Complex Lipid Metabolism UNIT III: Lipid Metabolism

 The degradation of phosphoglycerides is performed by phospholipases found in all tissues and pancreatic juice.  A number of toxins and venoms have phospholipase activity, and several pathogenic bacteria produce phospholipases that dissolve cell membranes and allow the spread of infection.  Sphingomyelin is degraded by the lysosomal phospholipase, sphingomyelinase. 4- Degradation of Phospholipids 2

 Phospholipases hydrolyze the phosphodiester bonds of phosphoglycerides, with each enzyme cleaving the phospholipid at a specific site.  The major enzymes responsible for degrading phosphoglycerides are shown in Figure Note:  Removal of the fatty acid from carbon 1 or 2 of a phosphoglyceride produces a lysophosphoglyceride, which is the substrate for lysophospholipases. A- Degradation of phosphoglycerides 3

 Phospholipases release molecules that can serve as:  messengers (for example, DAG and IP3),  or that are the substrates for synthesis of messengers (for example, arachidonic acid). Note:  Phospholipases are responsible not only for degrading phospholipids, but also for “remodeling” them.  For example, phospholipases A1 and A2 remove specific fatty acids from membrane-bound phospholipids; these can be replaced with alternative fatty acids using fatty acyl CoA transferase.  This mechanism is used as one way to create the unique lung surfactant, DPCC, and to insure that carbon 2 of PI (and sometimes of PC) is bound to arachidonic acid. A- Degradation of phosphoglycerides 4

 Figure Degradation of glycerophospholipids by phospholipases. 5

B- Degradation of sphingomyelin  Sphingomyelin is degraded by sphingomyelinase, a lysosomal enzyme that hydrolytically removes phosphorylcholine, leaving a ceramide.  The ceramide is cleaved by ceramidase into sphingosine and a free fatty acid (Figure 17.12). Note:  The ceramide and sphingosine released by the degradation of sphingomyelin play a role as intracellular messengers.  Ceramides appear to be involved in the response to stress, and sphingosine inhibits protein kinase C. 6

 Niemann-Pick disease (Types A and B) is an autosomal recessive disease caused by the inability to degrade sphingomyelin.  The deficient enzyme is sphingomyelinase—a type of phospholipase C.  In the severe infantile form (Type A—less than 1% normal activity), the liver and spleen are the primary sites of lipid deposits and are, therefore, tremendously enlarged.  The lipid consists primarily of the sphingomyelin that cannot be degraded (Figure 17.13).  Infants with this disease experience rapid and progressive neurodegeneration as a result of deposition of sphingomyelin in the central nervous system, and they die in early childhood. B- Degradation of sphingomyelin 7

Figure Degradation of sphingomyelin. Figure Accumulation of lipids in spleen cells from a patient with Niemann-Pick disease. 8

 A less severe variant (Type B—5% or more) causes little to no damage to neural tissue, but lungs, spleen, liver, and bone marrow are affected, resulting in a chronic form of the disease, with a life expectancy into adulthood.  Although Niemann-Pick disease occurs in all ethnic groups, both Type A and B occur with greater frequency in the Ashkenazi Jewish population than in the general population B- Degradation of sphingomyelin 9

 Glycolipids are molecules that contain both carbohydrate and lipid components.  Like the phospholipid sphingomyelin, glycolipids are derivatives of ceramides in which a long-chain fatty acid is attached to the amino alcohol sphingosine.  They are, therefore, more precisely called glycosphingolipids. Note:  Ceramides, then, are the precursors of both phosphorylated and glycosylated sphingolipids.  Like the phospholipids, glycosphingolipids are essential components of all membranes in the body, but they are found in greatest amounts in nerve tissue. 5- Overview of Glycolipids 10

 They are located in the outer leaflet of the plasma membrane, where they interact with the extracellular environment. As such, they play a role in the regulation of cellular interactions, growth, and development. Note:  When cells are transformed (that is, when they lose control of cell division and growth), there is a dramatic change in the glycosphingolipid composition of the membrane. 5- Overview of Glycolipids 11

