LIPOLYSIS: FAT OXIDATION & KETONES BIOC DR. TISCHLER LECTURE 33

OBJECTIVES 1.Lipolysis a) describe the pathway; b) location c) principal enzyme d) role e) role of albumin and FABP in transport/metabolism of FFA 2.Degradation of fatty acyl CoA a) roles of acyl CoA synthetase, CPT-I and CPT-II, and CAT b) relationship of  -oxidation products to energy production. c) degradation of odd- vs even-chain FA d) vitamins for metabolizing propionyl CoA to succinyl CoA 3.Ketone body metabolism a) where ketogenesis occurs b) when ketogenesis occurs c) role of keotgenesis d) why normal individuals do not usually develop ketacidosis even when producing ketone bodies.

FAT FACTS fat (lipid) makes up 37% of the calories in the American diet energy rich and provides 9 kcal/gm dietary lipids 90% triacylglycerols (TAGs) also include cholesterol esters, phospholipids, essential unsaturated fatty acids; fat-soluble vitamins most dietary fat transported to adipose for storage dietary TAGs hydrolyzed in the intestine by pancreatic lipases; then reassembled in the intestinal cells dietary fats transported to tissues as TAG or cholesterol via chylomicrons at peripheral tissues (e.g., adipose or muscle), FA removed from the TAG by a lipoprotein lipase in the capillary walls; released fatty acids diffuse into the cell

saturated fatty acid: CH 3 -(CH 2 ) n -COOH unsaturated fatty acid: CH 3 -CH=CH-(CH 2 ) n -COOH polyunsaturated fatty acid: CH 3 -CH=CH-CH 2 -CH=CH-(CH 2 ) n -COOH  CH OOC-R 1 CH 2 OH HOOC-R 1  | | R 2 -COO----CH CHOH HOOC-R 2 |  | CH OOC-R 3 CH 2 OH HOOC-R 3 Figure 1. General structures of fatty acids and triacylglycerol. Lipolysis of stored triacylglycerol by lipases produces fatty acids plus glycerol. Lipolysis Triacylglycerol Glycerol Fatty acids

LIPOLYSIS fatty acids hydrolytically cleaved from triacylglycerol largely in adipose to release fatty acids as a fuel may also occur in muscle or liver - smaller amounts of fatty acids are stored hormone-sensitive (cyclic AMP-regulated) lipase initiates lipolysis – cleaves first fatty acid this lipase and others remove remaining fatty acids fatty acids/glycerol released from adipose to the blood hydrophobic fatty acids bind to albumin, in the blood, for transport

MITOCHONDRION cell membrane FA = fatty acid LPL = lipoprotein lipase FABP = fatty acid binding protein A C S FABP FA [3] FABP acyl-CoA [4] CYTOPLASM CAPILLARY FA albumin FA from fat cell FA [1] acetyl-CoA TCA cycle  -oxidation [6] [7] carnitine transporter acyl-CoA [5] Figure 2. Overview of fatty acid degradation ACS = acyl CoA synthetase L P L Lipoproteins (Chylomicrons or VLDL) [2]

Figure 3 (top). Activation of palmitate to palmitoyl CoA (step 4, Fig. 2) and conversion to palmitoyl carnitine Intermembrane Space OUTER MITOCHONDRIAL MEMBRANE palmitoyl-carnitine CoA palmitoyl-CoA carnitine Cytoplasm palmitoyl-CoA AMP + PP i ATP + CoA palmitate CPT-I [2] ACS [1]

Figure 3 (bottom). Mitochondrial uptake via of palmitoyl- carnitine via the carnitine-acylcarnitine translocase (CAT) (step 5 in Fig. 2). Matrix INNER MITOCHONDRIAL MEMBRANE Intermembrane Space palmitoyl-carnitine carnitine CoA palmitoyl-CoA CAT [3] palmitoyl-carnitine CPT-II carnitine CoApalmitoyl-CoA [4] CPT-I

CAT Intermembrane Space OUTER MITOCHONDRIAL MEMBRANE palmitoyl-carnitine CoA carnitine Cytoplasm palmitoyl-CoA AMP + PP i ATP + CoA palmitate palmitoyl-CoA Matrix INNER MITOCHONDRIAL MEMBRANE [3] palmitoyl-carnitinecarnitine CoApalmitoyl-CoA [4] CPT-I [2] ACS [1] CPT-II

Figure 4. Processing and  -oxidation of palmitoyl CoA matrix side inner mitochondrial membrane 2 ATP 3 ATP respiratory chain recycle 6 times Carnitine translocase Palmitoylcarnitine Palmitoyl-CoA + Acetyl CoA CH 3 -(CH) 12 -C-S-CoA O oxidation FAD FADH 2 hydration H2OH2O cleavage CoA oxidation NAD + NADH Citric acid cycle 2 CO 2

propionyl CoA carboxylase: (biotin-dependent) propionyl CoA + ATP + CO 2  methylmalonyl CoA + AMP + PP i methylmalonyl CoA mutase: (adenosyl cobalamin-dependent) methylmalonyl CoA  succinyl CoA Figure 5. Reactions in the metabolism of propionyl CoA derived from odd-chain fatty acids OXIDATION OF ODD-CHAIN FATTY ACIDS Final step of  -oxidation produces: propionyl CoA + acetyl CoA

Figure 6. Ketone body formation (ketogenesis) in liver mitochondria from excess acetyl CoA derived from the  - oxidation of fatty acids MITOCHONDRION (excess acetyl CoA) Hydroxymethylglutaryl CoA HMG-CoA synthase acetyl CoA CoA Acetoacetate HMG-CoA-lyase acetyl CoA  -Hydroxybutyrate dehydrogenase NAD + NADH Acetone (non-enzymatic) 2 Acetyl CoA Fatty acid  -oxidation Citric acid cycle oxidation to CO 2 Acetoacetyl CoA CoA Thiolase

high rates of lipolysis (e.g., long ‑ term starvation or in uncontrolled diabetes) produce sufficient ketones in the blood to be effective as a fuel ketones are the preferred fuel if glucose, ketones, fatty acids all available in the blood primary tissues: using ketones, when available, are brain, muscle, kidney and intestine, but NOT the liver.  -Hydroxybutyrate + NAD +  acetoacetate + NADH  -hydroxybutyrate dehydrogenase in mitochondria; reverse of ketogenesis KETONE BODY OXIDATION

KETOACIDOSIS Excessive build-up of ketone bodies results in ketosis eventually leading to a fall in blood pH due to the acidic ketone bodies. In diabetic patients the events that can lead to ketosis are: ñ Relative or absolute (most common cause) deficiency of insulin ñ Mobilization of free fatty acids (from adipose lipolysis)ñ Increased delivery of free fatty acids to the liver ñ Increased uptake and oxidation of free fatty acids by the liverñ Accelerated production of ketone bodies by the liver

X Adipose Tissue Free fatty acids Liver Ketone Bodies Insulin Pancreas Figure 7. Mechanism for prevention of ketosis due to excess ketone body production that can lead to ketoacidosis