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Metabolism of Fat, Carbohydrate, and Nucleic Acids


Gaucher’s Disease: Most Common Lipid Storage Disease

This is an autosomal recessive disease resulting from a mutation in the gene for glucocerebrosidase . The genetic mutation is received from both parents as shown in Fig. 14.1 . As shown in the fourth panel of the figure ( lower left ), when both parents are carriers but do not exhibit the overt disease, one in four offsprings will have the overt disease, one in four will be normal, and two in four will be carriers but not have the overt disease.

Figure 14.1, Outcomes for the children of parents who are either carriers or who have overt Gaucher’s disease. All of the possibilities are shown. Upper left : both parents are normal and are not carriers in which case, if there are four children, all are normal. Upper center panel shows both parents with the overt disease: all four children would have the overt disease. Upper right panel: one parent has the overt disease, the other is normal in which case all children would be carriers but not have the overt disease. Lower left panel: both parents are carriers: one in four is normal, one in four has the overt disease, and two in four are carriers. Lower center panel: one parent is a carrier, while the other is normal: two children in four will be normal and two will be carriers. Lower right panel: one parent is a carrier and the other has the overt disease: two of four will be carriers and two of four will have the overt disease.

The symptomology is complex in that there are three types of the disease: Types 1–3, as shown in Fig. 14.2 .

Figure 14.2, A continuum of the symptoms of Gaucher’s disease.

The variation in intensity of the disease may be a function of the location of the mutation(s) within the gene. There are more than 250 possible mutations of the glucocerebrosidase gene, including 203 missense mutations and 18 nonsense mutations. Certain mutations are more common than others and Fig. 14.3 shows the 15 common mutations.

Figure 14.3, The structure and distribution of mutations in the glucocerebrosidase gene. (A) The 62-kb region surrounding the cerebrosidase (GBA) gene ( GBA ) along chromosome 1q showing the known genes and pseudogenes and their transcriptional direction. (B) The exonic structure of GBA, with the two ATGs and positions of 15 common mutations indicated. (C) Number of reported substitution, deletion, insertion, and splice-site mutations per exon. C1orf2 , Chromosome 1 open reading frame 2 (cote1); GBA , glucocerebrosidase; GBAP , glucocerebrosidase pseudogene; MTX1 , metaxin 1; MTXP , metaxin 1 pseudogene; THBS3 , thrombospondin 3.

There are 34 mutations known to cause Gaucher’s disease, 4 of which account for 95% of the disease in the Ashkenazi [Germany, France, Poland, Lithuania, Russia but not Spain (Sephardic Jews)] Jewish population and 50% in the general population (among Ashkenazi Jews, 1 in 10 is a carrier; among the general population, 1 in 200 is a carrier). Gaucher’s disease is the most common genetic defect in the Ashkenazi Jewish population. Both parents must contribute when a child has the overt disease. The genetic mutation occurs equally in males and females ( Fig. 14.1 ).

The glucocerebrosidase enzyme catalyzes the hydrolysis of glucocerebroside, yielding ceramide and glucose as shown in Fig. 14.4 .

Figure 14.4, Action of glucocerebrosidase.

Within 3 months after birth, there are severe symptoms: sucking and swallowing are impaired, the liver and spleen are enlarged (fat accumulates in the spleen, liver, kidneys, lungs, brain, and bone marrow); there can be extensive brain damage with spasticity, seizures, abnormal eye movements, and rigidity of limbs with death occurring before age 2. There is also a chronic milder neuropathological form of Gaucher’s disease that can start anytime in childhood or adulthood. This can exhibit symptoms described earlier, including enlarged spleen and liver together with seizures, poor coordination, skeletal irregularities (bone disease), abnormal eye movements, anemia, and respiratory problems. Persons so affected can survive into the early teen years or, in some cases, into adulthood.

Enzyme (glucocerebrosidase) replacement therapy has been developed and is administered every 2 weeks that alleviates many symptoms. A vector ( Fig. 14.5 ) has been developed that can be injected intravenously. It is able to cross the blood–brain barrier.

