Lipids are essential biomolecules involved in various physiological processes, including energy storage, cell membrane structure and function, and signalling pathways. A good understanding of lipids is therefore important for understanding how the body functions at a molecular level.
Besides, Lipids are one of the most heavily tested topics in the NEET PG exam, and questions related to lipid metabolism and function carry significant weightage.
Read this blog further to get a quick overview of this high-yield biochemistry topic for
NEET PG/NExT exam preparations. In this blog we’ll cover classification of lipids, PUFAs, lipoproteins, ketone body synthesis and ketone body utilization.
Lipid: Any compound which is insoluble in water & soluble in non-polar organic solvent
Made of fatty acid and alcohol joined by ester bond formed by removal of H
CLASSIFICATION OF LIPIDS
Phospholipases: Enzymes which hydrolyse phospholipids at different places
Phosphatidic acid = Glycerol + 2 FAs + P
So, product of hydrolysis of cardiolipins =
3 glycerol + 4 FA + 2 P Can be antigenic due to its complex nature.
Occurs due to Anticardiolipin antibodies formed in body
C/f are thrombotic condition and disease females with history of recurrent abortions
1. GlyceroPhospholipids Parent alcohol: Glycerol 2. SphingoPhospholipids Parent alcohol: Sphingosine Sphingosine is an unsaturated amino alcohol
Alcohol + FA + Carbohydrate Phosphate & base is never present
Alcohol glycerol is never present
Alcohol used is sphingosine (so, aka glycosphingolipids)
Glucosylceramide / Glucocerebrosid
Sphingosine + FA + glucose
Sphingosine + FA = ceramide
Ceramide + Glucose = Glucosylceramide or Glucocerebroside
Never found in CNS but always found in extra neural tissues
Sphingosine + FA + Galactose
Ceramide + Galactose = Galactosylceramide/ Galactocerebroside
Always found in CNS
Polyunsaturated Fatty Acids (PUFAs) means FAs having ≥ 2 double bonds
aka Essential FAs (EFA) - they cannot be synthesized in our body and needs to be essentially taken in diet
Categories of PUFAs
Cervonic acid/DHA (DocosaHexaenoic Acid)
22 C (docosa) & 6 = (hexaenoic)
Source: Fish oil
18 C & 3 =
Source: Flaxseed oil, Soybean oil)
Timnodonic acid/EPA (EicosaPentaenoic Acid)
20 C (eicosa) & 5 = (pentaenoic)
Source: Fish oil
18 C & 3 =
Source: oil of evening primrose
18 C & 2 =
Source: Safflower oil
20 C & 4 =
Source: Animal Fats
Any fatty acid starting from “L” have same number of carbons e.g. α-Linolenic acid, γ–Linolenic acid and Linoleic acid all have 18 carbons
Smaller Name- Linoleic = 2 double bonds
Bigger name - Linolenic = 3 double bonds
is required for brain development of first 2-3 years of life so, Health drinks are fortified with DHA.
Breast milk always contains DHA.
α–Linolenic acid is precursor of ω-3 category means if α–Linolenic acid is taken in diet, other two ω-3 FAs can be made form it in the body.
Linoleic acid is precursor of ω-6 category and can be used to make other two ω-6 FAs in the body
Most Essential FA is - Linoleic acid as it can make arachidonic acid which is required for PGs and Leukotrienes synthesis.
PUFAs which are cardioprotective: ω-3 PUFAs
Polar substance is soluble in polar medium and non-polar substance is soluble in non-polar medium
Transport medium in our body is blood and Blood is water based, hence it is polar
So, carbohydrate & proteins which are polar can easily dissolve in blood and thus can be easily transported from one place of body to another
But dietary Lipids due to their non-polar nature cannot be dissolved directly in blood and need special transport structures known as Lipoproteins e.g.
HDL, LDL, VLDL
In Lipoproteins, Lipid is present towards the core surrounded by proteins in the periphery.
