BENG0004 2021 Lecture 16_beta_oxidation.pdf

BENG0004 Biochemistry and Molecular Biology Emily Kostas Lecture 16 Cells have to eat – Metabolism, the major catabolic pathways β-oxidation  How lipids are degraded – the b-oxid...

BENG0004 Biochemistry and Molecular Biology Emily Kostas Lecture 16 Cells have to eat – Metabolism, the major catabolic pathways β-oxidation  How lipids are degraded – the b-oxidation pathway Lipids are fats and fatty acids. Fatty acids are long chains of carbon and hydrogen and are hydrophobic with a mildly hydrophilic carboxylate group at one end. They come in different chain lengths in food. The major ones are C16, C18 and can have one or more double bonds which makes their melting temperature lower. Fats are when three fatty acids are esterified to a glycerol molecule and are called triacylglycerols A triacylglycerol (fat) Fatty acids Storage of Fat Fat storage cells or adipocytes are found all over the body. Most of the space in an adipocyte is given over to a large fat reservoir Fatty acids are stored in adipocytes, where they are reassembled into triacylglycerides by esterification onto glycerol.  Double bond creates a kink in the fatty acid Trans fatty acids are uncommon in natural fatty acids, they form readily when polyunsaturated fatty acids from plants are "partially hydrogenated" chemically. This is done commercially to make plant fatty acids more solid and to improve shelf-life. Epidemiological studies correlate consumption of trans fatty acids with increased risk of heart disease. The Roles of Fatty Acids Fatty acids have two main roles – as a food giving energy and carbon compounds and as components of the membranes that surround cells. Triacylglycerols are ways of storing fatty acids in cells. The fatty acids in membranes are esterified to glycerol, but only two fatty acid chains are present. The third OH group of the glycerol is esterified to a polar or charged group to make an amphipathic molecule Phospholipid: Phospholipids will spontaneously self assemble into sheets and spheres and vesicles when placed in water with salts and buffer. The type of fatty acid at the two positions on glycerol will determine the fluidity of the membrane and give the assembled phospholipid layer a curvature which makes it easy for a cell membrane to form. The plasma membrane (cytoplasmic membrane) of a cell Cholesterol is only present in eukaryotic membranes – not in bacteria. Integral membrane proteins are embedded within the hydrophobic lipid bilayer. Many of these are channels or pores to allow specific uptake of compounds such as sugars, amino acids, ions, fatty acids etc. Fats as fuels – how are they utilised to make energy? In eukaryotic (e.g. mammalian) cells fatty acids are metabolised in the mitochondria. In prokaryotic cells (bacteria) fatty acids are metabolised in the cytoplasm. In humans fats are digested in the gut and lipases convert the fats into fatty acids and glycerol. The fatty acids are taken up into the blood and transported around the body in lipoprotein vesicles. They are transported into cells by the fatty acid transporter, an integral membrane protein. A similar situation happens in bacteria where lipases on the outside of the cell and near the cytoplasmic membrane convert the triacylglycerol to free fatty acids and the fatty acids are transported across the membrane. In bacteria the fatty acid is converted to a CoA ester as it is being taken up into the cell. Fatty acid uptake in E. coli LCFA = long chain fatty acid Outer membrane of E. coli. The FadL protein is a transport protein in the outer membrane. FadD is a transporter that also uses ATP to join the fatty acid to Coenzyme A in a high energy thioester bond. So that inside the cell the fatty acid is always coupled to CoA. This activates the fatty acid ready for oxidation. Coenzyme A or CoA The thiol group is used to form thio esters to many different kinds of carboxylic acid containing compounds. The most common of these is Acetyl CoA which is the product of many reactions in metabolism e.g. pyruvate metabolism and fatty acid metabolism The thio ester bond in acetyl CoA and in a fatty acyl CoA is a high energy bond. The CoA also acts as a ‘handle’ for enzymes and carrier proteins to carry lipids around the cell. In fatty acid activation ATP is hydrolysed first to AMP, then the PP (pyrophosphate) is also hydrolysed giving a lot of driving force to make the fatty acyl CoA ester. Fatty acid uptake in mammalian cells In mammalian cells, fatty acid metabolism takes place in the Mitochondria. This means the fatty acyl CoA has to get across the two mitochondrial membranes. It is converted to a carnitine derivative at the outer face of the mitochondrial outer membrane and transported across both membranes as this carnitine derivative. Inside the matrix of the mitochondria the carnitine is removed and replaced with CoA to reform a Fatty acyl CoA  The b-oxidation cycle for even 1. The first dehydrogenase uses a FAD (Flavin Adenine numbered carbon chains: Dinucleotide) cofactor instead of NAD. This creates a double bond. 1. 2. Water is added and forms a hydroxyl at the beta position. 2. 3. A second dehydrogenase removes an electron and a proton forming a keto group at the beta position. 3. 4. A fresh CoA is used to attack the beta-keto group. This left cleaves the carbon chain to form one molecule of Acetyl 4. CoA and a new fatty acyl CoA chain that is 2 carbons shorter. The energy yield of b-oxidation If you had a C18 fatty acid: Activate to Acyl-CoA (ATP → AMP + PPi) -2ATP 8 cycles of b-oxidation 8 FADH2 +16 ATP 8 NADH +24 ATP 9 acetyl CoA put into TCA cycle 9x3=27 NADH +81 ATP 9 GTP +9 ATP 9 FADH2 +18 ATP Total gain = 146 ATP Compare this to 3 glucose (C6) 3x36=108 ATP Unlike glycolysis, there is no net ATP generation in the actual pathway. The products from b-oxidation are reduced cofactors NADH and FADH2 and acetyl CoA. These need to go through other pathways to yield their energy. But ultimately fatty acids yield more energy per molecule than glucose.  Beta oxidation of odd numbered carbon chains The succinyl CoA goes into the TCA cycle  b-oxidation of fatty acids with double bonds When the double bond is 2 carbons away from the CoA ester an isomerase moves it so that it is now the same as the trans enoyl CoA in the beta oxidation step. Biochemistry. 8th Edition. Berg, Tymoczko, Gatto Jr. and Stryer. W.H Freeman and Co Lipids and beta-oxidation Chapter 22 pages 643-646 and 648-656