Figure 14.5, A schematic drawing of lentiviral vectors. The lentivirus is long-lasting (months–years). The enhanced glucocerebrosidase gene (eGFP) was cloned with the preprotrypsin secretory signal (SS) at the N terminus and the myc epitope tag was cloned at the C terminus. The ApoB LDLR-binding domain was cloned at the C terminus of these genes, generating the constructs LV-sGCmApoB and LV-sGFPmApoB. (A) These constructs were cloned into the self-inactivating LV vector under the control of the CAG promoter and containing the central purine tract (PPT) as well as the WPRE to aid in transduction and transcription of the LV vector. (B) As a control, the sGCm gene without the addition of the ApoB LDLR-binding domain was generated. The LV-sGCmApoB vector is 6190 base pairs, whereas the control LV-sGCm vector is 6103 bp. The LV-sGFPmApoB vector is 5360 bp. CAG , Chicken β-actin/globin; eGFP , enhanced glucocerebrosidase; LDLR , low-density lipoprotein receptor; LTR , long terminal repeat; PPT , polypurine tract; WPRE , woodchuck hepatitis virus posttranscriptional regulatory element.

In the bloodstream the recombinant protein binds to the low-density lipoprotein receptor and transcytoses to the central nervous system. By injecting into the bloodstream, the genes are also delivered to the liver and spleen allowing these organs to be the sites of expression and secretion of the therapeutic enzyme.

Prior to gene therapy, the following treatments could have been administered: bone marrow transplant, spleen removal, blood transfusions, and/or joint replacement.

Lipid Metabolism

Many aspects of lipid metabolism have been covered in Chapter 9 , Lipids. Some of the topics reported in that chapter are the action of pancreatic lipase on a triglyceride and on a phosphoglyceride; β-oxidation of acetyl coenzyme A (acyl-CoA) and the β-oxidation of fatty acids to acyl-CoA as well as the β-oxidation of very long-chain fatty acids in the peroxisome ; an activation of a fatty acid by the attachment of CoA on the outer mitochondrial membrane; the transport of long-chain acyl-CoA into mitochondria; and an overview of fatty acid metabolism (Fig. 9.36).

There are four phospholipases capable of degrading a triglyceride. Their actions on the triglyceride structure are shown in Fig. 14.6 .

Figure 14.6, Sites of actions of various phospholipases (A1, A2, C, and D) in hydrolyzing portions of a triglyceride.

One of the fates of fatty acids is the formation of double bonds in their chains. There are four major human fatty acid desaturases (Δ9-desaturase, Δ5-desaturase, Δ4-desaturase, and Δ6-desaturase). Their names reflect the position in the fatty acid chain being desaturated and forming a double bond. The desaturase associates with cytochrome b5 and cytochrome b5 reductases ; the latter employs NADH and O 2 for the introduction of the double bond. An overall reaction would be


R 1 − CH 2 − CH 2 − R 2 + O 2 + 2 e − + 2 H + → R 1 − CH = CH − R 2 + 2 H 2 O

A general reaction catalyzed by fatty acid desaturases is represented schematically in Fig. 14.7 .

Figure 14.7, The general reaction catalyzed by fatty acid desaturases. The first number in the designation of a fatty acid is the total number of carbons; the second number is the number of double bonds in the molecule, and the omega (ω) number is the position of the double bond counting from the last carbon in the chain. For example, linoleic acid=9,12-octadecadienoic acid=18:2Δ9,12=18:2ω6. AA , Arachidonic acid; ALA , α-linolenic acid; DGLA , dihomo-γ-linolenic acid; EPA , eicosapentaenoic acid; LA , linolenic acid; OA , oleic acid.

Oleic and linoleic acids are essential fatty acids in the diet. The common omega fatty acids with their designations are summarized in Table 14.1 .

Table 14.1
Structures and Designations of the Common Omega Fatty Acids.
Source: Reproduced from http://supplementscience.org/pufas.html .
Numerical Symbol Common Name and Structure Comments
18:1Δ9 An omega-9 monounsaturated fatty acid
18:2Δ9,12 An omega-6 polyunsaturated fatty acid
18:3Δ9,12,15 An omega-3 polyunsaturated fatty acid
20:4Δ5,8,11,14 An omega-6 polyunsaturated fatty acid
20:5Δ5,8,11,14,17 An omega-3 polyunsaturated fatty add enriched in fish oils
22:6Δ4,7,10,13,16,19 An omega-3 polyunsaturated fatty acid enriched in fish oils

Fatty Acid Degradation

The mitochondrion is the site of the β-oxidation pathway that generates acyl-CoA from fatty acids. Fatty acids, as the CoA derivatives, are transported into the mitochondrion by carnitine acyltransferase located in the outer mitochondrial membrane. The product of the reaction is acyl-carnitine from acyl-CoA plus carnitine derived from the mitochondrial matrix (carnitine is passed from the mitochondrial matrix through the inner mitochondrial membrane to the outer membrane where the carnitine acyltransferase reaction occurs). The system is summarized in Fig. 9.32A. As indicated, the first step in fatty acid degradation is the conversion of the fatty acid to the fatty acyl-CoA derivative. A flavin adenine dinucleotide (FAD) dehydrogenase catalyzes the subsequent oxidative step and this step is followed by hydration, oxidation by NAD + -dehydrogenase, and a cleavage of the chain to release acyl-CoA and a fatty acyl-CoA reduced in length by two carbons. This set of reactions is repeated until the fatty acid is fully degraded to acyl-CoA (the system is referred to as the fatty acid spiral). The degradation system is reported in Fig. 14.8 .

