Lipids Pt 3.docx

Lipids: Fatty Acid Metabolism Fatty acids (FA) are synthesized from and oxidized to acetyl-coa. Fatty acids are oxidized in the mitochondria and synthesized in the cytoplasm. Physiological conditions that promote fatty acid synthesis will inhibit oxidation (and vise-versa). This can prevent futile r...

Lipids: Fatty Acid Metabolism Fatty acids (FA) are synthesized from and oxidized to acetyl-coa. Fatty acids are oxidized in the mitochondria and synthesized in the cytoplasm. Physiological conditions that promote fatty acid synthesis will inhibit oxidation (and vise-versa). This can prevent futile recycling. De novo synthesis of Fatty Acids: General Info Fatty acid (de novo) synthesis normally occurs in the cytosol of adipocytes, hepatocytes, and mammary glands (for lactation). Short fatty acids are specifically produced in the mammary glands (where butiric acid and caproic acid are converted into milk fat). The kidney, brain, and lungs can synthesize FA in small quantities. Animals can synthesize all fatty acids they need, except for essential fatty acids which must be obtained by diet. The substrates of de novo synthesis are excess carbohydrate and proteins from the diet. Acetyl Coenzyme A (ACoA) from the mitochondria is essential for de novo synthesis of fatty acids. De novo synthesis of fatty acids also requires ATP and NADPH. The primary end product of de novo synthesis is palmitate or palmitic acid (16carbons). Palmitate can be further elongated in the smooth ER. Enzymes in the smooth ER can further desaturate LCFA by adding cis double bounds. The brain can produce VLCFA for synthesis of brain phospholipids. A variety of poly unsaturated fatty acids (PUFA) can be reduced by desaturation and elongation within de novo synthesis. De novo synthesis of Fatty Acids: Pathway Step 1: Cytosolic Acetyl Coa Production Mitochondrial ACoA is produced by pyruvate oxidation. Acetyl-CoA can’t leave the mitochondria because “CoA” can’t get through the inner mitochondrial membrane Therefore, the acetyl group is incorporated into citrate for membrane transport. Citrate is produced by the condensation of ACoA and OAA (oxaloacetate). In the cytosol, citrate is then cleaved into ACoA and OAA by ATP-citrate lyase. This whole process is stimulated when mitochondrial citrate is high because citrate is only high why ATP concentrations are high, so this triggers a high energy signal to be produced. Step 2: ACoA carboxylation to Malonyl CoA ACoA carboxylation to Malonyl CoA is catalyzed by acetyl-CoA carboxylase (ACC). ACC is allosterically activated by citrate and is inactivated by palmitoyl CoA (via pathway end product negative feedback). ACC synthesis is stimulated by high calorie and high carbohydrate diets, as well as hormonally by insulin. Biotin (Vit H or B7) and ATP are required for the carboxylation process. ACoA carboxylation to Malonyl CoA is the rate limiting step and regulated step of de novo fatty acid synthesis. Step 3: Synthesis of Palmitate (16:0) Most reactions in fatty acid synthesis are driven by the enzyme fatty acid synthase (FAS). Palmitate is synthesized by adding 2 carbons from malonyl CoA to the carboxyl end of a series of acyl acceptors (amino acids such as cysteine). The PPP (pentose phosphate pathway) provides reductant NADPH. Lipid: Storage Lipids are stored as TAG (triacylglycerol: molecule of 3 fatty acids). The 3 fatty acids in TAG are generally not the same. Often, carbon 1 is saturated, carbon 2 is unsaturated, and carbon 3 can be either saturated or unsaturated. The presence of unsaturated FA lowers the Tm of the lipid. Newly synthesized FA can be stored as mono-, di-, or TAG. Lipid caloric mass is over twice that of carbohydrates and proteins, making them more efficient for storage. Ex: 9kCal/gm of TAG compared to 4kCal/g for carbs and proteins. Lipids are stored as liquid droplets (of TAG) in adipocytes. A small amount of lipids are stored in the liver and are released as VLDL into the blood. Mobilization of fat from adipocytes requires release from their TAG form via lipolysis, with the help of HSL (hormone sensitive lipase). This is stimulated by epinephrine and glucagon. Fatty Acid Beta Oxidation: Carnitine Carnitine is a compound synthesized by methionine and lysine amino acids in the liver and kidneys (where they are abundant in mitochondrial membranes of muscle tissue). Carnitine can be obtained via diet from animal products such as red meat, poultry, and dairy. Carnitine deficiency can cause a