Voet Biochemistry 3e John Wiley & Sons, Inc.

Transcription

1 * * Voet Biochemistry 3e

2 Lipid Metabolism Part I: (Chap. 25, sec.1-3) Glucose C 6 H 12 O O 2 6 CO H 2 O G o = kj/mol Fats (palmitic acid) C 16 H 32 O O 2 16 CO H 2 O G o = kj/mol 1. Digestion and Absorption Voet Biochemistry 3e 2. Storage and Mobilization

3 ~4 Cal ~9 Cal ~4 Cal Table 25-1 Energy Content of Food Constituents. Voet Biochemistry 3e Page 910

4 Table 27-1 Fuel Reserves for a Normal 70-kg Man. Voet Biochemistry 3e Page 1065

5 Lipids can be ingested, released from storage, or synthesized (liver). Voet Biochemistry 3e

6 Lipid Uptake Voet Biochemistry 3e Page 929

7 Figure 25-1 Mechanism of interfacial activation of triacylglycerol lipase in complex with colipase. Voet Biochemistry 3e Page 910 Pancreatic (origin) lipase acts on mixed micelles (substrate). Lipid interaction exposes the active site to allow ester hydrolysis of triacylglycerides, generating free fatty acids.

8 Substrate here is phospholipid, not triacyl-glyceride. A2 Fig Catalytic action of phospholipase A 2. Voet Biochemistry 3e Page 911

9 Voet Biochemistry 3e Page 911 Figure 25-3a Substrate binding to phospholipase A 2. (a) A hypothetical model of phospholipase A 2 in complex with a micelle of lysophosphatidylethanolamine.

10 D99 H48 Figure 25-4a (a) The X-ray structure of the 124-residue monomeric porcine phospholipase A 2 (lavender) in complex with the tetrahedral intermediate mimic MJ33. (b) The catalytic mechanism of phospholipase A 2. Voet Biochemistry 3e Page 912 Mechanism resembles catalytic triad of serine proteases, with H 2 O playing the role of Ser. There is no covalent intermediate.

11 Figure 25-5 X-Ray structure of rat intestinal fatty acid binding protein. Voet Biochemistry 3e Page 913 This cytoplasmic protein helps carry fatty acids through intestinal cells, where they are assembled into chylomicrons. The fatty acid carboxylate ion pairs with Arg.

12 Glucagon, which signals a need to increase blood glucose, also calls for fatty acid release. It really signals we need more energy rich molecules. Glycerol-3-P Voet Biochemistry 3e Page 929 DHAP Glycolysis

13 7 1 Voet Biochemistry 3e Page Solubility of free fatty acids ~ 10-6 M In serum complex with albumin ~ 2 mm Figure 25-7 X-Ray structure of human serum albumin in complex with 7 molecules of palmitic acid. 2

14 Lipid Metabolism Part I: (Chap. 25, sec.1-3) Glucose C 6 H 12 O O 2 6 CO H 2 O G o = kj/mol Fats (palmitic acid) C 16 H 32 O O 2 16 CO H 2 O G o = kj/mol Triglycerides are broken down to glycerol and free fatty acids. 1. Metabolism of Glycerol Voet Biochemistry 3e 2. Fatty Acid Oxidation (Knoop 1904)

15 Glycerol part: ( -ATP + NADH ) / DHAP glycolysis Voet Biochemistry 3e Page 913 This looks familiar! Figure 25-6 Conversion of glycerol to the glycolytic intermediate dihydroxyacetone phosphate.

16 Step 1: Activation (cytosol) Figure 25-9 Mechanism of fatty acid activation catalyzed by acyl-coa synthetase. (many vary by FA length) Acyl~AMP Voet Biochemistry 3e Page 915 CoA derivative (fatty acyl CoA)

17 Step 2: Transport Transesterification rxn Keq ~ 1 Voet Biochemistry 3e Page 915 Figure Acylation of carnitine catalyzed by carnitine palmitoyltransferase.

18 Step 2: Transport Voet Biochemistry 3e Page 916 Transferase is inhibited by malonyl-coa Figure Transport of fatty acids into the mitochondrion.

