Biochemistry - 36 - Synthesis of TG and Membrane Lipids 2022.pdf

Synthesis of Triacylglycerols and Major Membrane Lipids Lecture 36 Reference: Lieberman and Peet, Chapter 31 (pp 695 – 705) Bluefield University – VCOM Campus MABS Program Biochemistry Jim Mahaney, PhD 1 Lecture Objectives a. Compare and contrast the synthesis of phosphatidic acid from glycerol i...

Synthesis of Triacylglycerols and Major Membrane Lipids Lecture 36 Reference: Lieberman and Peet, Chapter 31 (pp 695 – 705) Bluefield University – VCOM Campus MABS Program Biochemistry Jim Mahaney, PhD 1 Lecture Objectives a. Compare and contrast the synthesis of phosphatidic acid from glycerol in liver versus glucose (liver and adipose tissue) and interpret why adipose tissue can’t utilize glycerol for this purpose. b. Recall the processing of phosphatidic acid into a triglyceride. c. Recall the synthesis of glycerophospholipids from phosphatidic acid, and the synthesis of sphingolipids and glycolipids from serine and palmitoyl CoA. d. Relate the process by which VLDLs are synthesized by the liver. Identify the major enzymes involved in the process. e. Relate the role of lipoprotein lipase in muscle and adipose tissues for taking up fatty acids from VLDLs and chylomicrons. f. Compare and contrast the affinity of lipoprotein lipase in muscle versus adipose tissue and how that affects the function of each enzyme at each tissue. g. Recall triglyceride degradation in adipose tissue by hormone sensitive lipase and identify the hormone that regulates the hormone sensitive lipase. h. Relate the use of gluconeogenesis (early steps) for recycling free fatty acids back to triglycerides in adipose cells i. Recall the four phospholipase enzymes that degrade glycerophospholipids and recall the cellular organelle where sphingolipids are degraded. 2 2 Synthesis Pathways of TG and Glycerophospholipid: Phosphatidic Acid Objective A • synthesis of phosphatidic acid starts ultimately with glucose, but locally with dihydroxyacetone phosphate (DHAP) • DHAP is reduced to glycerol 3-P by G-3-P dehydrogenase. Preexisiting glycerol can also be phosphorylated in liver to glycerol 3-P. • Adipose lacks this enzyme to make G-3-P: Why? • Don’t want adipose tissue to convert glycerol back to lipid when FA’s are mobilized for energy. Rather, use the glycerol for glucose synthesis! • Acyl groups are added at C-1 and C-2 by acyltransferases using acyl CoA, saturated FAs are preferentially added at C-1 and unsaturated FAs at C-2 • Different enzymes attach FAs at C1 and C2 3 3 Objective B TG Synthesis and Transport • Carbon 3 of phosphatidic acid is dephosphorylated, forming diacyl glycerol (DAG) • A third FA is attached at C-3 to complete the TG synthesis • TGs are packaged into VLDLs, processed in the Golgi, then secreted into the blood (see next slide) • Destination is adipose tissue for storage AND muscle for energy use. • Remember: VLDL also as apoCII, which activates LPL • LPL favors muscle use over fat storage (covered later). 4 4 Glycerophospholipid Synthesis from Phosphatidic Acid Objective C • Ethanolamine and choline are rapidly phosphorylated upon entering cells and then react with CTP to form CDPethanolamine and CDP-choline, releasing PPi • Phosphoethanolamine and phosphocholine are transferred from the activated nucleotide derivative to DAG, releasing CMP 5 5 Objective C Interconversion of PLs • Phosphatidylserine is produced from PE • Liver can interconvert PE to PC by the sequential transfer of three methyl groups from S-adenosyl-methionine (SAM) • SAM is a methyl group donor, a major player in biochemical reactions. 6 6 Objective C Phosphatidylinositol and Cardiolipin • For PI and cardiolipin, activate the DAG itself first • React the activated DAG with with inositol (PI) or another DAG (cardiolipin). 7 7 Synthesis of Sphingolipids • Sphingolipid synthesis can be divided into two parts: • the formation of sphingosine then the ceramide • the conversion of ceramide to sphingomyelin and glycolipids • The precursors to sphingosine are seine and palmitoyl CoA whose condensation forms 3-ketosphinganine with decarboxylation of the serine Sphingosine à • 3-ketosphinganine is converted to a ceramide in three reactions • the 3-keto group is reduced to a 3-hydroxy group by NADPH Ceramide à • A Δ4 trans double bond is introduced by oxidation involving FAD • The amino group is acylated by transferring an FA (usually palmitate) from acyl CoA Objective C 8 8 Objective C Sphingolipids: Signaling and Cell Recognition 18 carbon amino alcohol backbone Primary difference between sphingolipids and phospholipids Note amide linkage of one fatty acid make a “ceramide” Addition of a phosphate group and a head group conveys structure and function properties Sphingomyelin commonly found in cell membranes, especially in the myelin sheath 9 9 Objective C Glycolipids: Cerebrosides and Gangliosides Several Varieties of Gangliosides Attach a single sugar and a cerebroside is formed Note: no phosphate Attach a polysaccharide and a ganglioside is formed Very specific structures and functions: primarily cell surface recognition and binding of specific proteins and factors to cell surface. 