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Biochemistry-II - Lipid Biosynthesis

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    Study Guide Biochemistry - II – Lipid Biosynthesis 1. Cholesterol Biosynthesis and Its Control 1.1 Why Cholesterol Is Important Cholesterol often gets a bad reputation, but it is actually an essential molecule in the human body. Without cholesterol, many normal body functions would not be possible. In animal cells, cholesterol makes up as much as half of the membrane lipids . Its main role in cell membranes is to: • Maintain proper membrane fluidity • Help stabilize electrical properties of the membrane Cholesterol is especially abundant in the nervous system , where stable membranes are critical for nerve signal transmission. 1.2 Cholesterol as a Building Block Cholesterol is not only important on its own — it also acts as a precursor for several vital molecules. Some important cholesterol - derived compounds include: Bile Acids • Help with digestion and absorption of fats • Essential for absorbing fat - soluble vitamins Steroid Hormones All 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 Guide Vitamin D • Produced from cholesterol • Plays a key role in calcium regulation and bone health Because of these roles, humans cannot survive without cholesterol and its derivatives. 1.3 The Downside of Cholesterol Although cholesterol is essential, excess cholesterol can cause problems. Cholesterol and its derivatives are: • Poorly soluble in water • Prone to forming deposits in blood vessels Doctors often find cholesterol - rich deposits called plaques in diseased arteries. These plaques: • Narrow blood vessels • Reduce blood flow • Increase the risk of heart disease and stroke Key Takeaway • Cholesterol is vital for life • It is essential for cell membranes , especially in nerve cells • It is the starting material for bile acids, steroid hormones, and vitamin D • Too much cholesterol can contribute to arterial plaque formation Understanding cholesterol biosynthesis and its regulation helps explain both its benefits and its risks .

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    Study Guide 2. Isoprenoid Compounds 2.1 What Are Isoprenoid Compounds? Isoprenoid compounds are a large and important group of biomolecules. They include cholesterol in animals and many natural products from plants, such as rubber . All isoprenoid compounds are built from a basic 5 - carbon unit , called the isoprenoid unit . By linking many of these units together, the body can form large and complex molecules like cholesterol. 2.2 The Building Block: Isoprenoid Unit The isoprenoid unit contains five carbon atoms arranged in a characteristic pattern. This small unit is repeated many times to build larger molecules. 2.3 Where Cholesterol Synthesis Begins In humans, cholesterol is synthesized in the liver . The starting material for cholesterol synthesis is acetyl - CoA , a key molecule produced during metabolism, especially from: • Lipid (fatty acid) degradation • Other energy - producing pathways 2.4 Common Intermediate: Mevalonic Acid All isoprenoid compounds are synthesized through a common intermediate called mevalonic acid (mevalonate) .

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    Study Guide The pathway can be summarized as: acetyl - CoA → mevalonate → isoprenoid units → cholesterol and other compounds Mevalonate acts as the direct precursor of isoprenoid units. 2.5 Formation of Acetyl - CoA Acetyl - CoA can be formed: • Directly from metabolism • Or by the acetate thiokinase reaction , which converts acetate into acetyl - CoA using ATP and Coenzyme A This ensures a steady supply of acetyl - CoA for biosynthetic pathways. 2.6 Condensation of Acetyl - CoA Molecules The first major step in the pathway is the condensation of two acetyl - CoA molecules . • This reaction produces acetoacetyl - CoA • It is similar to early steps seen in ketone body formation Formation of HMG - CoA

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    Study Guide Next, a third acetyl - CoA molecule is added to acetoacetyl - CoA by the enzyme 3 - hydroxy - 3 - methylglutaryl - CoA synthase (HMG - CoA synthase) . This reaction forms HMG - CoA , an important branching point in metabolism. Role of HMG - CoA and Key Enzyme HMG - CoA plays a central role because: • It is used in ketone body formation • It is also the starting point for cholesterol synthesis The key regulatory enzyme in isoprenoid and cholesterol synthesis is HMG - CoA reductase , which converts HMG - CoA into mevalonic acid . Because of its importance, HMG - CoA reductase is a major target of cholesterol - lowering drugs (statins) . Key Takeaway • Isoprenoid compounds include cholesterol and many plant products • All isoprenoids are built from a 5 - carbon unit • Acetyl - CoA is the starting molecule • Mevalonate is the common intermediate • HMG - CoA synthase and HMG - CoA reductase are key enzymes • Early steps are shared with ketone body metabolism 3. HMG - CoA Reductase 3.1 Why HMG - CoA Reductase Is Important HMG - CoA reductase is the most important enzyme in cholesterol biosynthesis.

