PHOSPHOLIPIDS

These are complex or compound lipids containing phosphoric acid, in addition to fatty acids, nitrogenous base and alcohol 

There are two  classes of phospholipids

1. Glycerophospholipids (or phosphoglycerides) that contain glycerol as the alcohol.

2. Sphingophospholipids (or sphingomyelins) that contain sphingosine as the alcohol

Glycerophospholipids

Glycerophospholipids are the major lipids that occur in biological membranes. They consist of glycerol 3-phosphate esterified at its C1 and C2 with fatty acids. Usually, C1 contains a saturated fatty acid while C2 contains an unsaturated fatty acid.

In glycerophospholipids, we refer to the glycerol residue (highlighted red above) as the "glycerol backbone."

Glycerophospholipids are Amphipathic

Glycerophospholipids are sub classified as

1. Phosphatidylethanolamine or cephalin also abbreviated as PE is found in biological membranes and composed of ethanolamine bonded to phosphate group on diglyceride.

2. Phosphatidylcholine or lecithin or PC which has chloline bonded with phosphate group and glycerophosphoric acid with different fatty acids like palmitic or hexadecanoic acid, margaric acid, oleic acid. It is a major component of cell membrane and mainly present in egg yolk and soy beans.

3. Phosphatidic acid (phosphatidate) (PA)

It consists of a glycerol with one saturated fatty acid bonded to carbon-1 of glycerol and an unsaturated fatty acid bonded to carbon-2 with a phosphate group bonded to carbon-3.

4.Phosphatidylserine (PS)

This phospholipid contains serine as an organic compound with other main components of phospholipids. Generally it found on the cytosolic side of cell membranes.

5. Phosphoinositides

It is a group of phospholipids which are negatively charged and act as a a minor component in the cytosolic side of eukaryotic cell membranes. On the basis of different number of phosphate groups they can be different types like phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate(PIP2) and phosphatidylinositol trisphosphate (PIP3). PIP, PIP2 and PIP3 and collectively termed as phosphoinositide.

6. Cardiolipin :

lt is so named as it was first isolated from heart muscle. Structurally, a cardiolipin consists of two molecules of phosphatidic acid held by an additional glycerol through phosphate groups. lt is an important component of inner mitochondrial membrane. Cardiolipin is the only phosphoglyceride that possesses antigenic properties.

The Phosphate Buffer System

This system, which acts in the cytoplasm of all cells, consists of H₂PO₄^–  as proton donor and HPO₄ ^2– as proton acceptor :

H₂PO₄^–  = H⁺ + H₂PO₄^–

The phosphate buffer system works exactly like the acetate buffer system, except for the pH range in which it functions. The phosphate buffer system is maximally effective at a pH close to its pKa of 6.86 and thus tends to resist pH changes in the range between 6.4 and 7.4. It is, therefore, effective in providing buffering power in intracellular fluids.

PROPERTIES OF TRIACYLGTYCEROLS

1. Hydrolysis : Triacylglycerols undergo stepwise enzymatic hydrolysis to finally liberate free fatty acids and glycerol.

The process of hydrolysis, catalysed by lipases is important for digestion of fat in the gastrointestinal tract and fat mobilization from the adipose tissues.

2. Saponification : The hydrolysis of triacylglycerols by alkali to produce glycerol and soaps is known as saponification.

3.Rancidity: Rancidity is the term used to represent the deterioration of fats and oils resulting in an unpleasant taste. Fats containing unsaturated fatty acids are more susceptible to rancidity.

Hydrolytic rancidity occurs due to partial hydrolysis of triacylglycerols by bacterial enzymes.

Oxidative rancidity is due to oxidation of unsaturated fatty acids.

This results in the formation of unpleasant products such as dicarboxylic acids, aldehydes, ketones etc.

Antioxidants : The substances which can prevent the occurrence of oxidative rancidity are known as antioxidants.

Trace amounts of antioxidants such as tocopherols  (vitamin E), hydroquinone, gallic acid and c,-naphthol are added to the commercial preparations of fats and oils to prevent rancidity. Propylgallate, butylatedhydroxyanisole (BHA)  and butylated hydroxytoluene (BHT) are the antioxidants used in food preservation.