Figure 14.8, The β-oxidation of fatty acids occurring in the mitochondrion. The fatty acid is degraded by two carbons at a time to form acetyl-CoA, and the steps are repeated until the entire fatty acid is converted to the acetyl-CoA two-carbon fragments. The acetyl-CoA produced can enter the citric acid cycle (Fig. 9.29) for conversion to ATP. ATP , Adenosine triphosphate; CoA , coenzyme A.

Fatty acid degradation sometimes can occur at a rate faster than glycolysis; in this case an excess of acyl-CoA would be produced (there would be less pyruvate formed from glycolysis). With a low pyruvate concentration little oxaloacetate would be produced so that the utilization of acyl-CoA in the tricarboxylic acid cycle (TCA cycle) would be limited. In this case the excess acyl-CoA would be converted to ketone bodies : acetone, acetoacetate, and β-hydroxybutyrate (Fig. 9.38).

Fat as Storage Energy

As indicated earlier, fatty acid degradation to acyl-CoA allows this product to enter the TCA cycle for the production of energy [adenosine triphosphate (ATP)]. Fatty acids can be released from chylomicrons and circulate in the bloodstream bound to albumin . However, the major source of fatty acids is found in the adipocyte in the form of triglycerides. In the adipocyte, hormone-sensitive lipase ( HSL ) converts stored fat into free fatty acids in response to the lipolytic hormones norepinephrine and glucagon . The action of the opposing hormone, insulin , leads to the depression of HSL. Norepinephrine and glucagon lead to an increase of the phosphorylation in the regulatory domain of HSL, since this enzyme is regulated by a phosphorylation–dephosphorylation mechanism. Consequently, elevated levels of insulin lead to an inhibition of adipocyte HSL by decreasing the phosphorylation in the regulatory domain of the enzyme. Phosphorylation facilitates a translocation of HSL with the accessory protein, perilipin . Perilipin has two forms, 57 and 46 kDa derived from RNA splicing of a single gene, where the 46-kDa product forms from the skipping of exon 6. Perilipin forms a coat on the lipid storage droplets in adipocytes and perilipin protects the adipocyte until HSL is transported from the cytoplasm and breaks down the stored fat.

Perilipin is activated through phosphorylation by adipocyte protein kinase A ( PKA ) in response to the stimulating hormones (glucagon and/or norepinephrine). Perilipin that is confined to adipocytes is localized to the surface of intracellular neutral lipid droplets. Perilipin is a member of the “ PAT ” protein family ( P erilipin, A dipophilin, and T IP47) and the other members of this family adipophilin and TIP47, in contrast to perilipin, have broad tissue distributions. When HSL is underphosphorylated, it resides in the cytosol. Under the influence of glucagon or norepinephrine, cyclic adenosine monophosphate (cAMP) is formed in the adipocyte, and PKA is activated and phosphorylates both perilipin and HSL. Phosphorylated perilipin translocates HSL to the surface of the lipid droplet facilitating enzymatic action on the stored triglycerides resulting in a stimulation of lipolysis 30-fold over unstimulated cells (a similar mechanism occurs in steroidogenic cells where, in the case of cortisol production, adrenocorticotropic hormone acts on the cell membrane, increasing cytoplasmic cAMP, activating a protein kinase that phosphorylates and activates cholesterol esterase to release free cholesterol from cholesterol esters in the lipid droplet). An overview of the perilipin-HSL system is shown in Fig. 9.42. The free fatty acids, thus, liberated from the adipocyte cross the plasma membrane (PM) and enter the bloodstream. Various tissues take up the fatty acids from the blood and the fatty acids cross the cell membrane. The uptake of the fatty acids is mediated by three distinct proteins: fatty acid translocase , PM fatty acid–binding protein , and fatty acid transport protein ( FATP ; FATPs may also exist in the mitochondrion to assist in the mitochondrial uptake of fatty acids). Catalysis of the activation of fatty acids to acyl-CoA esters by acyl-CoA synthase enhances the cellular uptake process by moving the acyl-CoA derivatives into anabolic or catabolic pathways, creating a mass action effect (by removing product) on the PM uptake process.