decreased ability of tissues to use LCFA for energy. Carnitine deficiency can be due to cellular defects, genetic or medical conditions, or due to liver or kidney pathologies. If someone has a deficiency of lysine or methionine, they can also develop a carnitine deficiency. The carnitine shuttle (CPT1) can be inhibited by malonyl CoA, so newly synthesized FA cannot be transferred to the mitochondria to be degraded. Fatty Acid Beta Oxidation Beta oxidation is the major catabolism pathway for fatty acids and it takes place in the mitochondria. LCFA must form an active intermediate (fatty acyl CoA) before being oxidized in the mitochondria. The carnitine shuttle is required to transport fatty acyl CoA into the mitochondria (rate limiting transport). Beta oxidation is a cyclic process, and each cycle is catalyzed by enzymes with chain length specificity. Each cycle of beta oxidation produces: 1 acetyl-CoA, 1 NADH, 1 FADH2. The first cycle of beta oxidation is a sequence of 4 reactions that involve the beta carbon and cause shortening of the FA by 2 carbons at the carboxyl group. These 4 reactions include: Reaction 1: Oxidation reaction producing FADH2 Reaction 2: Hydration reaction Reaction 3: Oxidation reaction producing NADH Reaction 4: CoA-dependent thiolytic cleavage reaction that frees a molecule of acetyl-CoA Oxidation of 1 palmitoyl CoA will result in the production of: 8 acetyl-CoA, 7 NADH, 7 FADH2. The final products after full oxidation (beta oxidation, TCA cycle, and OxPhos completed) will produce ATP, CO2, and H2O. 2 ATP: activation od palmate to palmitoyl CoA (fatty acyl CoA) 1 fatty acid (palmitoyl CoA) can produce 129 ATP, in comparison to 1 glucose which will produce 36 ATP. Lipids: Ketones Ketones are used as alternative fuel for the cells. The adult live mitochondria can convert ACoA from fatty acid (beta) oxidation into ketone bodies. Types of ketones include: acetoacetate, beta hydroxybutyrate, and acetone. Acetone is metabolically inert and can cause fruity smell on the breath and urine of ketonic patients. Acetoacetate and beta hydroxybutyrate are free soluble lipids. They are transported in the blood plasma to peripheral tissues (muscle, brain, kidney, mammary gland, small intestine, fetal liver). In peripheral tissue cells, acetoacetate and beta hydroxybutyrate can be converted back to acetyl CoA which will enter the TCA cycle for ATP production. Acetoacetate and beta hydroxybutyrate can be used in biosynthesis of glycerophospholipids, sphingolipids, and steroid. Acetoacetate and beta hydroxybutyrate cause negative feedback on hormone sensitive lipase (HSL) activity within the adipocytes. Ketones are an important energy supply for the peripheral tissues. Ketones are water soluble (because they are polar, hydrophillic molecules), and can be transported freely, without albumin or lipoproteins. Ketones can cross the blood brain barrier and placental barrier. Ketones are used proportionally to their concentration in the blood by extrahepatic tissue. If concentrations of ketones are high enough, ketones can be used in the cardiac and skeletal muscle, intestinal mucosa cells, renal cortex, brain, and even by a fetus. The production of ketone bodies is stimulated by the lipolysis of TAG in adipocytes. Increased acetyl-CoA (due to beta oxidation of FFA) can cause the liver to exceed oxidative capacity (meaning that there is not enough oxygen available to continue the ETC). This then stimulate the production of ketone bodies. Ketone’s major function is to save glucose, which is important during episodes of fasting and prolonged exercise. During fasting, fatty acids mobilized form adipose tissue move to the liver. Fatty acid oxidation produces high amounts of NADH (exceeding the oxidative capacity of TCA/OxPhos in the liver) which causes acetyl CoA to go into ketogenesis (ketone body production. Mammalian RBC and liver cannot use ketones as an energy source because RBC lack mitochondria, and hepatocytes lack the thiophorase enzyme. Ketolysis Ketolysis is when ketones are used by peripheral tissues. In normal conditions, the liver produces low levels of ketone bodies. During fasting, ketone body production increases in the liver because the ketone bodies are required as energy for peripheral tissue. Fasting can be caused by diabetes mellitus. Step 1: 3-hydroxybutyrate is oxidated to acetoacetate. Step 2: Acetoacetate and a CoA molecule combine to form Acetoacetyl CoA. Step 3: Acetoacetyl CoA is converted into 2 acetyl CoA’s.