Mevalonate is then activated by three successive phosphorylations, yielding 5-pyrophosphomevalonate. In addition to activating mevalonate, the phosphorylations maintain its solubility, since otherwise it is insoluble in water. After phosphorylation, an ATP-dependent decarboxylation yields isopentenyl pyrophosphate, IPP, an activated isoprenoid molecule. Isopentenyl pyrophosphate is in equilibrium with its isomer, dimethylallyl pyrophosphate, DMPP. One molecule of IPP condenses with one molecule of DMPP to generate geranyl pyrophosphate, GPP. GPP further condenses with another IPP molecule to yield farnesyl pyrophosphate, FPP. Finally, the NADPH-requiring enzyme, squalene synthase catalyzes the head-to-tail condensation of two molecules of FPP, yielding squalene (squalene synthase also is tightly associated with the endoplasmic reticulum). Squalene undergoes a two step cyclization to yield lanosterol. The first reaction is catalyzed by squalene monooxygenase. This enzyme uses NADPH as a cofactor to introduce molecular oxygen as an epoxide at the 2,3 position of squalene. Through a series of 19 additional reactions, lanosterol is converted to cholesterol.
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Regulating Cholesterol Synthesis
Normal healthy adults synthesize cholesterol at a rate of approximately 1g/day and consume approximately 0.3g/day. A relatively constant level of cholesterol in the body (150 - 200 mg/dL) is maintained primarily by controlling the level of de novo synthesis. The level of cholesterol synthesis is regulated in part by the dietary intake of cholesterol. Cholesterol from both diet and synthesis is utilized in the formation of membranes and in the synthesis of the steroid hormones and bile acids (see below). The greatest proportion of cholesterol is used in bile acid synthesis.
The cellular supply of cholesterol is maintained at a steady level by three distinct mechanisms:
- 1. Regulation of HMG-CoA reductase activity and levels
- 2. Regulation of excess intracellular free cholesterol through the activity of acyl-CoA:cholesterol acyltransferase, ACAT
- 3. Regulation of plasma cholesterol levels via LDL receptor-mediated uptake and HDL-mediated reverse transport.
Regulation of HMG-CoA reductase activity is the primary means for controlling the level of cholesterol biosynthesis.
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| Regulation of HMG-CoA reductase by covalent modification. HMG-CoA reductase is most active in the dephosphorylated state. Phosphorylation is catalyzed by reductase kinase (RK), an enzymes whose activity is also regulated by phosphorylation. Phosphorylation of RK is catalyzed by reductase kinase kinase (RKK). Hormones such as glucagon and epinephrine negatively affect cholesterol biosynthesis by increasing the activity of the inhibitor of phosphoprotein phosphatase-1, PPI-1. Conversely, insulin stimulates the removal of phosphates and, thereby, activates HMG-CoA reductase activity. Additional regulation of HMG-CoA reductase occurs through an inhibition of synthesis of the enzyme by elevation in intracellular cholesterol levels.
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Regulation of HMG-CoA reductase takes place both through covalent modification and through control of the absolute level of the enzyme within cells. HMG-CoA reductase is a single polypeptide embedded in microsomal membranes; it is most active in its unmodified form. Phosphorylation of the enzyme decreases its activity.
HMG-CoA reductase is phosphorylated by reductase kinase, RK. RK itself is activated via phosphorylation. The phosphorylation of RK is catalyzed by reductase kinase kinase, RKK. There are two isoforms of RKK, one independent of cAMP and one dependent upon cAMP. The cAMP-dependent RKK is activated in the presence of cAMP. Since the intracellular level of cAMP is regulated by hormonal stimuli, regulation of cholesterol biosynthesis is hormonally controlled. Insulin leads to a decrease in cAMP, which in turn activates cholesterol synthesis. Alternatively, glucagon and epinephrine--- which increase the level of cAMP--- inhibit cholesterol synthesis.