Joint Regulation of Lipid and Carbohydrate Metabolism

The metabolism of lipids and carbohydrates varies from one organ to the next. The rates of entry of metabolic products into the brain, muscle, and adipose tissues are under the control of the blood level of glucose by the liver and also controlled by the levels of insulin and glucagon from the pancreas. Fatty acid metabolism in adipose tissue is coordinated with glycolysis in the liver determining the blood levels of glucose that is converted in the adipose cell to glycerol for esterification of fatty acids to triglycerides. Glucose, as well as fatty acids and ketone bodies, can be oxidized in muscle that also forms lactate that can be transported to the liver for gluconeogenesis to generate glucose. Glucose is the main source of energy for the brain unless the body is fasting or starving in which case the brain can utilize ketone bodies effectively. The Atkins diet uses the condition of low or absent carbohydrate intake (and high fat and protein intake) that leads to ketosis , allowing heart and skeletal muscles to burn ketone bodies derived from fat, instead of using glucose. This is quite effective for weight loss but the high protein intake can put a strain on the kidney.

The energy charge (ADP/ATP) of the cell is a determinant of β-oxidation of fatty acids because this process is coupled to oxidative phosphorylation. With a low cellular energy charge [(ADP) is relatively high compared to (ATP)], the degradation of fatty acids (β-oxidation) to acyl-CoA is stimulated. Acyl-CoA enters the TCA cycle and regenerates ATP. Conversely, when the energy charge of a cell is high [(ATP) is relatively high compared to (ADP)], there is a stimulation of the synthesis of fatty acids and phosphatidic acid.

Another point of regulation is the transport of fatty acids across the mitochondrial inner membranes mediated by carnitine acyltransferase . When glucose is high, malonyl-CoA levels are high. Malonyl-CoA is an inhibitor of mitochondrial carnitine acyltransferase I in the mitochondrial outer membrane. This results in an inhibition of fatty acid transport into the mitochondrion and acyl-CoA is used for the synthesis of fatty acids.

Glucagon and Glucagon-Like Peptides

Glucagon is a 29-amino acid peptide synthesized and secreted from the α-cells of the pancreas. It is secreted from these cells when the blood levels of glucose fall (also when blood levels of amino acids are high and when exercise depletes circulating glucose) and its actions are to increase the level of blood glucose. The stimulation of the formation of glucose occurs through the binding of glucagon to its receptor (seven-membrane spanning receptor) to increase the level of cAMP that activates PKA. As a result, glycogen synthase a is phosphorylated and converted to inactive glycogen synthase b, inhibiting the formation of glycogen from glucose. Second, the protein kinase phosphorylates inactive phosphorylase kinase , converting it to the active phosphorylase kinase that subsequently phosphorylates glycogen phosphorylase b to the active a form that causes the degradation of glycogen ultimately to free glucose. When the level of blood glucose is elevated, insulin is released from the β-cell of the pancreas and exerts actions that are opposite to those of glucagon in terms of the reduction of blood glucose through the stimulation of the formation of glycogen plus its other actions (see Fig. 6.28).

In the intestine the reaction to a meal is to release glucagon-like peptides (GLPs) : GLP-1 and the gastric inhibitory peptide . GLP-1 binds to an intestinal receptor and to a receptor in the pancreatic β-cell and stimulates the glucose-dependent release of insulin. A summary of these effects is shown in Fig. 6.53 and the interplay between these hormones at the level of the pancreas is shown in Fig. 14.9 .

Figure 14.9, A model depicting the roles of glucagon in insulin-secreting islet β-cells, glucagon secreting α-cells, and somatostatin ( SS ) secreting δ-cells. Major actions are either stimulatory ( green circles with + sign ) or inhibitory ( red circles with − sign ). cAMP , Cyclic adenosine monophosphate; GIP , gastric acid inhibitory peptide; GLP , glucagon-like peptide; GLU-R , glucagon receptor.

