Lipid Structures and Functions - Biochemistry

Alpha-linolenic acid is an essential fatty acid that must be consumed in the diet (cannot be synthesized by the body). It is an eighteen-carbon omega-3 fatty acid that is used to synthesize eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), two important long chain polyunsaturated fatty acids. Alpha-linoleic acid is the precursor to arachodonic acid (AA).

Omega-6 fatty acids are primarily found in vegetable oils (ex. soybean oil or corn oil), chicken, and eggs. Note that poultry feed is very heavy on corn products and thus increases the omega-6 fatty acid proportion of almost all farm raised animals. Fish, chia seeds, and flax seeds are high in omega-3 fatty acids.

Dihomo-gamma-linolenic acid (DGLA) is a twenty-carbon omega-6 fatty acid that is a desaturation product of linoleic acid. The rest of the answer choices are indeed omega-3 fatty acids.

Omega-3 fatty acids can be utilized as a substrate for the synthesis of series 3 prostaglandins (a subclass of eicosanoids), and are generally considered "anti-inflammatory" for this reason. Furthermore, omega-3 fatty acids are so named for having a double bond at the 3rd carbon from the omega end. They are generally found in fish and flax and chia seeds, while omega-6 fatty acids are in poultry and vegetable oils.

Arachidonic acid (AA) is used as a substrate for series 2 eicosanoid synthesis, including thromboxane A2 (TXA2). Series 3 eicosanoids are synthesized from alpha-linolenic acid (ALA), and series 1 are synthesized from dihomo-gamma-linolenic acid (DGLA).

Alpha-linolenic acid (ALA) is an omega-3 fatty acid that is utilized to produce series 3 eicosanoids. Series 3 eicosanoids lead to a weak platelet aggregation response, and are considered anti or less-inflammatory. Thus, a diet rich in omega-3 fatty acids is recommended for those with heart disease risk. Omega-3 fatty acids are primarily found in fish or flax and chia seeds, not vegetable oil, which is rich in omega-6 fatty acids.

The hydrocarbon tail of free fatty acids is nonpolar, and so it interacts unfavorably with water molecules. So, when free fatty acids are placed in a polar environment, the nonpolar tails are driven together and inward in order to avoid water molecules. A micelle is formed from the circular formation of free fatty acids.

A greater degree of unsaturation results in a disruption of van der Waals forces between fatty acid chains. This is due to the "kinks" that occur within an unsaturated fatty acid. It is easy to visualize a fatty acid membrane with kinks being less tightly packed, and therefore more fluid. This increase in fluidity is independent of temperature.

The rules for fatty acid nomenclature state that Omega 6 means that a double bond is located at the sixth position of the fatty acid - counting from the methyl end.

Linoleic acid's nomenclature is C18:29,12. Linolenic acid's nomenclature is C18:39,12,15. They both have 18 carbons but differ by one extra double bond in linolenic acid.

Steroid hormones are derivatives of cholesterol and therefore can pass through the plasma membrane of cells since they are lipophilic. Steroid hormones generally don't bind on the surface of plasma membranes, but instead to receptors inside the cell itself. Since they are nonpolar, they are carried by proteins in the blood.

Steroids are derivatives of the molecule cyclopentanoperhydrophenanthrene which is composed of 4 fused carbon rings. There are three hexane rings and one pentane ring in the structure of steroids.

Although it is one of the three main types of membrane lipids (along with phospholipids and glycolipids) cholesterol's structure is quite different. It is a steroid, and so it is made from four linked hydrocarbon rings, including three six-membered rings and one five-membered ring.

Cholesterol is a very important molecule for a number of reasons. One function of cholesterol is to regulate membrane fluidity in the plasma membrane. Another very important role that cholesterol plays is as the precursor to a number of steroid hormones including cortisol, testosterone, estradiol, and progesterone. Insulin, however, is a peptide hormone made in the pancreas.

Cholesterol is a molecule with 27 carbon atoms. In order to synthesize this steroid, a three step process must be completed. First, isopentenyl pyrophosphate is synthesized. Second, six isopentenyl pyrophosphates form squalene. Third, squalene cyclizes and the product of this is converted to cholesterol. The building blocks that began this whole process were carbons from acetyl-CoA.

The fist step of cholesterol synthesis is the synthesis of isopentenyl pyrophosphate from acetyl-CoA molecules, and this occurs in the cytoplasm. The second step of cholesterol synthesis is six isopentenyl pyrophosphate molecules converging to form squalene. This occurs in the endoplasmic reticulum. The third step of cholesterol synthesis is the cyclization of squalene and the final conversion of the molecule to cholesterol. This occurs in the endoplasmic reticulum.

Once acetyl-CoA molecules have been converted to 3-hydroxy-3-methylglutary-CoA (HMG-CoA), reduction to mevalonate occurs. This synthesis is an irreversible step in the pathway to create cholesterol, and therefore it is the committed step in cholesterol synthesis.

Oleic acid (which composes much of olive oil) has a double bond between its 9th and 10th atoms. Hence it has an omega-9 unsaturation. It has 18 carbon atoms, not 16, which gives it a condensed molecular formula:. Because it is a cis-isomer, the IUPAC name contains a Z, and becuse it is an unsaturated carboxylic acid, it ends in -enoic acid.

In a saturated fatty acid, all of the covalent carbon to carbon bonds are single bonds. So a carbon atom in the middle of the chain will have two covalent bonds to other carbon atoms, and can therefore bond to two hydrogen atoms.