The shell also contains a single copy of apo B, which is recognized by target cells. The role of LDL is to transport cholesterol to peripheral tissues and regulate de novo cholesterol synthesis at these sites, as described in Section A different purpose is served by high-density lipoprotein 1.
Schematic Model of Low-Density Lipoprotein. The Blood Levels of Certain Lipoproteins Can Serve Diagnostic Purposes High serum levels of cholesterol cause disease and death by contributing to the formation of atherosclerotic plaques in arteries throughout the body. High-density lipoprotein functions as a shuttle that moves cholesterol throughout the body.
HDL binds and esterifies cholesterol released from the peripheral tissues and then transfers cholesteryl esters to the liver or to tissues that use cholesterol to synthesize steroid hormones. A specific receptor mediates the docking of the HDL to these tissues. The exact nature of the protective effect of HDL levels is not known; however, a possible mechanism is discussed in Section The mode of control in the liver, the primary site of cholesterol synthesis, has already been discussed: dietary cholesterol reduces the activity and amount of 3-hydroxymethylglutaryl CoA reductase, the enzyme catalyzing the committed step.
The results of studies by Michael Brown and Joseph Goldstein are sources of insight into the control of cholesterol metabolism in nonhepatic cells. In general, cells outside the liver and intestine obtain cholesterol from the plasma rather than synthesizing it de novo. Specifically, their primary source of cholesterol is the low-density lipoprotein.
The process of LDL uptake, called receptor-mediated endocytosis, serves as a paradigm for the uptake of many molecules. The steps in the receptor-mediated endocytosis of LDL are as follows see Figure Apolipoprotein B on the surface of an LDL particle binds to a specific receptor protein on the plasma membrane of nonhepatic cells.
The receptors for LDL are localized in specialized regions called coated pits, which contain a specialized protein called clathrin.
The receptor- LDL complex is internalized by endocytosis, that is, the plasma membrane in the vicinity of the complex invaginates and then fuses to form an endocytic vesicle Figure These vesicles, containing LDL , subsequently fuse with lysosomes, acidic vesicles that carry a wide array of degradative enzymes.
The protein component of the LDL is hydrolyzed to free amino acids. This distribution is important in that cholesterol promotes negative curvature of membranes and may be a significant factor in bringing about membrane fusion as in the process of exocytosis. Cholesterol also has a key role in the lateral organization of membranes and their free volume distribution, factors permitting more intimate protein-cholesterol interactions that may regulate the activities of membrane proteins.
Some proteins bind to cholesterol deep within the hydrophobic core of the membrane via binding sites on the membrane-spanning surfaces or in cavities or pores in the proteins, driven by hydrogen bond formation.
The last is the single most important consumer of ATP in cells and is responsible for the ion gradients across membranes that are essential for many cellular functions; depletion of cholesterol in the plasma membrane deactivates these ion pumps. The agonist affinities of other GPCRs, including the oxytocin and serotonin receptors, are dramatically increased by membrane cholesterol, while the inactive state of rhodopsin is stabilized through indirect effects on plasma membrane curvature.
In the brain in addition to being essential for the structure of the myelin sheath, cholesterol is a major component of synaptic vesicles and controls their shape and functional properties, and it also has an important role in the organization and positioning of neurotransmitter receptors. In the nucleus of cells, cholesterol is intimately involved in chromatin structure and function.
Cholesterol forms a well-defined and essential association with the sphingolipids in the formation of the membrane sub-domains known as rafts see the specific web page , which are so important in the function of cells. It appears that the synthesis of cholesterol and of the sphingolipids, especially sphingomyelin, is regulated coordinately to satisfy the requirements of membrane composition and function. The interaction of cholesterol with ceramides is essential for the barrier function of the skin.
Simplistically, the higher cholesterol concentrations in the plasma membrane support its barrier function by increasing membrane thickness and reducing its permeability to small molecules.
In contrast, the endoplasmic reticulum has increased membrane flexibility because of its lower cholesterol concentrations and thus enables the insertion and folding of proteins in its lipid bilayer. Cholesterol Biosynthesis Cholesterol biosynthesis involves a highly complex series of at least thirty different enzymatic reactions, which were unravelled in large measure by Konrad Bloch and Fyodor Lynen, who received the Nobel Prize for their work on the topic in When the various regulatory, transport and genetic studies of more recent years are taken into account, it is obvious that this is a subject that cannot be treated in depth here.
