Biochemistry-II - Lipid Biosynthesis - Quick Guides | Biochemistry

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Study GuideBiochemistry-II–Lipid Biosynthesis1.Cholesterol Biosynthesis and Its Control1.1Why Cholesterol Is ImportantCholesterol often gets a bad reputation, but it is actually anessential moleculein the human body.Without cholesterol, many normal body functions would not be possible.In animal cells, cholesterol makes upas much as half of the membrane lipids.Its main role in cell membranes is to:•Maintainproper membrane fluidity•Help stabilizeelectrical propertiesof the membraneCholesterol is especially abundant in thenervous system, where stable membranes are critical fornerve signal transmission.1.2Cholesterol as a Building BlockCholesterol is not only important on its own—it also acts as aprecursorfor several vital molecules.Some important cholesterol-derived compounds include:Bile Acids•Help withdigestion and absorption of fats•Essential for absorbingfat-soluble vitaminsSteroid HormonesAll steroid hormones are made from cholesterol, including:•Adrenal hormones, which regulate salt and water balance•Male and female sex hormones, which control reproduction and development

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Study GuideVitamin D•Produced from cholesterol•Plays a key role incalcium regulation and bone healthBecause of these roles, humanscannot survivewithout cholesterol and its derivatives.1.3The Downside of CholesterolAlthough cholesterol is essential, excess cholesterol can cause problems.Cholesterol and its derivatives are:•Poorly soluble in water•Prone to forming deposits in blood vesselsDoctors often find cholesterol-rich deposits calledplaquesin diseased arteries.These plaques:•Narrow blood vessels•Reduce blood flow•Increase the risk ofheart disease and strokeKey Takeaway•Cholesterol isvital for life•It is essential forcell membranes, especially in nerve cells•It is the starting material forbile acids, steroid hormones, and vitamin D•Too much cholesterol can contribute toarterial plaque formationUnderstanding cholesterol biosynthesis and its regulation helps explainboth its benefits and itsrisks.

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Study Guide2.Isoprenoid Compounds2.1What Are Isoprenoid Compounds?Isoprenoid compoundsare a large and important group of biomolecules.They includecholesterolin animals and many natural products from plants, such asrubber.All isoprenoid compounds are built from a basic5-carbon unit, called theisoprenoid unit.By linking many of these units together, the body can form large and complex molecules likecholesterol.2.2The Building Block: Isoprenoid UnitThe isoprenoid unit containsfive carbon atomsarranged in a characteristic pattern.This small unit is repeated many times to build larger molecules.2.3Where Cholesterol Synthesis BeginsIn humans,cholesterol is synthesized in the liver.The starting material for cholesterol synthesis isacetyl-CoA, a key molecule produced duringmetabolism, especially from:•Lipid (fatty acid) degradation•Other energy-producing pathways2.4Common Intermediate: Mevalonic AcidAll isoprenoid compounds are synthesized through acommon intermediatecalledmevalonic acid(mevalonate).

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Study GuideThe pathway can be summarized as:acetyl-CoA → mevalonate → isoprenoid units → cholesterol and other compoundsMevalonate acts as thedirect precursorof isoprenoid units.2.5Formation of Acetyl-CoAAcetyl-CoA can be formed:•Directly from metabolism•Or by theacetate thiokinase reaction, which converts acetate into acetyl-CoA using ATPand Coenzyme AThis ensures a steady supply of acetyl-CoA for biosynthetic pathways.2.6Condensation of Acetyl-CoA MoleculesThe first major step in the pathway is thecondensation of two acetyl-CoA molecules.•This reaction producesacetoacetyl-CoA•It is similar to early steps seen inketone body formationFormation of HMG-CoA