10 10 Sphingolipids to Glycolipids…and More! Objective C • Sphingomyelin is formed by the transfer of phosphocholine from phosphatidylcholine to the C1 hydroxy group of ceramide • Gluco- and galactocerebrosides are formed by the transfer of the sugar group from UDP-glucose or UCP-galactose • Complex glycolipids are formed by adding additional sugars one at a time 11 11 Review…Glycolipids: Lipid “Rafts” and Cell Surface Recognition • Glycoproteins, glycolipids and cholesterol on the extracellular face of the plasma membrane group together to form “rafts.” • Can be long lived or formed transiently based on cell signaling • Create a “surface” for the binding of proteins or protein complexes From the Lipids and Membranes lecture 12 12 Objective D Fatty Acid and Triglyceride Synthesis and VLDLs • VLDL assembly nearly identical to chylomicron assembly. • VLDL: liver • Chylomicron: intestine Note: first look at the blood lipoproteins… VLDL! 13 13 Objectives E, F Storage of TG in Adipose Tissue • TG in blood (in chylomicrons and VLDL) are hydrolyzed to FA and glycerol by lipoprotein lipase (LPL) • LPL is on the outer surface of the plasma membrane of cells lining the capillary tissues. • LPL is synthesized in adipose cells and secreted into capillaries of adipose tissue in response to insulin • FAs liberated by LPL are taken up by adipose cells – free glycerol is not utilized by the cells • Adipose cells don’t have glycerol kinase • Prevents futile cycle of attaching FAs immediately back onto glycerol 3-P • Muscle VERY similar, but fats liberated by LPL are taken up by the muscle cell to be used for energy production. Glycerol eliminated. • Muscle LPL much higher affinity than adipose LPL • Allows muscle to use fat when fat levels are low. Adipose will only store when fat levels are higher Note: Fed state…lipid for chylomicrons and glucose for insulin 14 14 Lipoprotein Lipase • Notice the Apo-CII protein is recognized by the LPL enzyme • Different cell types have different LPL isoforms • Anchors the lipoprotein to the endothelial cell membrane. • Lipoprotein lipase protrudes into lipoprotein and binds TG • TG is hydrolyzed and transported across the membrane 15 15 Objective G Degradation of Triacylglycerols • During fasting, stored TG in adipose tissue is broken down by process called lipolysis. • lipases cleave fatty acids from the TG: a hormone sensitive lipase (HSL) starts the process. • • • • signaled by decreasing insulin and increasing glucagon cAMP levels rise, protein kinase A is activated phosphorylation of hormone-sensitive lipase turns it on other lipases cleave remaining FAs from glycerol backbone • free fatty acids and glycerol are released into the blood • FAs are oxidized for energy in muscle and kidney • Free glycerol is used by liver for gluconeogenesis • FAs are carried by albumin in the blood 16 16 Objective H Recycling of Free FA in Adipose • Not all FA released from adipose cell during fasting. • Amount regulated by glucagon or epinephrine • Allows fine control of free FA level in blood • Adipose doesn’t have glycerol kinase – must use DHAP/NADH to make glycerol 3phosphate. • Note also precursors that lead to G-3-P and pathway very similar to gluconeogenesis and 1st bypass. 17 17 Objective I Degradation of Glycerophospholipids and Sphingolipids • Glycerophospholipids are hydrolyzed by phospholipases • phospholipase A1 releases FA at position 1 on glycerol • phospholipase A2 releases FA at position 2 on glycerol • phospholipase C releases the phosphorylated headgroup at position 3 • phopholipase D releases the headgroup Like most cell components, sphingolipids are degraded in the lysosomes. There are individual enzymes for each type of sphingolipid. If any of these enzymes are deficient, there are specific disease states associated with each one. 18 18 Aside: Glycolipids and Disease 19 19 Sample Question Glycerol can serve as a starting point for triacylglycerol synthesis in liver but not in adipose tissue. What is the starting point for triacylglycerol synthesis in adipose tissue? a. 1,2-diacylglycerol b. phosphatidic acid c. pyruvate d. dihydroxyacetone phosphate 20 Sample Question An obese teenager presents with a metabolic disorder manifesting as an inability to liberate free fatty acids from triacylglycerol stores in adipose tissues. Clinical tests show that the patient is able to digest, absorb, utilize and store dietary triacylglcerols and fatty acids normally. However, following a fast and a glucagon challenge, free fatty acids do not appear in the blood stream. A defect in what enzyme could cause this problem? a. b. c. d. Acetyl CoA carboxylase Hormone sensitive lipase Lipoprotein lipase Lecithin-cholesterol acyltransferase 21 Another One A patient is very concerned that his adipose cells cannot import glycerol from the blood, since he knows glycerol is the backbone of triglycerides. What can you tell him that will allay his concerns? a. His glycerol kinase will supply all the glycerol needed by adipose cells . b. Adipose cells do not synthesize triglycerides: liver does this. c. His adipose cells can use DHAP for TG synthesis instead of glycerol. c. His adipose cells only use glycerol for gluconeogenesis. d. His adipose cells will synthesize the glycerol needed for triglyceride formation. 22 Yet Another One A patient has a PEPCK defect in his adipose cells. What will be the effect of this defect on adipose cell function? a. Glycerol in his adipose cells will not get phosphorylated to support TG synthesis. b. His adipose cells will not bind VLDL or chylomicrons properly. c. His adipose cells will have a decreased ability to synthesize DHAP needed for TG formation. c. His adipose cells will not be able to carry out gluconeogenesis. d. His adipose not export free glycerol. 23 One More? A patient has a disorder in her adipose cell Lipoprotein Lipase (LPL) enzyme, where the enzyme does not recognize apoC-II on VLDL or chylomicron particles. Which of the following would be a likely consequence of this disorder? a. Her adipose cells will be much smaller than normal. b. Her blood lipoprotein level will be much higher than normal. c. Her blood vessels will have increased level of fatty streaks. d. Her liver may have more fatty deposits than normal. e. All of the listed effects are likely consequences. 24 Thank you! 25 25