The bare bones of mechanistic aspects are therefore delineated, which with the references listed below should serve as a guide to further study.
In plants , cholesterol synthesis occurs by a somewhat different pathway with cycloartenol as the key intermediate. Almost all nucleated cells are able to synthesise their full complement of cholesterol. The first steps involve the synthesis of the important intermediate mevalonic acid from acetyl-CoA and acetoacetyl-CoA, both of which are in fact derived from acetate, in two enzymatic steps.
The second enzyme HMG-CoA reductase is a particularly important control point, and is widely regarded as the rate-limiting step in the overall synthesis of sterols; its activity is regulated at the transcriptional level and by many more factors including a cycle of phosphorylation-dephosphorylation.
This and subsequent enzymes are membrane-bound and are located in the endoplasmic reticulum. An isomerase converts part of the latter to 3,3-dimethylallyl pyrophosphoric acid. This reacts with another molecule of 5-isopentenyl pyrophosphate to produce the sesquiterpene derivative C15 farnesyl pyrophosphate, two molecules of which are condensed to yield presqualene pyrophosphate.
Both of the last steps are catalysed by the enzyme squalene synthase, which regulates the flow of metabolites into either the sterol or non-sterol pathways with farnesyl pyrophosphate as the branch point and is considered to be the first committed enzyme in cholesterol biosynthesis. In the next important step, squalene is first oxidized by a squalene monooxygenase to squalene 2,3-epoxide, which undergoes cyclization catalysed by the enzyme squalene epoxide lanosterol-cyclase to form the first steroidal intermediate lanosterol or cycloartenol en route to phytosterols in photosynthetic organisms.
In this remarkable reaction, there is a series of concerted 1,2-methyl group and hydride shifts along the chain of the squalene molecule to bring about the formation of the four rings. No intermediate compounds have been found. This is believed to be one of the most complex single enzymatic reactions ever to have been identified, although the enzyme involved is only 90 kDa in size. Again, the reaction takes place in the endoplasmic reticulum, but a cytosolic protein, sterol carrier protein 1, is required to bind squalene in an appropriate orientation in the presence of the cofactors NADPH, flavin adenine dinucleotide FAD and O2; the reaction is promoted by the presence of phosphatidylserine.
In subsequent steps, lanosterol is converted to cholesterol by a series of demethylations, desaturations, isomerizations and reductions, involving 19 separate reactions.
Desmosterol is the key intermediate in the co-called 'Bloch' pathway, while 7-dehydrocholesterol is the immediate precursor in the 'Kandutsch-Russel' pathway. Chylomicrons contain triglycerides, cholesterol molecules, and other apolipoproteins protein molecules. They function to carry these water-insoluble molecules from the intestine, through the lymphatic system, and into the bloodstream, which carries the lipids to adipose tissue for storage.
Lipolysis To obtain energy from fat, triglycerides must first be broken down by hydrolysis into their two principal components, fatty acids and glycerol. This process, called lipolysis, takes place in the cytoplasm. The glycerol that is released from triglycerides after lipolysis directly enters the glycolysis pathway as DHAP. Because one triglyceride molecule yields three fatty acid molecules with as much as 16 or more carbons in each one, fat molecules yield more energy than carbohydrates and are an important source of energy for the human body.
Triglycerides yield more than twice the energy per unit mass when compared to carbohydrates and proteins. Therefore, when glucose levels are low, triglycerides can be converted into acetyl CoA molecules and used to generate ATP through aerobic respiration.
This fatty acyl CoA combines with carnitine to create a fatty acyl carnitine molecule, which helps to transport the fatty acid across the mitochondrial membrane. Once inside the mitochondrial matrix, the fatty acyl carnitine molecule is converted back into fatty acyl CoA and then into acetyl CoA Figure 3. Figure 3. Breakdown of Fatty Acids. During fatty acid oxidation, triglycerides can be broken down into acetyl CoA molecules and used for energy when glucose levels are low.
Ketogenesis If excessive acetyl CoA is created from the oxidation of fatty acids and the Krebs cycle is overloaded and cannot handle it, the acetyl CoA is diverted to create ketone bodies.