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Study GuideNext, a third acetyl-CoA molecule is added to acetoacetyl-CoA by the enzyme3-hydroxy-3-methylglutaryl-CoA synthase (HMG-CoA synthase).This reaction formsHMG-CoA, an important branching point in metabolism.Role of HMG-CoA and Key EnzymeHMG-CoA plays a central role because:•It is used inketone body formation•It is also the starting point forcholesterol synthesisThekey regulatory enzymein isoprenoid and cholesterol synthesis isHMG-CoA reductase, which converts HMG-CoA intomevalonic acid.Because of its importance, HMG-CoA reductase is a major target ofcholesterol-lowering drugs(statins).Key Takeaway•Isoprenoid compounds includecholesteroland many plant products•All isoprenoids are built from a5-carbon unit•Acetyl-CoAis the starting molecule•Mevalonateis the common intermediate•HMG-CoA synthaseandHMG-CoA reductaseare key enzymes•Early steps are shared withketone body metabolism3.HMG-CoA Reductase3.1Why HMG-CoA Reductase Is ImportantHMG-CoA reductaseis themost important enzymein cholesterol biosynthesis.

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Study GuideIt controls thecommitted stepof the pathway.Acommitted stepmeans that:•Once this reaction occurs, the pathway canonlylead tocholesterol synthesis•No other major metabolic products can be formed from the intermediateBecause of this, HMG-CoA reductase is also themain regulatory pointof cholesterol production inthe body.3.2What Reaction Does HMG-CoA Reductase Catalyze?HMG-CoA reductase convertsHMG-CoAintomevalonate.This reaction:•RemovesCoenzyme A (CoA-SH)•Converts athiol-bound carboxyl groupinto afree alcohol•Marks the first step that is unique to cholesterol biosynthesis3.3A Two-Step Reduction ProcessThe reduction of HMG-CoA to mevalonate occurs intwo steps:1.The thioester group of HMG-CoA is reduced to analdehyde2.The aldehyde is further reduced to analcoholEach reduction step requires reducing power.

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Study Guide3.4Role of NADPHThis reaction usestwo molecules of NADPHas reducing agents.•NADPH provides thereducing equivalents(electrons)•Two NADPH molecules are required because:oThe molecule must be reducedtwiceoFirst to an aldehyde, then to an alcoholAfter the reaction:•NADPH is converted toNADP⁺•Coenzyme A is released and recycled3.5Why This Step Is Tightly RegulatedBecause HMG-CoA reductase controls the committed step:•Its activity directly determines how much cholesterol is made•Cells carefully regulate this enzyme to maintain cholesterol balanceThis is why many cholesterol-lowering drugs, such asstatins, work byinhibiting HMG-CoAreductase.Key Takeaway•HMG-CoA reductase catalyzes thecommitted stepof cholesterol biosynthesis•It convertsHMG-CoA → mevalonate•The reaction releasesCoA-SH•TwoNADPHmolecules are required•The reduction occurs intwo stages(aldehyde → alcohol)•This enzyme is themain control pointof cholesterol synthesis

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Study Guide4.Mevalonate Squalene4.1Overview: From Mevalonate to SqualeneIn cholesterol biosynthesis,mevalonateis converted into a large30-carbon molecule calledsqualene.This process involves:•Phosphorylation•Decarboxylation•Isomerization•Condensation reactionsThese steps gradually build small 5-carbon units into much larger molecules that eventually formcholesterol.Step 1: Phosphorylation of MevalonateMevalonate containsalcohol (–OH) groups, which must be activated before further reactions canoccur.•First, one hydroxyl group of mevalonate accepts aphosphate from ATP•A second phosphorylation also occurs, again usingATP as the phosphate donorThis produces:•Phosphomevalonate•ThenpyrophosphomevalonateThese phosphorylation steps prepare the molecule for the next major transformation.

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Study GuideStep 2: Decarboxylation to Form IPPgeranyl and farnesyl pyrophosphates → squaleneThe multiply phosphorylated mevalonate now undergoes anenzyme-catalyzed rearrangement.During this reaction:•Carbon dioxide (CO₂)is released•Aphosphate groupis removed•A5-carbon compoundis formedThe product isisopentenyl pyrophosphate (IPP).IPP is a highly reactive molecule and serves as abasic building block for isoprenoid compounds.Step 3: Interconversion of IPP and DMAPPIPP is rapidly converted back and forth with another 5-carbon compound calleddimethylallyl pyrophosphate (DMAPP).•This conversion is carried out by anisomerase enzyme•IPP and DMAPP are chemically different but function together in synthesis reactionsBoth molecules are essential for building larger isoprenoid structures.

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