 Glycosphingolipids are antigenic, and they have been identified as a source of blood group antigens, various embryonic antigens specific for particular stages of fetal development, and some tumor antigens. Note:  The carbohydrate portion of a glycolipid is the antigenic determinant.  They also serve as cell surface receptors for cholera and tetanus toxins, as well as for certain viruses and microbes.  Genetic disorders associated with an inability to properly degrade the glycosphingolipids result in intracellular accumulation of these compounds. 5- Overview of Glycolipids 12

 The glycosphingolipids differ from sphingomyelin in that they do not contain phosphate, and the polar head function is provided by a monosaccharide or oligosaccharide attached directly to the ceramide by an O-glycosidic bond (Figure 17.14).  The number and type of carbohydrate moieties present help determine the type of glycosphingolipid. 6- Structure of Glycosphingolipids 13

 The simplest neutral (uncharged) glycosphingolipids are the cerebrosides.  These are ceramide monosaccharides that contain either a molecule of galactose (galactocerebroside—the most common cerebroside found in membranes) or glucose (glucocerebroside, which serves primarily as an intermediate in the synthesis and degradation of the more complex glycosphingolipids). Note:  Members of a group of galactocerebrosides (or glucocerebrosides) may also differ from each other in the type of fatty acid attached to the sphingosine.  As their name implies, cerebrosides are found predominantly in the brain and peripheral nervous tissue, with high concentrations in the myelin sheath. A- Neutral glycosphingolipids 14

 Ceramide oligosaccharides (or globosides) are produced by attaching additional monosaccharides (including GalNAc) to a glucocerebroside. Examples of these compounds include: A- Neutral glycosphingolipids 15 Cer-Glc-Gal-Gal-GalNac-GalNacGloboside (Forssman antigen): Cer-Glc-GalGloboside (lactosylceramide): Cer-GlcCerebroside (glucocerebroside): (Cer = ceramide, Glc = glucose, Gal = galactose, GalNac = N-acetylgalactosamine)

 Acidic glycosphingolipids are negatively charged at physiologic pH.  The negative charge is provided by N-acetylneuraminic acid (NANA) in gangliosides, or by sulfate groups in sulfatides. 1.Gangliosides:  These are the most complex glycosphingolipids, and are found primarily in the ganglion cells of the central nervous system, particularly at the nerve endings.  They are derivatives of ceramide oligosaccharides, and contain one or more molecules of NANA.  The notation for these compounds is G (for ganglioside), plus a subscript M, D, T, or Q to indicate whether there is one (mono), two, three, or four (quatro) molecules of NANA in the ganglioside, respectively. B- Acidic glycosphingolipids 16

 Additional numbers and letters in the subscript designate the sequence of the carbohydrate attached to the ceramide. (See Figure for the structure of GM2.)  Gangliosides are of medical interest because several lipid storage disorders involve the accumulation of NANA- containing glycosphingolipids in cells. B- Acidic glycosphingolipids 17 Figure Structure of the ganglioside GM2.

2.Sulfatides:  Sulfoglycosphingolipids (sulfatides) are cerebrosides that contain sulfated galactosyl residues, and are therefore negatively charged at physiologic pH.  Sulfatides are found predominantly in nerve tissue and kidney. Figure Structure of 3 ′ - phosphoadenosine-5 ′ - phosphosulfate (PAPS). 18 Figure Structure of galactocerebroside 3-sulfate.

