MCB Final Exam

*A hydrophobic, planar molecule
*27-30 carbons
*steroid nucleus
-3 six-membered rings, 1 five membered right (makes hydrophobic and planar)
*hydroxyl group hanging off 6-membered rings
-affects membrane fluidity in mammalian cells

*Acetyl CoA: hydrolysis of high energy bonds
-chosterol synthesis if very energetically demanding
-can only be synthesized in fed state (stimulate glycolysis)
*ATP: formation of activated isoprene units
-3 ATP to form a SINGLE 5-carbon isoprene unit
-link isopene units together until 30-carbons is achieved
*NADPH: pentose phosphate pathway

*begins in cytosol with synthesis of mevalonate from acetyl-CoA (combine two acetyl Co-A together, split off a Co-A
- synthesis primarily in liver
- some levels of synthesis in tissues that make steroid hormones, but not enough to meet demands
1. Thiolase
-catalyzes fusion of first 2 acetyl CoA
2. HMG-CoA synthase makes HMG-CoA
-addition of a third acetyl CoA
3. HMG-CoA reductase - is rate-limiting step in cholesterol biosynthesis
-located in E.R, but catalytic site faces cytoplasm
**production of mevalonate is COMMITTING, once formed it won't go anywhere but towards cholesterol synthesis

*site of action in cholesterol-lowering drugs
-body makes much more cholesterol compared to amount taken in from the diet
* HMG-CoA reductase is key rate-limiting step in biosynthesis
*Lovastain (meyinolin) blocks HMG-CoA reductase and prevents synthesis of cholesterol
-very structurally similar to HMG-CoA
*Lovastatin is an (inactive) lactone
*In the body, lactone is hydrolyzed to meyinolinic acid, a competitive inhibitor of the reductase, k1= .6 nM (50% inhibitory concentration)

*thiolase and HMG CoA reactions are shared reactions with production of ketone bodies
*HMG-CoA in absence of insulin can go towards synthesis of ketone bodies

* Mevalonate ---> 5-phospho-mevalonate ---> 5-pyrophospho-mevalonate ---> 3-ispoentyl pyrphosphate
-activation step
- ATP used in 3 reactions
-CO2 leaves in last step
*CoQ in ETC also needs isoprene units for synthesis (part of its hydrocarbon tail)
-may explain why people that take cholesterol lowering drugs, or statins, also take CoQ as a supplement to combat muscle weakness

1. specificity
-want to target an enzyme without broad consequences
2. figure out a minimum effective dose
- lower dose = reduce chances of side effects

1.isopentenyl pyrophosphate <---> dimethylallyl pyrophosphate
-need both of these forms for first step
2. Condense these two together to create geranyl pyrophosphate
-from 5 carbons to 10 carbons in this reaction
-release of pyrophosphate and its cleavage drives reaction (cleavage of a pyrophosphate is very exergonic)
3. use isopentenyl phosphate and attach it to geranyl pyrophosphate
-creates 15-C farnesyl pyrophosphate
4. condense two farnesyl pyrophosphate
- forms 30-C Squalene
-cleavage of PPi and 1 NADPH is used to drive reaction
-ring structure starts to develop

*occur in smooth ER
*family of enzymes called cytochrome P450 reductase
-involved in drug and environmental detox, and biosynthesis
-need NADPH reducing equivalents AND OXYGEN
-b/c oxygen is used, becomes a site for free radical formation
* over 20 reactions (19 reactions from lenosterol to cholesterol)

*as rate-limiting step, it is the principle site of regulation in cholesterol synthesis
-at level of HMG-CoA R, cholesterol regulates its own biosynthesis
1. Phosphorylation by AMP-activated kinase inactivates
- covalent modification
2. Degradation of HMG-CoA reductase: half-life is 3 hours and depends on cholesterol level
-regulation of amount
3. Gene expression (mRNA production) is controlled by cholesterol levels
-regulation of amount

*SREDP (sterol response element binding protein)
- as long as cholesterol is available (presence in cell), it binds to SCAP which is in association with SREDP and gene isn't turned on
- SCAP cleaves SREDP and releases its domain that binds onto DNA when cholesterol levels drop
-SREDP goes to nucleus and binds to sterol reponse element (special nucleotide sequence) and turns on gene expression for HMG-CoA reductase
-domain will eventually degrade once cholesterol rises again

*may cause a defect in feedback control of HMG-CoA reductase synthesis
-making too much of it
*cholesterol triggers degradation of HMG-CoA Reductase
-defect in degradation by sterols
*Glucagon and sterols will trigger activation of AMP-dependant protein kinase
-will phosphorylate HMG-CoA reductase, making it less active
-same kinase regulating FA synthesis

* LDL's are main carriers of cholesterol and cholesterol esters
-not made directly, but evolves from VLDL
-represents circulating cholesterol (delivers to tissues that cant make enough for themselves)
*relative amounts of HDL and LDL affect disposition of cholesterol and formation of arterial plaques

*production of steroid hormones
*production of bile
- a detergent to help absorb dietary fat
*steroid nucleus to make vitamin D
*problem rises when body doesn't sense LDL circulating in blood

*Cholesterol
-females = 157-167y
-males = 150-174
-healthy levels are under 200
*HDL
-females = 52-55, males 45
*LDL
-females= 100-106, males 97-116
*ratio of HDL to LDL is important, not absolute number
-LDL levels fluctuate a lot, HDL remains relatively constant