LipMetab_IV Phosphoglycerol Sphingo Metabolism Notes PDF

S.H. Ackerman, PhD Phosphoglycerol & sphingolipid metabolism 1 [email protected] 4213 Scott Hall LECTURE TITLE: PHOSPHOGLYCEROL & SPHINGOLIPID METABOLISM LEARNING OBJECTIVES 1. Outline the biosynthetic pathways for glycerolipids, with spe...

S.H. Ackerman, PhD Phosphoglycerol & sphingolipid metabolism 1 [email protected] 4213 Scott Hall LECTURE TITLE: PHOSPHOGLYCEROL & SPHINGOLIPID METABOLISM LEARNING OBJECTIVES 1. Outline the biosynthetic pathways for glycerolipids, with specific attention to the tissue-specific origin of glycerol-3-phosphate, the pathway bifurcation downstream phosphatidate synthesis, and the activated intermediates used to make TAG, PC, PE, PS, PI, and PG/CL. 2. Recall the names and actions of the phosphoglycerol phospholipases. 3. Outline the biosynthetic pathways for sphingolipids, with specific attention to the relationship between ceramide and sphingosine, and the activated intermediates used to make SM and the various glycolipids. 4. Explain the general defect behind sphingolipid storage diseases. 5. Identify a specific sphingolipid disease from the name of either the deficient enzyme or the accumulated storage material. OUTLINE I. Overview of membrane lipids and review of phosphoglycerol nomenclature/structure. II. Anabolic pathways to make phosphoglycerols. A. Origin of glycerol-3-phosphate B. Bifurcation downstream phosphatidate synthesis III. Review of the names and cleavage sites of the phospholipases that hydrolyze phosphoglycerols. IV. Anabolic pathways to make sphingolipids. A. Ceramide synthesis B. Activated donors for sphingomyelin and glycolipids V. Sphingolipid catabolism and storage diseases.  Phosphoglycerol & sphingolipid metabolism 2 Membrane lipids (review) The 3 classes of membrane lipids are highlighted in yellow in the upper panel of the adjacent figure). Cholesterol comprises ~35-40% of total membrane lipids in humans, but shows an asymmetric distribution with ~90% or more of the sterol located in the plasma membrane. The unique character of lipid bilayers is provided by members of the fatty acid superfamily of lipids, which are sub-classified as either glycerolipids or sphingolipids by the presence of either glycerol or sphingosine (shaded purple in lower panel of adjacent figure). All of the sphingolipids are derivatives of ceramide (sphingosine + N-acyl FA) (described in the second half of the lecture), and all are membrane lipids. Phosphatidate (1,2-diacylglycerol-3-phosphate) is the precursor to all glycerolipids, and is the major branch Phosphatidate Ceramide point in the biosynthesis of these complex molecules. Phosphoglycerol nomenclature, structure, and sub-structures are shown in the adjacent figure.  Phosphoglycerol & sphingolipid metabolism 3 Anabolic pathways to make phosphoglycerols Phosphatidate Synthesis Whereas de novo synthesis of TAG from surplus carbon is limited to hepatocytes and adipocytes, nearly all cells make phosphoglycerols. Glycerol-3-P is the starting material for making all of the glycerolipids, and gets acylated at positions C-1 and C-2 in a common pathway that terminates with phosphatidate (see adjacent figure). These reactions were described earlier when de novo TAG synthesis in liver and adipose tissue was presented. The most important point to note here relates to the origin of glycerol-3-P. Reduction of DHAP by the NAD+-dependent glycerol3P dehydrogenase is the generic/default pathway that is shared in common by all cells, using the DHAP produced during glycolysis. Liver is the only tissue that expresses the gene for glycerol kinase, which transfers the g-phosphoryl from ATP to the glycerol C3 hydroxyl oxygen to make glycerol-3P. Glycerolipid synthesis bifurcates at phosphatidate. There are two different group transfer strategies For PI and PG synthesis, phosphatidate is For TAG, PC, PE, and PS, phosphatidate is used to derivatize glycerol at the C3 position (see activated in a reaction where CTP is cleaved at the a-phosphate, releasing PPi, and the desphosphorylated yielding DAG. Synthesis proceeds using chemically activated adjacent slide). One (right-hand side of figure) is cytidylate moiety is condensed with phosphatidate, yielding CDP-diacylglycerol forms of the polar head groups; FA-CoA for TAG and CDP-derivatives of choline or (CDP-DAG). Phosphoanhydride bond cleavage ethanolamine. to dephosphorylate phosphatidate, which yields drives the reaction with inositol or glycerol. 