 Synthesis of glycosphingolipids occurs primarily in the Golgi by sequential addition of glycosyl monomers transferred from UDP–sugar donors to the acceptor molecule.  The mechanism is similar to that used in glycoprotein synthesis. 7- Synthesis and Degradation of Glycosphingolipids 19

 The enzymes involved in the synthesis of glycosphingolipids are glycosyl transferases, each specific for a particular sugar nucleotide and acceptor. Note: These enzymes may recognize both glycosphingolipids and glycoproteins as substrates. A. Enzymes involved in synthesis 20

 A sulfate group from the sulfate carrier, 3'- phosphoadenosine-5'-phosphosulfate (PAPS), is added by a sulfotransferase to the 3'-hydroxyl group of the galactose in a galactocerebroside.  Galactocerebroside 3-sulfate is the major sulfatide in the brain.  An overview of the synthesis of sphingolipids is shown in Figure Note:  PAPS is also the sulfur donor in glycosaminoglycan synthesis, and steroid hormone catabolism. B. Addition of sulfate groups 21

Figure Overview of sphingolipid synthesis. 22

 Glycosphingolipids are internalized by endocytosis as described for the glycosaminoglycans.  All of the enzymes required for the degradative process are present in lysosomes, which fuse with the endocytotic vesicles.  The lysosomal enzymes hydrolytically and irreversibly cleave specific bonds in the glycosphingolipid.  As seen with the glycosaminoglycans and glycoproteins, degradation is a sequential process following the rule “last on, first off,” in which the last group added during synthesis is the first group removed in degradation. C. Degradation of glycosphingolipids 23

 In a normal individual, synthesis and degradation of sphingolipids are balanced, so that the amount of these compounds present in membranes is constant.  If a specific hydrolase required for the degradation process is partially or totally missing, a sphingolipid accumulates in the lysosomes.  Lipid storage diseases caused by these deficiencies are called sphingolipidoses. The result of a specific hydrolase deficiency may be seen dramatically in nerve tissue, where neurologic deterioration can lead to early death. Note: Ganglioside turnover in the central nervous system is extensive during neonatal development. 24 D. Sphingolipidoses

1.Common properties:  A specific lysosomal hydrolytic enzyme is deficient in each disorder.  Therefore, usually only a single sphingolipid (the substrate for the deficient enzyme) accumulates in the involved organs in each disease.  The disorders are progressive and, although many are fatal in childhood, extensive phenotypic variability is seen leading to the designation of different clinical types, such as Types A and B in Niemann-Pick disease.  Genetic variability is also seen because a given disorder can be caused by any one of a variety of mutations within a single gene.  The sphingolipidoses are autosomal recessive diseases, except for Fabry disease, which is X-linked.  The incidence of the sphingolipidoses is low in most populations, except for Gaucher and Tay-Sachs diseases, which, like Niemann-Pick disease, show a high frequency in the Ashkenazi Jewish population. D. Sphingolipidoses 25

2.Diagnosis and treatment:  A specific sphingolipidosis can be diagnosed by measuring enzyme activity in cultured fibroblasts or peripheral leukocytes, or by analysis of DNA.  Histologic examination of the affected tissue is also useful.  Prenatal diagnosis, using cultured amniocytes or chorionic villi, is available. Note:  Shell-like inclusion bodies are seen in Tay-Sachs, and a wrinkled tissue paper appearance of the cytosol is seen in Gaucher disease. D. Sphingolipidoses 26 Figure Aspirated bone marrow cells from patient with Gaucher disease.

 The sphingolipid that accumulates in the lysosomes in each disease is the structure that cannot be further degraded as a result of the specific enzyme deficiency.  Gaucher disease, in which macrophages become engorged with glucocerebroside, and Fabry disease, in which globosides accumulate in the vascular endothelial lysosomes of the brain, heart, kidneys, and skin, are treated by recombinant human enzyme replacement therapy, but the monetary cost is extremely high.  Gaucher has also been treated by bone marrow transplantation (because macrophages are derived from hematopoietic stem cells). D. Sphingolipidoses 27

Figure Degradation of sphingolipids showing the enzymes affected in related genetic diseases, the sphingolipidoses. All of the diseases are autosomal recessive except Fabry disease, which is X-linked, and all can be fatal in early life. (Cer = ceramide). 28