Lipids present in Lipoproteins
Triglyceride ? NP
Phospholipid ? Amphipathic
Cholesterol ? Amphipathic
Cholesterol ester (cholesterol + FA) ? NP
Proteins are called Apo-proteins
Arrangement of lipids in Lipoprotein
Non-polar Lipids (TG, Cholesterol ester)
Amphipathic Lipids (Cholesterol, phospholipids)
Polar portion will be towards outside
Non-polar portion will be towards inside
Composition of various lipoproteins
Lipoprotein Lipid present Protein/Apoprotein present Chylomicron TG (exogenous) Apo B48 Chylomicron Remnant TG + Cholesterol Apo B48 + Apo E VLDL TG (endogenous) Apo B100 VLDL Remnant (IDL) TG + Cholesterol Apo B100 + Apo E LDL Cholesterol Apo B100 + Apo E HDL Cholesterol ester Apo-A, Apo-C and Apo-E
Related Biochemistry Articles:
High Density Lipoproteins (HDL)
Reverse cholesterol Transport i.e. transport of excess cholesterol from peripheral tissues and blood vessels to Liver
How HDL adds Fatty acid
Exogenous TG is transported to peripheral tissues by Chylomicron.
Endogenous TG is transported from liver to peripheral tissues by: VLDL
Cholesterol is transported from liver to peripheral tissues by: LDL
Cholesterol is transported from peripheral tissues to liver by: HDL
Anabolic enzyme present in endothelium cells
Breaks TG present in chylomicrons and VLDL to convert them to their remnants.
Ligands on lipoproteins for uptake by liver
Ligands are proteins present on lipoprotein by which liver will recognise particular Lipoprotein
Hyperlipoproteinemia (Fredrickson classification)
Or Apo C-II defect
Chylo > VLDL
or Apo B100
Familial combined hyperlipoproteinemia
↑ VLDL remnant
Broad β diseases / Dys β lipoproteinemia
Clinical Features recognition to tackle hyperlipoproteinemia related clinical questions
Tendon xanthoma ↑ Cholesterol
Eruptive xanthoma ↑ TG
Palmar & Tubero eruptive xanthoma ↑ Chylo-remnant & ↑ VLDL remnant
Milky plasma ↑ Chylomicrons
Acute pain in abdomen ↑ TG
Abnormal Lipoprotein found in two conditions:
Rich in amphipathic lipids (PL & cholesterol)
Poor in neutral lipids (TG & cholesterol ester)
FATTY ACID SYNTHESIS
Occurs in cytoplasm
Activated by Insulin
FA synthase complex
Main enzyme and a multienzyme complex (6 enzyme activities)
composed of 2 identical subunits)
Q. FA is synthesized from?
Ans: Acetyl Co A and not malonyl CoA
Because extra Carbon of Malonyl CoA is not getting added in newly synthesized FA
st enzyme carboxylase adds one CO 2 to form malonyl CoA but 2 nd enzyme FA synthase removes the CO 2. So, only CO 2 of acetyl CoA are used. But, Main Donor of carbon for fatty acid synthesis – Malonyl CoA
Used for the transport of Acetyl CoA from mitochondria to cytoplasm for fatty acid synthesis
Provides two things for FA synthesis
KETONE BODY SYNTHESIS AND KETONE BODY UTILIZATION
Both are catabolic pathway
Both occur in mitochondria
Both are activated by glucagon and inhibited by insulin
KB synthesis occurs only in Liver whereas KB utilization occurs in vital organs brain and heart & also in muscles but never occurs in liver.
Note: In the first step of KB utilization, high energy CoA is added without using ATP. So, the rule of business (i.e. activation using ATP) is not followed.
Thiolase is a common enzyme for 4 lipid metabolic pathways
Ketone body synthesis
Ketone body utilization
β-oxidation of FA
Liver cannot use ketone bodies as it lacks Thiophorase.
Previous Year’s Question
Q. Active metabolite form in synthesis of fatty acid is? (AIIMS Nov 2017)
Malonyl CoA Stearate
Q. Major product of fatty acid synthesis is? (AIIMS Nov 2017)
Q. Which of the following is an essential fatty acid? (FMGE JUNE 2018)
Linoleic acid Stearic acid
Q. Type I hyperlipoproteinemia is characterized by? (NEET Jan 2019)
Elevated lipoprotein lipase
Q. Ketone bodies are not utilized by? (JIPMER May 2019)
Q. A patient has multiple tendon xanthomas. Serum cholesterol (398 mg/dl) & LDL (220 mg/dl) were found to be raised. Statins were given to this patient. What is the diagnosis? (NEET Sep 2021)
Lipoprotein lipase deficiency
Familial hypercholesterolemia Tangier’s disease
Q. Eicosanoids are formed from? (INICET May 2022)
Arachidonic acid Platelet aggregation
4 fused rings
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