The activity of HMG-CoA reductase is further controlled by the cAMP signaling pathway. Increases in cAMP lead to phosphorylation and activation of phosphoprotein phosphatase inhibitor-1 (PPI-1). HMG-CoA reductase is dephosphorylated and, thereby, activated by phosphoprotein phosphatase-1 (PP-1). At the same time, PP-1 dephosphorylates and inactivates RK. The cAMP-induced activation of PPI-1 results in a decrease in the level of active PP-1, resulting in a reduced ability of PP-1 to dephosphorylate HMG-CoA reductase and RK. This maintains RK in the phosphorylated and active state, and HMG-CoA reductase in the phosphorylated and inactive state. As the stimulus leading to increased cAMP production is removed, the level of phosphorylations decreases and that of dephosphorylations increases. The net result is a return to a higher level of HMG-CoA reductase activity. This regulatory cascade is additionally affected by PKA-mediated phosphorylation of PPI, which leads to reduced dephosphorylation by PP-1.
The ability of insulin to stimulate, and glucagon to inhibit, HMG-CoA reductase activity is consistent with the effects of these hormones on other metabolic pathways. The basic function of these two hormones is to control the availability and delivery of energy to all cells of the body.
Long-term control of HMG-CoA reductase activity is exerted primarily through control over the synthesis and degradation of the enzyme. When levels of cholesterol are high, the level of expression of the HMG-CoA gene is reduced. Conversely, reduced levels of cholesterol activate expression of the gene. Insulin also brings about long-term regulation of cholesterol metabolism by increasing the level of HMG-CoA reductase synthesis. The rate of HMG-CoA turn-over is also regulated by the supply of cholesterol. When cholesterol is abundant, the rate of HMG-CoA reductase degradation increases.
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The Utilization of Cholesterol
Cholesterol is transported in the plasma predominantly as cholesteryl esters associated with lipoproteins. Dietary cholesterol is transported from the small intestine to the liver within chylomicrons. Cholesterol synthesized by the liver, as well as any dietary cholesterol in the liver that exceeds hepatic needs, is transported in the serum within LDLs. The liver synthesizes VLDLs and these are converted to LDLs through the action of endothelial cell-associated lipoprotein lipase. Cholesterol found in plasma membranes can be extracted by HDLs and esterified by the HDL-associated enzyme LCAT. The cholesterol acquired from peripheral tissues by HDLs can then be transferred to VLDLs and LDLs via the action of cholesteryl ester transfer protein (apo-D) which is associated with HDLs. Reverse cholesterol transport allows peripheral cholesterol to be returned to the liver in LDLs. Ultimately, cholesterol is excreted in the bile as free cholesterol or as bile salts following conversion to bile acids in the liver.
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Bile Acids Synthesis and Utilization
The end products of cholesterol utilization are the bile acids, synthesized in the liver. Synthesis of bile acids is the predominant mechanisms for the excretion of excess cholesterol. However, the excretion of cholesterol in the form of bile acids is insufficient to compensate for an excess dietary intake of cholesterol.
The most abundant bile acids in human bile are chenodeoxycholic acid (45%) and cholic acid (31%). The carboxyl group of bile acids is conjugated via an amide bond to either glycine or taurine before their secretion into the bile canaliculi. These conjugation reactions yield glycocholic acid and taurocholic acid, respectively. The bile canaliculi join with the bile ductules, which then form the bile ducts. Bile acids are carried from the liver through these ducts to the gallbladder, where they are stored for future use. The ultimate fate of bile acids is secretion into the intestine, where they aid in the emulsification of dietary lipids. In the gut the glycine and taurine residues are removed and the bile acids are either excreted (only a small percentage) or reabsorbed by the gut and returned to the liver. This process is termed the enterohepatic circulation.
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Clinical Significance of Bile Acid Synthesis
Bile acids perform four physiologically significant functions:
- 1. their synthesis and subsequent excretion in the feces represent the only significant mechanism for the elimination of excess cholesterol.
- 2. bile acids and phospholipids solubilize cholesterol in the bile, thereby preventing the precipitation of cholesterol in the gallbladder.
- 3. they facilitate the digestion of dietary triacylglycerols by acting as emulsifying agents that render fats accessible to pancreatic lipases.
- 4. they facilitate the intestinal absorption of fat-soluble vitamins.
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Return to THCME Medical Biochemistry Page
Michael W. King, Ph.D / Medical Biochemistry / Terre Haute Center for Medical Education / memwk@thcme.indstate.edu
Last modified: Wednesday, 12-Apr-00 15:24:21