1,2-diacylglycerol (DAG), and is used for de novo synthesis of storage lipid (TAG) and the phosphoglycerol membrane lipids, PC, PE, and CDP-ethanolamine PS. The structures are completed by condensing inositol glycerol CDP-choline DAG with either the fatty acid from an acyl-CoA or FA-CoA a phosphorylated alcohol that has been rendered chemically reactive by the transient attachment of CMP. (a more detailed description is given on the TAG following page. PI PG PC PE PS is synthesized from PE Exchange reaction Serine + PE PS + ethanolamine PS* Instead, for the synthesis of PI and PG, phosphatidate is covalently coupled to CMP creating a chemically-reactive metabolite (CDP-DAG) that can be condensed with an alcohol. Phosphatidylinositol is synthesized from CDP-DAG and inositol by PI synthase, with the release of the activating CMP group (upper and lower panels of adjacent figure). Once phosphatidylglycerol has been synthesized from CDP-DAG and glycerol, PG is used to make cardiolipin by condensing it with the phosphatidate component of another CDP-DAG, commensurate with the release of CMP (cardiolipin synthase The actions of downstream kinases create polyphosphorylated derivatives reaction). of PI that are essential components of signal transduction pathways.  Phosphoglycerol & sphingolipid metabolism 4 The primary pathways operating in human cells to synthesize PC, PE, and PS are shown in the adjacent figure. The alcohol head groups of PC and PE are activated for group transfer to DAG by the concerted actions of substrate-specific kinases and cytidyl transferases, yielding CDP choline or CDP-ethanolamine. Transferring the phosphocholine component from CDP-choline to DAG releases phosphatidylcholine and CMP as products. Phosphatidylethanolamine is made in essentially the same manner from CDP-ethanolamine and DAG. Phosphatidylserine is the product of an isoenergetic exchange reaction in which serine replaces ethanolamine in PE. Minor pathways interconvert between PC, PE, and PS. Decarboxylation of PS yields PE. Trimethylation of PE yields PC; uses S-adenosylmethionine (SAM) as the methyl donor. The biosynthetic pathways for glycerolipids (TAG and phosphoglycerols) is summarized below.  Phosphoglycerol & sphingolipid metabolism 5 Phospholipases are a high-yield topic Recall that there are position-specific phospholipases that cleave phosphoglycerols, releasing two sub-component products. FA #1 Phospholipase A1 hydrolyzes the bond to the FA at glycerol C1, yielding a free FA and a C2-acylated lysophosphatidate. Phospholipase A2 hydrolyzes the bond to the FA at glycerol C2, FA #2 yielding a free FA and a C1-acylated lysophosphatidate. Phospholipase C hydrolyzes the bond to the phosphorylated alcohol alcohol at glycerol C3, yielding a free phosphoalcohol and DAG. Phospholipase D hydrolyzes the bond to the alcohol at glycerol C3, yielding a free alcohol and phosphatidate. Review of sphingolipid structure sphingophospholipid Sphingomyelin (SM) Spingoglycolipid (GL) Cerebroside, globoside, ganglioside Ceramide + phosphocholine Ceramide Ceramide Ceramide + + + Monosaccharide Neutral Acidic (Glc, Gal) polysaccharide polysaccharide N-Acetyl- Neuraminic Acid  Phosphoglycerol & sphingolipid metabolism 6 Anabolic pathways to make sphingolipids Ceramide is a metabolic intermediate that is common all of the sphingolipid biosynthesis pathways. Ceramide is synthesized from an amino acid (serine) and a fatty acid (palmitate). The pathway is initiated by attaching the acyl component of palmitoyl-CoA to the Ser aC, commensurate with loss of the a-carboxylate group as CO2, in an irreversible reaction. The resultant sphingoid metabolite is converted to ★ ★ ceramide following attachment of a second fatty acid to the nitrogen of the former a-amino group of serine, flanked by a couple of redox reactions. Reciprocal reactions catalyzed by ceramidase and ceramide synthase, which, respectively, break and make the amide in ceramide interconvert ceramide with the metabolite, sphingosine. These reactions are interesting/important for several reasons. First, it reveals why another name for ceramide is N-acyl sphingosine. Second, it reveals the mechanism fatty acid exchange in the hydrophobic component of sphingolipids (analogous to PLA2 action in phosphoglycerols). Third, it reveals the first step in the pathway to make sphingosine-1-phosphate (S1P). S1P is a bioactive extracellular lysophospholipid (single tail) that elicits responses in target cells when it binds to an S1P receptor in the plasma membrane (S1P receptors are GPCR’s). Sphingomyelin and glycolipids are synthesized in reactions that transfer phosphocholine from PC or monosaccharides from activated sugars to the primary hydroxyl group of ceramide (see adjacent figure). Recall from previous lectures on glycogen synthesis and galactose entry to glycolysis that the activated form of neutral sugars is made by condensing UMP (from UTP) with 1-phosphoglucose (or with phospho derivatives of Gal or GalNac) to make UDP monosaccharides. UDP is released when the sugar is transferred to the acceptor molecule. In a similar reaction, the CMP moiety from CTP is transferred to sialic acid to make CMP-sialic acid. CMP is released when sialic acid is transferred to the acceptor molecule. The first sialic acid residue added to a neutral polysaccharide head group converts the lipid from a globosides to a ganglioside. It goes without saying that in all of these pathways to activate sugars, pyrophosphatase-catalyzed cleavage the PPi product makes the reactions between sugars and NTP’s irreversible.  