These ketone bodies can serve as a fuel source if glucose levels are too low in the body. Ketones serve as fuel in times of prolonged starvation or when patients suffer from uncontrolled diabetes and cannot utilize most of the circulating glucose. In both cases, fat stores are liberated to generate energy through the Krebs cycle and will generate ketone bodies when too much acetyl CoA accumulates.
Figure 4. Excess acetyl CoA is diverted from the Krebs cycle to the ketogenesis pathway. This reaction occurs in the mitochondria of liver cells. Ketone Body Oxidation Organs that have classically been thought to be dependent solely on glucose, such as the brain, can actually use ketones as an alternative energy source. The resulting acetyl-CoA is turned into acetoacetate along the pathway detailed in the next slide. Utilization of ketone bodies in the brain, muscle and other tissues is quite straightforward.
It involves the following steps: 1. As shown in this plot redrawn from [ 54 ] , the rate of decarboxylation is enhanced by unfractionated serum proteins. The catalytic activity of serum is greater when ketogenesis has been induced, suggesting the existence of a specific enzyme activity. A bacterial acetoacetate decarboxylase is known and has been studied in molecular detail; however, the hypothetical mammalian enzyme [ 55 ] has yet to be purified and characterized. Thus, acetone provides a back door for fatty acyl carbon to be turned back into glucose.
The first step of the pathway shown here is catalyzed by the enzyme cytochrome P type 2E1. This enzyme can also metabolize ethanol and is transcriptionally induced by both acetone and ethanol. The enzyme that converts acetol to propanediol has not been characterized; the subsequent steps are catalyzed by alcohol dehydrogenase and aldehyde dehydrogenase, respectively, which also function in the major pathway of ethanol degradation see slide 7.
This is also the case for some antiepileptic drugs. An important common target for anesthetics and antiepileptic drugs is the GABAA receptor, which is one of the two major inhibitory neurotransmitter receptors in the central nervous system.
The ketogenic diet restricts carbohydrates and protein, and it supplies most calories as triacylglycerol, much of which is then converted to ketone bodies. In animal experiments, acetone has greater antiepileptic potency than the two other ketone bodies and, as illustrated here, also than the metabolites formed in its own breakdown.
In the experiment shown here [ 60 ], the GABAA receptor antagonist pentylenetetrazole was used to induce epileptic seizures. Injection of acetone raises the dosage of pentylenetetrazole necessary to trigger seizures. It does this at lower concentrations than its metabolite acetol, indicating greater anticonvulsant potency. The ketogenic diet does not work in all patients, and it seems desirable to increase its effectiveness. A possible strategy to achieve this might be to combine the diet with inhibitors of metabolic breakdown of acetone [ 61 ].
This approach is currently undergoing experimental evaluation. This is the major pathway for utilizing excess dietary carbohydrates and protein. Fatty acid synthesis occurs mainly in the fat tissue and the liver. However, the mechanistic details are somewhat different. The bulk of the work in fatty acid synthesis is accomplished by a single enzyme, fatty acid synthase, which is quite an amazing molecule: it combines six active sites with eight distinct catalytic activities on a single polypeptide chain.
Its product is palmitic acid hexadecanoic acid. As stated before, fatty acids vary in their chain lengths and degree of bond saturation. These variants are derived from palmitate through chain elongation and desaturation, which are accomplished in the ER by separate enzymes called elongases and desaturases.
I hope that by now you recognize the pattern: CO2 is fixed, and ATP is expended—another biotin-dependent reaction, working in the same way as pyruvate carboxylase and propionyl-CoA carboxylase.
These two acetyl CoA molecules are then processed through the Krebs cycle to generate energy Figure 5. This keeps the brain functioning when glucose is limited. Cholesterol transport by HDL is facilitated by lecithin-cholesterol acyltransferase see next slide.
At the basolateral membranes of both intestinal and liver cells, organic anion transport proteins OATPs , which have a fairly low degree of substrate specificity, participate in bile acid transport. There is evidence that synthesis de novo is essential whatever the dietary intake. The results of studies of cultured fibroblasts show that, when cholesterol is abundant inside the cell, new LDL receptors are not synthesized, and so the uptake of additional cholesterol from plasma LDL is blocked. A Electron micrograph showing LDL conjugated to ferritin for visualization, dark spots bound to a coated-pit region on the surface of a cultured human fibroblast cell.