19 The Beta oxidation pathway. Note the resemblance of first 3 steps to TCA steps: Succ fum -mal OAA Voet Biochemistry 3e β-hydroxycodh is inhibited by NADH

20 Electron transfer flavoprotein β- Oxidation - more dehydrogenase Humans have 4 different DHs for different FA chain length. MCAD deficiency correlated with SIDS. hydratase There are 3 of these, for short, medium, and long chain fatty acids dehydrogenase Voet Biochemistry 3e Page 917 (spiral) thiolase (human mito β-oxid: Rxn steps 2,3,4 for long chain lipids is carried out by a multifunctional protein)

21 Voet Biochemistry 3e Page 917 Figure Ribbon diagram of the active site region in a subunit of medium-chain acyl-coa dehydrogenase from pig liver mitochondria in complex with octanoyl-coa.

22 Voet Biochemistry 3e Page 929

23 Regulation of β-oxidation Fatty acids are a crucial energy reserve and must be administered carefully. Carnitine transferase is inhibited by malonyl- CoA, the feed stock for fatty acid synthesis. Once a fatty acid is in the mitochondria, it is burned up. Voet Biochemistry 3e β-hydroxyacyl-coa dehydrogenase is inhibited by NADH, slowing catabolism when energy levels are high. Thiolase in inhibited by high concentrations of AcCoA

24 5. Other fatty acid oxidation pathways: Voet Biochemistry 3e

25 (ω-6) Fig Structures of two common unsaturated fatty acids. Voet Biochemistry 3e Page 919

26 β isomerase Problem #1: cis 3 vs. trans 2 Voet Biochemistry 3e Page 929

27 β-oxidation of poly unsaturated fatty acids: Linoleic acid Again, specialty enzymes are brought in to keep intermediates within the basic β- oxidation pathway. Voet Biochemistry 3e 3,2- enol-coa isomerase

28 Odd # C FA Figure Conversion of propionyl-coa to succinyl-coa. biotin Voet Biochemistry 3e Page 922 This rather convoluted path allows C-C bond formation at a C which is α to a carbonyl. You can t just stick it on C3 of propionyl CoA, because there is no way to stabilize the carbanion character needed for the attack. Ahh. This looks familiar B12

29 Figure The propionyl-coa carboxylase reaction. Voet Biochemistry 3e Page 922

30 Figure The rearrangement catalyzed by methylmalonyl-coa mutase. (only a couple of B12 dependent enzymes in mammals). Voet Biochemistry 3e Page 923

31 Only certain bacteria can synthesize B12; neither plants nor animals can do it. We get most of our B12 from meat (which got it from gut bacteria). A deficiency leads to pernicious anemia 5,6-dimethylbenzimidazole Voet Biochemistry 3e Page 923 Figure Structure of 5 - deoxyadenosylcobalamin (coenzyme B 12 ).

32 Facile homolytic cleavage of Co-C bond is key to this cofactor s utility. Voet Biochemistry 3e Page 926 Figure Proposed mechanism of methylmalonyl-coa mutase.

33 Figure 25-22a X-Ray structure of P. shermanii methylmalonyl- CoA mutase in complex with 2-carboxypropyl-CoA and AdoCbl. (a) The catalytically active α subunit. (b) The arrangement of AdoCbl and 2-carboxypropyl-CoA molecules. Voet Biochemistry 3e Page 925

34 β-oxidation in peroxisomes. First DH passes electrons from flavin to O 2, producing heat. In animals: Long Chain FA are used. Import does not require carnitine. Shortened FAs transfer to mitochondia. Voet Biochemistry 3e Page 929 In yeast and plants: all β-ox is done in Peroxisomes & Glyoxysomes

35 Lipid Metabolism 8. Ketone Bodies Voet Biochemistry 3e The ketone bodies are a form of soluble lipid ; they reach fairly high blood concentrations, 0.3 mm. The ketone bodies are readily converted to AcCoA and drop straight into the TCA cycle. They are used by heart and kidney, and the brain can adapt to use ketone bodies after several days of starvation.

36 . Note: We will see these first 2 steps later in steroid biosynthesis. Ketone bodies Voet Biochemistry 3e Page 929

37 Figure The metabolic conversion of ketone bodies to acetyl-coa. Voet Biochemistry 3e Page ketoacyl-CoA transferase is an alternative to the action of succinylcoa synthetase in TCA. Each ketone body activation therefore costs a GTP. The liver lacks the transferase since it only wants to make ketone bodies for other tissues.

38 Conditions that accelerate gluconeogenesis, like starvation or diabetes, drain TCA intermediates and slow that cycle. Malonyl CoA is not formed and so carnitine transferase shuttles fatty acids through β-ox. AcCoA accumulates and ketone bodies pour out of the liver. They are acidic and lead to acidosis and ketosis. Voet Biochemistry 3e Page 929 Recall, in mammals there can be no NET synthesis of glucose for fatty acids.