Phosphoglycerol & sphingolipid metabolism 7 Sphingolipid breakdown and storage diseases Sphingolipids are hydrolyzed sequentially to basic components in the lumen of lysosomes by acid-stable hydrolases. These enzymes are glycoproteins, which are synthesized in the lumen of the ER and Golgi with a mannose-6-phosphate tag. The M6P tag is recognized by proteins that direct vesicle traffic inside cells and route these enzymes to lysosomes. Similar to what is observed for the catabolic pathway that breaks down galactose, a genetic mutation in any of the sphingolipid-specific hydralases causes upstream metabolites to accumulate and produces the lysosomal storage diseases known as sphingolipidoses. Sphingolipidoses The characteristics of the better-known sphingolipidoses are given in the adjacent table. The mutation in Fabry’s disease is X-linked; all of the sphingolipid storage diseases are linked to a genetic mutation that is are inherited as an autosomal recessive trait. Diagnosis is made by assaying for a particular enzyme activity or accumulated lipid species in tissue biopsies, cultured fibroblasts, peripheral blood leukocytes, N.B. This table is not inclusive of all known sphingolipid storage plasma, and/or amniotic fluid. diseases. As is typical of diseases caused by inborne errors in metabolism, sphingolipidoses occur only rarely in the general population. However, there is a high incidence of Tay Sachs disease in the Ashkenazi Jewish population. Because sphingolipids are particularly abundant in cells of the central nervous system, sphingolipid storage diseases typically present with severe neurological defects, a poor prognosis, and fatality in early life. There are specific phenotypic traits associated with sphingolipid disease. For example, a cherry red spot in the retinal macula of the eye (adjacent figure) is commonly associated with sphingolipid storage diseases. Formerly, this trait was considered to be highly diagnostic of Tay Sachs disease. However, it is routinely observed in patients who present with one of the other sphingolipidoses (e.g. Niemann-Pick, Gaucher’s).  Phosphoglycerol & sphingolipid metabolism 8 I have included the following information familiarize you with traits that are fairly diagnostic of a particular type of sphingolipid storage disease. You do not need to use this information to differentiate between sphingolipidoses on the GI summative exam, but a question of this type can be anticipated for the USMLE Step 1 exam. When the time comes to hunker down and study for the Boards, I recommend chapters 14 (Glycogen metabolism) and 21 (Sphingolipid metabolism) of Michael King’s book, Integrative Medical Biochemistry Examination and Board Review, Michael W. King (Indiana University SOM), 2014 McGraw-Hill Education; the end-of-chapter questions focus on differentiating between glycogen and sphingolipid storage diseases. In addition to a cherry red spot in the retinal macula, which may or may not be present, the following defines some of the specific features associated with Niemann-Pick, Gaucher’s, Tay Sachs, and Fabry’s diseases. Niemann-Pick types A and B are caused by deficiency for lysosomal sphingomyelinase, also known as ASMD for acid sphingomyelinase deficiency. Type A is severe with onset in early infancy. The hallmark feature is hepatosplenomegaly; there might be accompanying jaundice and/or ascites in newborns. Red arrows denote Affected infants will typically start showing signs of neurologic deterioration and Niemann-Pick foam cells muscle spasticity by 9-12 months of age. The disorder is often fatal by 3 years Patiño-Escobar B, Solano M H, Zarabanda L, of age. et al. (2019) Cureus 11):e4767 The accumulation of lysosomal sphingomyelin in macrophages and monocytes underscores the appearance of “Niemann-Pick foam cells” revealed by histological stains. Patients with Gaucher’s disease accumulate glucocerebroside due to deficiency for glucocerebrosidase. The build-up of this lipid gives rise to the striated tubular pattern observed in the cytoplasm of phagocytic cells known as Gaucher’s cells. Infants with Tay Sachs disease typically appear normal at birth, but show signs of motor weakness between 3 and 5 months of age. A hallmark trait of Tay Sachs is an exaggerated startle response. Fabry’s disease is characterized by cardiomyopathy that results from the continual deposition of glycosphingolipid in cardiovascular structures. Fabry’s disease is typically diagnosed in male offspring because the mutation in a-galactosidase A affects males is X-linked.