Lipid peroxidation in vivo: In the living cells, lipids undergo oxidation to produce peroxides and free radicals which can damage the tissue. .

The free radicals are believed to cause inflammatory diseases, ageing, cancer , atherosclerosis etc

Iodine number : lt is defined as the grams (number)  of iodine absorbed by 100 g of fat or oil. lodine number is useful to know the relative

unsaturation of fats, and is directly proportional to the content of unsaturated fatty acids

Determination of iodine number will help to know the degree of adulteration of a given oil

Saponification number : lt is defined as the mg  (number) of KOH required to hydrolyse (saponify) one gram of fat or oiL

Reichert-Meissl (RM)  number: lt is defined as the number of ml 0.1 N KOH required to completely neutralize the soluble volatile fatty acids distilled from 5 g fat. RM number is useful in testing the purity of butter since it contains a good concentration of volatile fatty acids (butyric acid, caproic acid and caprylic acid).

Acid number : lt is defined as the number of mg of KOH required to completely neutralize free fatty acids present in one gram fat or oil. In normal circumstances, refined oils should be free from any free fatty acids.

CHOLESTEROL AND ITS IMPORTANCE

Cholesterol is an important lipid found in the cell membrane. It is a sterol, which means that cholesterol is a combination of a steroid and an alcohol .

It is an important component of cell membranes and is also the basis for the synthesis of other steroids, including the sex hormones estradiol and testosterone, as well as other steroids such as cortisone and vitamin D.

In the cell membrane, the steroid ring structure of cholesterol provides a rigid hydrophobic structure that helps boost the rigidity of the cell membrane.

Without cholesterol the cell membrane would be too fluid. In the human body, cholesterol is synthesized in the liver.

Cholesterol is insoluble in the blood, so when it is released into the blood stream it forms complexes with lipoproteins.

Cholesterol can bind to two types of lipoprotein, called high-density lipoprotein (HDL) and low-density lipoprotein (LDL).

A lipoprotein is a spherical molecule with water soluble proteins on the exterior. Therefore, when cholesterol is bound to a lipoprotein, it becomes blood soluble and can be transported throughout the body.

HDL cholesterol is transported back to the liver. If HDL levels are low, then the blood level of cholesterol will increase.

High levels of blood cholesterol are associated with plaque formation in the arteries, which can lead to heart disease and stroke.

Thyroid Hormones

Thyroid hormones (T4 and T3) are tyrosine-based hormones produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland, are primarily responsible for regulation of metabolism. Iodine is necessary for the production of T3 (triiodothyronine) and T4 (thyroxine).

A deficiency of iodine leads to decreased production of T3 and T4, enlarges  the thyroid tissue and will cause the disease known as goitre.

Thyroid hormones are transported by Thyroid-Binding Globulin

Thyroxine binding globulin (TBG), a glycoprotein binds T4 and T3 and has the capacity to bind 20 μg/dL of plasma.

Diseases

1. Hyperthyroidism (an example is Graves Disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.

2 Hypothyroidism (an example is Hashimoto’s thyroiditis) is the case where there is a deficiency of thyroxine, triiodiothyronine, or both.

The Effects of Enzyme Inhibitors

Enzymes can be inhibited

• competitively, when the substrate and inhibitor compete for binding to the same active site or
• noncompetitively, when the inhibitor binds somewhere else on the enzyme molecule reducing its efficiency.

The distinction can be determined by plotting enzyme activity with and without the inhibitor present.

Competitive Inhibition

In the presence of a competitive inhibitor, it takes a higher substrate concentration to achieve the same velocities that were reached in its absence. So while V_max can still be reached if sufficient substrate is available, one-half V_max requires a higher [S] than before and thus K_m is larger.

Noncompetitive Inhibition

With noncompetitive inhibition, enzyme molecules that have been bound by the inhibitor are taken out

• enzyme rate (velocity) is reduced for all values of [S], including
• V_max and one-half V_max but
• K_m remains unchanged because the active site of those enzyme molecules that have not been inhibited is unchanged.