Biochemistry 20 - Structure And Functions Of Lipids

unlike the other macromolecules (peptides, carbohydrates, and nucleic acids) come in a variety of forms. By definition, they are a group of molecules that can be extracted in organic solvents. These include fatty acids, acylglycerols, phosphoacylglycerols, sphingolipid, and steroids.

Fatty Acids - Long hydrophobic hydrocarbon chains with a negatively charged carboxyl group at physiological pH. In humans, fatty acids have an even number and mainly 16-20 carbons. Longer fatty acids are found in the brain. Free fatty acids have detergent character and are inside of cells esterified in mainly triacylglycerols (TAGs) or membrane lipids. Linoleic acid (18:2,ω6) and α-linoleic acid (18:3,ω3) are dietary essential. They can cross the blood brain barrier.
Cholesterol - Lipid molecule. The structure of cholesterol is composed of a steroid body with a hydroxyl polar "head" and short hydrocarbon non-polar "tail". It is found in the plasma membrane in both leaflets. It can either be ingested or synthesized by the liver. Cholesterol regulates and stabilizes the correct fluidity of the plasma membrane which is important due to changed fatty acid fluidity at different temperatures. They are also precursors to hormone steroids.

Long hydrophobic hydrocarbon chains with a negatively charged carboxyl group at physiological pH. In humans, fatty acids have an even number and mainly 16-20 carbons. Longer fatty acids are found in the brain. Free fatty acids have detergent character and are inside of cells esterified in mainly triacylglycerols (TAGs) or membrane lipids. Linoleic acid (18:2,ω6) and α-linoleic acid (18:3,ω3) are dietary essential. They can cross the blood brain barrier.

Lipid molecule. The structure of cholesterol is composed of a steroid body with a hydroxyl polar "head" and short hydrocarbon non-polar "tail". It is found in the plasma membrane in both leaflets. It can either be ingested or synthesized by the liver. Cholesterol regulates and stabilizes the correct fluidity of the plasma membrane which is important due to changed fatty acid fluidity at different temperatures. They are also precursors to hormone steroids.

Fatty acids are long hydrophobic hydrocarbon chains with a negatively charged carboxyl group at physiological pH. The melting point of fatty acids increases with chain length and decreases with degrees of unsaturation. The lower the melting point, the more fluid-like characteristic the fatty acid provides for the lipid membranes. Membrane fluidity is an important part of its function.

Dietary Essential Fatty Acids - In humans, fatty acids can only be desaturated between carbons 9 and 10 (Δ9) and the carboxyl end. They can be elongated at the carboxyl carbon. However, since humans can't desaturate past the 10th carbon, fatty acids which are unsaturated past carbon ten must be incorporated into our bodies through our diet. This includes: linoleic acid and α-linolenic acid. In addition to being precursors of physiological important fatty acids, they are components in biological membranes and modify the fluidity of the membrane either by themselves or by their longer family members (20:4,w6 and 22:6,w3). Essential fatty acids can pass through the adult blood-brain barrier. In addition to essential fatty acids, arachidonic acid, EPA and DHA are found in breast milk and can enter the developing brain in infants.
Linoleic acid (18:2, ω-6) - A polyunsaturated omega-6 dietary essential fatty acid. It is a precursor to other omega-6 fatty acids that can be synthesized in our bodies like Arachidonic acid (20:4, ω-6)
α-Linolenic acid (18:3, ω-3) - A polyunsaturated omega-3 dietary essential fatty acid. It is a precursor to other omega-3 fatty acids that can be synthesized in our bodies like Eicosapentaenoic acid (EPA) 20:5,ω-3 and Docosahexaenoic acid (DHA) 22:6,ω-3.

A polyunsaturated omega-6 dietary essential fatty acid. It is a precursor to other omega-6 fatty acids that can be synthesized in our bodies like Arachidonic acid (20:4, ω-6)

A polyunsaturated omega-3 dietary essential fatty acid. It is a precursor to other omega-3 fatty acids that can be synthesized in our bodies like Eicosapentaenoic acid (EPA) 20:5,ω-3 and Docosahexaenoic acid (DHA) 22:6,ω-3.

Humans cannot change a fatty acid of the w-6 family to a fatty acid of the w-3 family, but a smaller member of the same family can be used to form a larger fatty acid.
Arachidonic acid (20:4,ω-6) - is an omega-6 fatty acid synthesized from Linoleic acid via adding 2 more points of unsaturation and elongating it by 2 carbons.
Docosahexaenoic acid DHA (22:6,ω-3) - is an omega-3 fatty acid. To synthesize, a-Linolenic acid (18:3,ω-3) is first converted into Eicosapentaenoic acid (EPA) (20:5,ω-3) which in turn is converted into DHA. DHA in phospholipids of the brain provides a high fluid microenvironment. DHA is important for brain functions and the visual cycle in the retina.

is an omega-6 fatty acid synthesized from Linoleic acid via adding 2 more points of unsaturation and elongating it by 2 carbons.

is an omega-3 fatty acid. To synthesize, a-Linolenic acid (18:3,ω-3) is first converted into Eicosapentaenoic acid (EPA) (20:5,ω-3) which in turn is converted into DHA. DHA in phospholipids of the brain provides a high fluid microenvironment. DHA is important for brain functions and the visual cycle in the retina.

Triacylglycerol (TAG)- Lipid molecule. Its structure is composed of 3 fatty acids attached to a glycerol backbone. They are completely non-polar. TAGs represent about 40% of the normal diet. TAGs are the main storage form of fatty acids. TAG synthesis occurs mainly in the liver, adipose tissue, lactating mammary gland and intestinal mucosal cells.
Phospholipids - Lipids containing phosphate are phospholipids. They are characterized by a polar phosphate polar head and 2 fatty acid non-polar tails. Most phospholipids contain a glycerol backbone. Typically: Position 1 on the glycerol contains a saturated fatty acyl ester, position 2 contains a unsaturated fatty acyl ester, and position 3 contains the phosphoryl group with the specific head group.

