pravastatin has a structure similar to that of hydroxy-methyl-glutarate on the CoA molecule. Also the double cyclic ring structure of pravastatin is important in its ability to bind to the enzyme active site.]

Distribution of cholesterol

Cholesterol is an essential component of mammalian cell membranes. It is, however, not evenly distributed. Plasma membranes have a cholesterol content of up to 45% of all lipids found, while ER membranes contain as little as 10%. Inner mitochondrial membranes are almost devoid of cholesterol. Actual cholesterol levels are cell type specific and may vary. As for phospholipids and membrane proteins, cholesterol is transported from its site of synthesis to other membranes by vesicle transport. Cholesterol designated for secretion and transport to other cells in the body  is acylated before incorporated into lipoprotein particles. The low density lipoprotein particle (LDL) is the major particle for cholesterol transport. LDL particles are remnant of VLDL that contain both triglycerides and cholesterol. VLDLs are produced in liver cells - lipids, cholesterol, proteins  - and transported from there to target organs to unload triglycerides (to adipocyte, muscle). The remaining cholesterol rich LDLs are recognized by other cell types by cell surface receptor specific for LDLs. They are taken into the cells by endocyteoses and the lysosomal compartment separates the components extracting cholesterol for membrane biosynthesis in ER membranes and recycles the LDL receptor to the cell surface.

Steroid hormone synthesis
KEGG pathway MAP00140 and MAP00150)

Steroid hormones are cholesterol derivatives in animals that are used for a broad range of signaling mechanisms. Cholesterol is hydroxylated and shortened (removing the the C6 hydrophobic side chain at position C21)  to the C21 intermediates pregnenolone and progestagen. The latter is a hormone secreted in the uterus controlling ovum implantation. It is also the precursor for the male and female sex hormones androgens (C19) and estrogens (C18), respectively. Other hormones for both sexes include the mineralcorticoids (e.g. aldosterone; C21) used to control kidney function (sodium, potassium, proton absorption), and glucocorticoids (e.g. cortisol C21), which are stress activators of gluconeogenesis,  glycogen, fat, and protein degradation, similar to the peptide hormone glucagon.

One important enzyme type in sterol metabolism are the monooxygenases. They catalyze an oxidation-reduction step in the presence of molecular oxygen. One oxygen atom is used for hydroxyl (or epoxide) formation, while the other is reduced to H2O. Monooxygenases are also known as cytochrome P450 or mixed function oxygenases.

Bile acid synthesis
(KEGG pathway MAP00120)

Bile acids are amphipathic derivatives of cholesterol. They are synthesized in the liver and used as detergents in the gastro intestinal tract. They are stored in the gallbladder from where they are released into the small intestine to help solubilize dietary fats.

Cholesterol is converted to trihydroxycoprostanoate. This reaction includes the activity of 8 enzymes having either monooxygenase or dehydrogenase activity. This includes EC Cholesterol 7alpha-monooxygenase. This P450 heme-thiolate protein is acting on paired donors with incorporation of molecular oxygen using NADH or NADPH as one donor, and incorporation of one atom of oxygen. A second type of oxidation is catalyzed by dehydrogenases like EC, Cholest-5-ene-3beta,7alpha-diol 3beta-dehydrogenase, acting on the CH-OH group of donors and using NAD+ or NADP+ as acceptor groups. Please note that some of the dehydrogenase and monooxygenases are used in bile acid synthesis as well as steroid hormone synthesis.

The hydroxylation renders the sterol ring structure more water soluble. The activated cholyl-CoA then reacts with the amino acids glycine or taurine to form glycocholate or taurocholate, respectively. These final bile acid structures are amphipathic molecules forming small, water soluble micelles. It is the latter micelle activity by which bile acids solubilize dietary lipids.

A small difference

Looking at the structure of cholesterol derivatives (hormones, bile acids) the hydrogen at position C5 can be in one of two stereo-conformations: alpha- or beta, trans or cis, respectively. The orientation is viewed with respect to the orientation of the methyl groups C19 and C18 between rings A-B and C-D. The alpha form is typical for steroid hormones, whereas the beta form is found only in bile acids. Stereo-chemical differences is a major property in sterol metabolism and selectivity of hormones for their receptors.

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