BMB 462 Lecture 12 Outline of Last Lecture I. Elongation and Desaturation of fatty acidsII. Mixed function oxidasesIII. Sources of Glycerol 3-phosphateIV. Synthesis of Phosphatidic Acid and TriacylglycerolV. Strategy for membrane lipid synthesisVI. Phosphatidylethanolamine and PhosphatidylcholineOutline of Current Lecture I. Overview of Cholesterol SynthesisII. Mevalonate pathway for IPP synthesisIII. Regulation of Cholesterol SynthesisIV. Lipoprotein Transport of Cholesterol/LipidsV. Apolipoprotein FunctionCurrent LectureConcepts to remembers from previous courses/lectures:- Volume increases faster than Surface Area, which could lead to cell lysisI. Overview of Cholesterol Synthesisa. 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 isoprenei. The cell uses an activated form of isoprene, just like acetate, that's going to allow us to do condensations by providing Eii. Isoprene = 5C building block. Have very typical repeated 5C structure withbranch (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 Ci. 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 Biosynthesisi. Condense 3 Acetyl-CoAs to make mevalonate1. Stage 1. Have three 2C units (3 acetyl-CoAs) that give you 6C mevalonate. Have CoA linked intermediatesii. Activation of Mevalonate to IPP and DMP1. 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 ATPiii. 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 15Ca. 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 system1. It takes 15 NAD(P)H and 22 reactions to achieve synthesis of cholesterol2. 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 synthesisa. IPP synthesis occurs in the cytosol of liver cellsb. When compared to ketone body synthesis: i. The first 2 reactions are essentially identicalii. 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-COAiv. The fate of HMG-CoA is to get reduced by HMG-CoA reductase, which uses 2 NADPH. e- go to Carbon in substratev. Mevalonate ends stage 1vi. 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 Synthesisa. Regulation of cholesterol occurs on local (within cell) and global (within organism) levelsb. Most of the regulation is at stage 1; it is typical to regulate at beginning of pathways.i. Regulatory enzyme is HMG-CoA reductasec. Regulation of Gene Expressioni. 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 SREBPii. 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 cleavageiii. Via Hormonal regulation1. Insulin and glucagon regulate on an organismal level.2. Insulin tells you blood sugar is high, so tells you to make cholesterol3. Insulin dephosphorylates HMG-CoA reductase to activate it4. Glucagon says there's low blood sugar. Phosphorylates HMG-CoA reductase and deactivates it.iv. Via Feedback1. Feedback in a particular cell.2. High cholesterol = breakdown of HMG-CoA reductase3. 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 storagev. Via Atherosclerosis and Statins1. 2 sources of cholesterol: those you eat and those you make. Statins help inhibit making cholesterolIV. Lipoprotein Transport of Cholesterol/Lipidsa. 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 thisi. ACAT - Acetyl-CoA acetyltransferase1. Fatty Acids come from Acyl-CoA. Used for formation of chylomicrons 2. Function: Formation of cholesterol for export and storage in cellsii. LCAT - (lecithin-cholesterol acyltransferase) - basically phosphatidylcholine1. Sometimes used as emulsifier because it’s amphipathic.2. Donates a Fatty Acid from C2 and attaches it to cholesterol3. 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 : volume1. Ratio decreases w/ size.2.
View Full Document
Unlocking...