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    Study Guide It controls the committed step of the pathway. A committed step means that: • Once this reaction occurs, the pathway can only lead to cholesterol synthesis • No other major metabolic products can be formed from the intermediate Because of this, HMG - CoA reductase is also the main regulatory point of cholesterol production in the body. 3.2 What Reaction Does HMG - CoA Reductase Catalyze? HMG - CoA reductase converts HMG - CoA into mevalonate . This reaction: • Removes Coenzyme A (CoA - SH) • Converts a thiol - bound carboxyl group into a free alcohol • Marks the first step that is unique to cholesterol biosynthesis 3.3 A Two - Step Reduction Process The reduction of HMG - CoA to mevalonate occurs in two steps : 1. The thioester group of HMG - CoA is reduced to an aldehyde 2. The aldehyde is further reduced to an alcohol Each reduction step requires reducing power.

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    Study Guide 3.4 Role of NADPH This reaction uses two molecules of NADPH as reducing agents. • NADPH provides the reducing equivalents (electrons) • Two NADPH molecules are required because: o The molecule must be reduced twice o First to an aldehyde, then to an alcohol After the reaction: • NADPH is converted to NADP ⁺ • Coenzyme A is released and recycled 3.5 Why This Step Is Tightly Regulated Because HMG - CoA reductase controls the committed step: • Its activity directly determines how much cholesterol is made • Cells carefully regulate this enzyme to maintain cholesterol balance This is why many cholesterol - lowering drugs, such as statins , work by inhibiting HMG - CoA reductase . Key Takeaway • HMG - CoA reductase catalyzes the committed step of cholesterol biosynthesis • It converts HMG - CoA → mevalonate • The reaction releases CoA - SH • Two NADPH molecules are required • The reduction occurs in two stages (aldehyde → alcohol) • This enzyme is the main control point of cholesterol synthesis

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    Study Guide 4. Mevalonate Squalene 4.1 Overview: From Mevalonate to Squalene In cholesterol biosynthesis, mevalonate is converted into a large 30 - carbon molecule called squalene . This process involves: • Phosphorylation • Decarboxylation • Isomerization • Condensation reactions These steps gradually build small 5 - carbon units into much larger molecules that eventually form cholesterol. Step 1: Phosphorylation of Mevalonate Mevalonate contains alcohol ( – OH) groups , which must be activated before further reactions can occur. • First, one hydroxyl group of mevalonate accepts a phosphate from ATP • A second phosphorylation also occurs, again using ATP as the phosphate donor This produces: • Phosphomevalonate • Then pyrophosphomevalonate These phosphorylation steps prepare the molecule for the next major transformation.

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    Study Guide Step 2: Decarboxylation to Form IPP geranyl and farnesyl pyrophosphates → squalene The multiply phosphorylated mevalonate now undergoes an enzyme - catalyzed rearrangement . During this reaction: • Carbon dioxide (CO ₂ ) is released • A phosphate group is removed • A 5 - carbon compound is formed The product is isopentenyl pyrophosphate (IPP) . IPP is a highly reactive molecule and serves as a basic building block for isoprenoid compounds . Step 3: Interconversion of IPP and DMAPP IPP is rapidly converted back and forth with another 5 - carbon compound called dimethylallyl pyrophosphate (DMAPP) . • This conversion is carried out by an isomerase enzyme • IPP and DMAPP are chemically different but function together in synthesis reactions Both molecules are essential for building larger isoprenoid structures.

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