Metabolism of Cholesterol

Cholesterol is subjected to oxidation when cholesterol is in relatively at large excess. The oxidation products usually are present in very low concentrations in the human compared to the levels of cholesterol (1/1000–1/1,000,000). The oxidation products of cholesterol are called oxysterols that have very short half-lives. They are intermediates in catabolic pathways in the liver and in other organs and lead to the formation of bile acids in the liver. Oxysterol receptors are a part of the nuclear receptor family and regulate sterol-sensitive genes. It has been postulated that 27-hydroxycholesterol may suppress the accumulation of cholesterol in the presence of 7-hydroperoxycholesterol or other lipids deemed to be toxic. A summary of the human oxysterols produced from cholesterol is shown in Fig. 14.10 . The most important oxysterols appear at the bottom of the figure and are underlined.

Figure 14.10, Primary cholesterol oxygenation reactions mediated by different CYP species or occurring nonenzymatically in the presence of ROS. The oxygenation of cholesterol into 25-hydroxycholesterol is catalyzed by cholesterol 25-hydroxylase , a nonheme iron protein. The quantitatively most important oxysterols present in the human circulation are underlined. CYP , Cytochrome P-450; ROS , reactive oxygen species.

The levels of oxysterols regulate the expression of cholesterologenic enzymes and thus act as a negative product feedback on the synthesis of cholesterol. The sterol regulatory element (SRE)-binding protein-2 is responsive both to sterols and oxysterols and controls the response of cholesterol-forming enzymes [lanosterol 14 α-demethylase ( CYP51A1 ) and squalene synthase ( farnesyl diphosphate farnesyl transferase 1 )] by way of the negative LXR DNA response elements ( nLXRE s), which are the components of each of the genes regulating these enzymes. Both the SRE and nLXRE are involved in the oxysterol-dependent repression of the CYP51A1 gene. This expands information on the negative feedback control of the synthesis of cholesterol discussed in the chapter on lipids. The summary actions of the two liver receptors (LXRα, LXRβ) that form heterodimers with RXR (the ligand of which can be 9- cis -retinoic acid) activate genes that regulate both cholesterol and fatty acid homeostasis.

Sunlight and Vitamin D

In the bowel wall epithelium, cholesterol is oxidized to 7-dehydrocholesterol that is then transported to the skin. In the skin, sufficient sunlight allows the conversion of 7-dehydrocholesterol to a precursor ( cholecalciferol ) of the activated form of the vitamin. 7-Dehydrocholesterol, with UV radiation, is converted to cholecalciferol ( vitamin D 3 ) in the skin and then to 25-hydroxycholecalciferol in the liver by 25-hydroxylase . This product is transported in the blood to the kidney where it is further hydroxylated to 1,25-dihydroxycholecalciferol (1,25-dihydroxy vitamin D 3 , the hormone), the active form that serves for the ligand of the vitamin D receptor (VDR) . The vitamin D–binding protein (induced by the VDR) carries the hormone in the blood to various cells where dissociation occurs outside the cell, the vitamin D 3 enters the target cell and is carried to the nucleus where the VDR resides. The complex (1,25-dihyroxycholecalciferol-VDR) then activates specific genes to bring about the vitamin D cellular response. One of the responses is the induction of calcium-transporting proteins that transport calcium from the intestinal lumen across the intestinal epithelial cell to the basolateral side of the cell and eventually into the bloodstream. The role of this vitamin is to ensure the proper balance between calcium and phosphorous for the mineralization of bone .

Many consider 10 minutes of sunlight per day to satisfy the human requirement for vitamin D . This is probably not nearly sufficient. In older persons (and in obese persons) the ability to convert 7-dehydrocholesterol in the skin to vitamin D 3 is decreased. Activated vitamin D ( 1,25-dihydroxy vitamin D3 ) is a hormone belonging to the steroid hormone class based on the nature of its receptor (VDR). In Chapter 16 , Steroid Hormones, the formation and action of the activated form of vitamin D is elaborated. Fig. 14.11 summarizes expectation for the amounts of vitamin D in relation to time of exposure to sunlight.

Figure 14.11, Obtaining adequate vitamin D from sunlight. The upper figure shows the amount of time in the sun to achieve adequate levels of vitamin D. The lower figure indicates the type of clothing, location, and age class that determine time in sunlight to attain adequate amounts of vitamin D.

Natural diets do not contain sufficient vitamin D, explaining the need for sunlight to activate the vitamin D precursor in skin. In climates with limited numbers of days of sunlight, children often grow up vitamin D-deficient and remain deficient in adulthood. This condition can lead to an increased predisposition to colorectal cancer as this vitamin is known to enhance the function of the immune system among other effects. A recent report ( ) indicates that 63 loci are involved in the action of vitamin D in the human, a finding that underscores the importance of this vitamin.

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