LIPIDS: CLASSIFICATION, PUFAS, LIPOPROTEINS, KETONE BODY SYNTHESIS & UTILIZATION
Apr 25, 2023
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.
BASICS
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 H2O
CLASSIFICATION OF LIPIDS
Simple Lipids
Base alcohol is glycerol
Phospholipid (PL)
Phospholipases: Enzymes which hydrolyse phospholipids at different places
Cardiolipin
Complex phospholipid
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.
E.g. Anti-Phospholipid Syndrome
Occurs due to Anticardiolipin antibodies formed in body
C/f are thrombotic condition and disease females with history of recurrent abortions
Phospholipids Types
1. GlyceroPhospholipids
Parent alcohol: Glycerol
2. SphingoPhospholipids Parent alcohol: Sphingosine Sphingosine is an unsaturated amino alcohol
Glycolipids
Made of Alcohol + FA + Carbohydrate
Phosphate & base is never present
Alcohol glycerol is never present
Alcohol used is sphingosine (so, aka glycosphingolipids)
Types
Glucosylceramide / Glucocerebrosid
Sphingosine + FA + glucose
Sphingosine + FA = ceramide
Ceramide + Glucose = Glucosylceramide or Glucocerebroside
Never found in CNS but always found in extra neural tissues
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
Omega-3 category
Omega-6 category
Cervonic acid/DHA (DocosaHexaenoic Acid)
22 C (docosa) & 6 = (hexaenoic)
Source: Fish oil
α-Linolenic acid
18 C & 3 =
Source: Flaxseed oil, Soybean oil)
Timnodonic acid/EPA (EicosaPentaenoic Acid)
20 C (eicosa) & 5 = (pentaenoic)
Source: Fish oil
γ–Linolenic acid
18 C & 3 =
Source: oil of evening primrose
Linoleic acid
18 C & 2 =
Source: Safflower oil
Arachidonic acid
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
Important information
DHA
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
LIPID TRANSPORT
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
LIPOPROTEINS
Structure
In Lipoproteins, Lipid is present towards the core surrounded by proteins in the periphery.
Main enzyme and a multienzyme complex (6 enzyme activities)
Dimer (composed of 2 identical subunits)
Important Information
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
1st enzyme carboxylase adds one CO2 to form malonyl CoA but 2nd enzyme FA synthase removes the CO2. So, only CO2 of acetyl CoA are used.
But, Main Donor of carbon for fatty acid synthesis – Malonyl CoA
CITRATE SHUTTLE
Used for the transport of Acetyl CoA from mitochondria to cytoplasm for fatty acid synthesis
Provides two things for FA synthesis
Acetyl CoA
NADPH
KETONE BODY SYNTHESIS AND KETONE BODY UTILIZATION
Similarities
Both are catabolic pathway
Both occur in mitochondria
Both are activated by glucagon and inhibited by insulin
Differences
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.
Important information
Thiolase is a common enzyme for 4 lipid metabolic pathways
Ketone body synthesis
Ketone body utilization
Cholesterol synthesis
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)
Acetyl CoA
Malonyl CoA
Stearate
Palmitate
Q. Major product of fatty acid synthesis is? (AIIMS Nov 2017)
Acetyl CoA
ATP
Citrate
Palmitate
Q. Which of the following is an essential fatty acid? (FMGE JUNE 2018)
Citric acid
Linoleic acid
Stearic acid
Palmitic acid
Q. Type I hyperlipoproteinemia is characterized by? (NEET Jan 2019)
Elevated LDL
Elevated HDL
Elevated lipoprotein lipase
Elevated chylomicrons.
Q. Ketone bodies are not utilized by? (JIPMER May 2019)
Brain
RBC
Heart
Skeletal muscle
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
Huntington’s disease
Q. Eicosanoids are formed from?(INICET May 2022)
Arachidonic acid
Platelet aggregation
4 fused rings
Arginine
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