BMB 462 Lecture 12 Outline of Last Lecture I Elongation and Desaturation of fatty acids II Mixed function oxidases III Sources of Glycerol 3 phosphate IV Synthesis of Phosphatidic Acid and Triacylglycerol V Strategy for membrane lipid synthesis VI Phosphatidylethanolamine and Phosphatidylcholine Outline of Current Lecture I Overview of Cholesterol Synthesis II Mevalonate pathway for IPP synthesis III Regulation of Cholesterol Synthesis IV Lipoprotein Transport of Cholesterol Lipids V Apolipoprotein Function Current Lecture Concepts to remembers from previous courses lectures Volume increases faster than Surface Area which could lead to cell lysis I Overview of Cholesterol Synthesis a All Carbons for cholesterol comes from the acetate in acetyl CoA the activated form of acetate need the activation to drive synthesis via the intermediate isoprene i The cell uses an activated form of isoprene just like acetate that s going to allow us to do condensations by providing E ii Isoprene 5C building block Have very typical repeated 5C structure with branch BE ABLE TO RECOGNIZE Make cholesterol and other isoprenoids b Cholesterol has steroid nucleus 4 fused rings first 3 have 6 C and 4th has 5 C i Attached to nucleus are a couple methyl groups Also have hydroxyl makes cholesterol amphipathic so it can go into membranes And have acyl saturated Carbon tail These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute c The 4 Stages of Biosynthesis i Condense 3 Acetyl CoAs to make mevalonate 1 Stage 1 Have three 2C units 3 acetyl CoAs that give you 6C mevalonate Have CoA linked intermediates ii Activation of Mevalonate to IPP and DMP 1 Stage 2 IPP and DMP are activated isoprenes These are needed in next reaction for condensation 2 Lose 1 Carbon from 6 C for phosphorylated 5C intermediate Source of Pi is ATP iii Condense IPPs and DMPs 5Cs to Squalene 30C 1 Stage 3 start sticking together 5 C activated isoprenes The cell adds a 5C and 5C to make 10C then add another 5C to make 15C a Then take 2 15Cs to make 30C linear chain squalene 2 The Energy for these is from releasing pyrophosphate which then becomes 2 phosphates and that drives the condensations generally putting things together is thermodynamically unfavourable so need driving force iv Cyclization to a 4 ring system 1 It takes 15 NAD P H and 22 reactions to achieve synthesis of cholesterol 2 The cell cyclizes the linear chain and loses 3 carbons in the process 27 C in cholesterol 3 If you re using NADPH NADH there will be oxidation reduction reactions II Mevalonate pathway for IPP synthesis a IPP synthesis occurs in the cytosol of liver cells b When compared to ketone body synthesis i The first 2 reactions are essentially identical ii Thiolase in beta oxidation in reverse in ketone body synth The reaction is running in same direction here as Ketone synthesis iii Uses CoA to add Cs to make HMG COA iv The fate of HMG CoA is to get reduced by HMG CoA reductase which uses 2 NADPH e go to Carbon in substrate v Mevalonate ends stage 1 vi Start stage 2 Use ATP to phosphorylate intermediate The kinase phosphotransferase Also carboxylase adding Pi causes loss of 1 C to make the 5 C intermediate i Have pyrophosphate attached to 5 C intermediate which is how you get activated isoprene III Regulation of Cholesterol Synthesis a Regulation of cholesterol occurs on local within cell and global within organism levels b Most of the regulation is at stage 1 it is typical to regulate at beginning of pathways i Regulatory enzyme is HMG CoA reductase c Regulation of Gene Expression i Via Sterol Regulatory Element Binding Protein SREBP 1 Bound to DNA and turns on genes that make proteins for cholesterol synthesis 2 Sits in ER membrane and only released when cleaved It is released from membranes and moves to nucleus then turns on genes 3 Cholesterol levels regulate low cholesterol levels releases SREBP ii SREBP Cleavage Activating Protein SCAP 1 SCAP binds to cholesterol so if cholesterol is present it s going to prevent SCAP from cleaving SREBP 2 Decrease in cholesterol increase in SREBP cleavage iii Via Hormonal regulation 1 Insulin and glucagon regulate on an organismal level 2 Insulin tells you blood sugar is high so tells you to make cholesterol 3 Insulin dephosphorylates HMG CoA reductase to activate it 4 Glucagon says there s low blood sugar Phosphorylates HMG CoA reductase and deactivates it iv Via Feedback 1 Feedback in a particular cell 2 High cholesterol breakdown of HMG CoA reductase 3 Since that s the key regulatory enzyme so when there s less of it less synthesis Not your typical allosteric feedback inhibition though because fully getting rid of enzyme 4 Increased cholesterol also stops LDL endocytosis stop bringing in cholesterol as LDL lipoprotein 5 Also activates ACAT which makes cholesterol esters for storage v Via Atherosclerosis and Statins 1 2 sources of cholesterol those you eat and those you make Statins help inhibit making cholesterol IV Lipoprotein Transport of Cholesterol Lipids a Esterification of Cholesterol The cholesterol needs to be completely hydrophobic the cell does that by esterifying it attaching to fatty acid via ester bond b Essentially connection to fatty acid occurs when you take an OH group and Fatty Acid and bind them together in esterification 2 enzymes are needed that do this i ACAT Acetyl CoA acetyltransferase 1 Fatty Acids come from Acyl CoA Used for formation of chylomicrons 2 Function Formation of cholesterol for export and storage in cells ii LCAT lecithin cholesterol acyltransferase basically phosphatidylcholine 1 Sometimes used as emulsifier because it s amphipathic 2 Donates a Fatty Acid from C2 and attaches it to cholesterol 3 Used for reverse transport by HDL Picks up cholesterol from out in excess tissue and brings it to liver 4 Can t just break it down and oxidize it for E have to turn it into bile acids bile salts 5 Some might be used to generate other molecules but it s mostly just hard to get rid of and will end up back in tissue blood Which is why people take statins eat too much cholesterol so controls it c Major Classes of lipoproteins are classified by physical Characteristics i Density Surface Area size and ratio Surface area volume 1 Ratio decreases w size 2 Classify lipoproteins based on density and size Chylomicrons are least dense and largest HDL are smallest and the most dense ii Chylomicron have most
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