Glycolipids - Lipids containing sugars are glycolipids. Glycolipids contain a sphingosinebackbone. Spingosine with a fatty acid joined to its amino group is named ceramide. Glycolipids can be grouped into neutral or acidic glycolipids.

Lipid molecule. Its structure is composed of 3 fatty acids attached to a glycerol backbone. They are completely non-polar. TAGs represent about 40% of the normal diet. TAGs are the main storage form of fatty acids. TAG synthesis occurs mainly in the liver, adipose tissue, lactating mammary gland and intestinal mucosal cells.

Lipids containing phosphate are phospholipids. They are characterized by a polar phosphate polar head and 2 fatty acid non-polar tails. Most phospholipids contain a glycerol backbone. Typically: Position 1 on the glycerol contains a saturated fatty acyl ester, position 2 contains a unsaturated fatty acyl ester, and position 3 contains the phosphoryl group with the specific head group.

Lipids containing sugars are glycolipids. Glycolipids contain a sphingosinebackbone. Spingosine with a fatty acid joined to its amino group is named ceramide. Glycolipids can be grouped into neutral or acidic glycolipids.

A phospholipid that is a major component of the plasma membrane. The specific head group is an ethanolamine and is characterized by having a overall non-charged head. Commonly found on the inner leaflet of cell membrane.

A phospholipid that is a major component of the plasma membrane. The specific head group is a choline group and is characterized by having a overall non-charged head. Commonly found on the outer leaflet of cell membrane. Hepatocytes can form PC using dietary choline and also by 3x methylation of PE. It provides monolayer in lipoproteins. It is also the precursor to dipalmitoyl-PC, the lung surfactant in the alveoli of the lung, and used in the synthesis of sphingomyelin.

These are phospholipid analogues that contain a long-chain fatty alcohol ether linkage at carbon-1 of the glycerol backbone. The fatty alcohol group has a double bond between its carbons 1 and 2 (vinyl-ether). The PC analogue is mainly found in the heart. The PE analogue is mainly found in nerve tissue.

A phospholipid where the phosphate head is bonded to the hydroxyl group of a serine amino acid. It contains an overall negative charge. PS is found in the inner leaflet of the plasma membrane. Exposure of PS on the surface of cells is an early event of apoptosis and leads to removal of cells by macrophages.

A phospholipid that is found in the inner leaflet of the plasma membrane. PI often contains arachidonic acid which can be released for eicosanoid synthesis directly by phospholipase A or indirectly by phospholipase C. PI can also be phosphorylated to PIP2 and be cleaved by phospholipase C to generate the second messengers DAG and inositol trisphosphate (IP3).

A special phospholipid found mainly in the inner mitochondrial membrane in eukaryotes. It is needed for the respiratory complexes of the electron transport chain.
Cardiolipin is also found in bacteria. It is also known as di-phosphatidylglycerol and contains 4 fatty acyl esters, mainly linoleic acid. Cardiolipin is a phospholipid that is antigenic and is recognized by antibodies raised against Treponema pallidum, the bacterium that causes syphilis. Deficiency of cardiolipin leads to Barth’s syndrome, a rare but severe cardiomyopathy.

These are polar membrane lipids that are found mostly in the outer leaflet of the plasma membrane and are major structural phospholipid in nerve tissues, RBC and ocular lenses. It may act as insulator of nerve fibers as part of the myelin sheath. it is involved in signal transduction, lipid rafts in the plasma membrane and may lead to apoptosis after cleavage to ceramide. The synthesis starts with palmitoyl CoA and serine. PLP (vitamin B6) is needed as coenzyme. Diseases related to sphingomyelin include: Multiple Sclerosis (demyelination) and Nieman-Pick disease (lysosomal storage disease)

A glycosphingolipid characterized by a ceramide body attached to a monosaccharide. Cerebrosides and globosides are found in brain and peripheral nervous tissue, with high concentration in the myelin sheath.

A glycosphingolipid characterized by a ceramide body attached to polysaccharide. Cerebrosides and globosides are found in brain and peripheral nervous tissue, with high concentration in the myelin sheath.

are cerebrosides that contain sulfated galactosyl residues. They are found in nerve tissue and may stabilize myelin.

are globosides after addition of N-acetyl-neuraminic acid (NANA) [sialic acid]. Gangliosides are primarily found in the CNS

Dipalmitoyl-PC is the lipid part of lung surfactant (90% lipid, 10% protein)released by alveolar Type II cells by active transport by specific ABC-transporters. Two esterified saturated fatty acids, palmitates, are needed in order to generate an extra- cellular fluid that prevents alveolar collapse during exhalation. Deficiency of lung surfactant can lead in preterm infants to Respiratory Distress Syndrome (RDS). Lung maturity of the fetus is often predicted by the lecithin/sphingomyelin ratio. Two or above is related to maturity. RDS is also found in adults due to damage of lung surfactant producing cells.

Nomenclature for unsaturated fatty acids of the form: (X:Y, ω-Z) where X is the number of carbons in the fatty acid chain, Y is the number of degrees of unsaturation (double-bonds), and Z is the number of terminal carbons before the first degree of unsaturation. Ex: Linoleic Acid is 18:2, ω-6, meaning it contains 18 carbons, 2 double bonds, and the 1st double bond is between the 6th and 7th terminal carbons.

Signaling molecules made by oxidation of 20-carbon fatty acids. Eicosanoids are derived from either omega-3 (ω-3) or omega-6 (ω-6) fatty acids.