CLASSIFICATION OF LIPIDS

Lipids are classified as follows:

1. Simple lipids: Esters of fatty acids with various alcohols.

(a) Fats: Esters of fatty acids with glycerol. Oils are fats in the liquid state. A long-chain carboxylic acid; those in animal fats and vegetable oils often have 12–22 carbon atoms.

(b) Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols. Waxes are carboxylic acid esters, RCOOR’ ,with long, straight hydrocarbon chains in both R groups

2. Complex lipids: Esters of fatty acids containing groups in addition to an alcohol and a fatty acid.

(a) Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue. They frequently have nitrogen containing bases and other substituents,

Eg  glycerophospholipids the alcohol is glycerol

     sphingophospholipids the alcohol is sphingosine.

(b) Glycolipids (glycosphingolipids): Lipids containing a fatty acid, sphingosine, and carbohydrate. These lipids contain a fatty acid, carbohydrate and nitrogenous base. The alcohol  is sphingosine, hence they are also called as glycosphingolipids. Clycerol  and phosphate  are absent  

e.g., cerebrosides, gangliosides.

(c) Other complex lipids: Lipids such as sulfolipids and aminolipids. Lipoproteins may also be placed in this category.

3. Precursor and derived lipids: These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid soluble vitamins, and hormones. Because they are uncharged, acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids

4. Miscellaneous lipids: These include a large number of compounds possessing the characteristics of lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane (in bees wax), terpenes etc.

NEUTRAL LIPIDS: The lipids which are uncharged are referred to as neutral lipids. These are mono-, di-, and triacylglycerols, cholesterol and cholesteryl esters.

The Bicarbonate Buffer System

This is the main extracellular buffer system which (also) provides a means for the necessary removal of the CO2 produced by tissue metabolism. The bicarbonate buffer system is the main buffer in blood plasma and consists of carbonic acid as proton donor and bicarbonate as proton acceptor :

 H₂CO₃ = H⁺ + HCO₃^–

If there is a change in the ratio in favour of H₂CO₃, acidosis results.

This change can result from a decrease in [HCO₃ ⁻ ] or from an increase in [H₂CO₃ ]

Most common forms of acidosis are metabolic or respiratory

Metabolic acidosis is caused by a decrease in [HCO₃ ⁻ ] and occurs, for example, in uncontrolled diabetes with ketosis or as a result of starvation.

Respiratory acidosis is brought about when there is an obstruction to respiration (emphysema, asthma or pneumonia) or depression of respiration (toxic doses of morphine or other respiratory depressants)

Alkalosis results when [HCO₃ ⁻ ] becomes favoured in the bicarbonate/carbonic acid ratio

Metabolic alkalosis occurs when the HCO₃ ⁻  fraction increases with little or no concomitant change in H₂CO₃

Severe vomiting (loss of H+ as HCl) or ingestion of excessive amounts of sodium bicarbonate (bicarbonate of soda) can produce this condition

Respiratory alkalosis is induced by hyperventilation because an excessive removal of CO2 from the blood results in a decrease in [H₂CO₃ ]

Alkalosis can produce convulsive seizures in children and tetany, hysteria, prolonged hot baths or lack of O2 as high altitudes.

The pH of blood is maintained at 7.4 when the buffer ratio [HCO3 − ] / [ H₂CO₃] becomes 20

Function of Calcium

The major functions of calcium are

(a) Excitation and contraction of muscle fibres needs calcium. The active transport system utilizing calcium binding protein is called Calsequestrin. Calcium decreases neuromuscular irritability.
(b) Calcium is necessary for transmission of nerve impulse from presynaptic to postsynaptic region.
(c) Calcium is used as second messenger in system involving protein and inositol triphosphate.
(d) Secretion of insulin, parathyroid hormone, calcium etc, from the cells requires calcium.
(e) Calcium decrease the passage of serum through capillaries thus, calcium is clinically used  to reduce allergic exudates.
(f) Calcium is also required for coagulation factors such as prothrombin.
(g) Calcium prolongs systole.
(h) Bone and teeth contains bulk quantity of calcium.

General structure of amino acids

• All organisms use same 20 amino acids.
• Variation in order of amino acids in polypeptides allow limitless variation.
• All amino acids made up of a chiral carbon attached to 4 different groups      

 - hydrogen
 - amino group
 - carboxyl
 - R group: varies between different amino acids

• Two stereoisomers (mirror images of one another) can exist for each amino acid. Such stereoisomers are called enantiomers. All amino acids found in proteins are in the L configuration.
• Amino acids are zwitterions at physiological pH 7.4. ( i.e. dipolar ions). Some side chains can also be ionized

Structures of the 20 common amino acids

• Side chains of the 20 amino acids vary. Properties of side chains greatly influence overall conformation of protein. E.g. hydrophobic side chains in water-soluble proteins fold into interior of protein
• Some side chains are nonpolar (hydrophobic), others are polar or ionizable at physiological pH (hydrophilic).
• Side chains fall into several chemical classes: aliphatic, aromatic, sulfur-containing, alcohols, bases, acids, and amides. Also catagorized as to hydrophobic vs hydrophilic.
• Must know 3-letter code for each amino acid.

Aliphatic R Groups

• Glycine: least complex structure. Not chiral. Side chain small enough to fit into niches too small for other amino acids.
• Alanine, Valine, Leucine, Isoleucine
  • no reactive functional groups      
  • highly hydrophobic: play important role in maintaining 3-D structures of proteins because of their tendency to cluster away from water
• Proline has cyclic side chain called a pyrolidine ring. Restricts geometry of polypeptides, sometimes introducing abrupt changes in direction of polypeptide chain.

Aromatic R Groups

• Phenylalanine, Tyrosine, Tryptophan
  • Phe has benzene ring therefore hydrophobic.  
  • Tyr and Trp have side chains with polar groups, therefore less hydrophobic than Phe.
  • Absorb UV  280 nm. Therefore used to estimate concentration of proteins.

Sulfur-containing R Groups

• Methionine and Cysteine)
  • Met is hydrophobic. Sulfur atom is nucleophilic.
  • Cys somewhat hydrophobic. Highly reactive. Form disulfide bridges and may stabilize 3-D structure of proteins by cross-linking Cys residues in peptide chains.

Side Chains with Alcohol Groups

• Serine and Threonine
  • have uncharged polar side chains. Alcohol groups give hydrophilic character.
  • weakly ionizable.

Basic R Groups

• Histidine, Lysine, and Arginine.
  • have hydrophilic side chains that are nitrogenous bases and positively charged at physiological pH.
  • Arg is most basic a.a., and contribute positive charges to proteins.

Acidic R Groups and their Amide derivatives

• Aspartate, Glutamate
  • are dicarboxylic acids, ionizable at physiological pH. Confer a negative charge on proteins.
• Asparagine, Glutamine
  • amides of Asp and Glu rspectively
  • highly polar and often found on surface of proteins
  • polar amide groups can form H-bonds with atoms in other amino acids with polar side chains.

CLASSIFICATION OF ENZYMES

1. Oxidoreductases : Act on many chemical groupings to add or remove hydrogen atoms. e.g. Lactate dehydrogenase

2. Transferases Transfer functional groups between donor and acceptor molecules. Kinases are specialized transferases that regulate metabolism by transferring phosphate from ATP to other molecules. e.g. Aminotransferase.

3. Hydrolases Add water across a bond, hydrolyzing it. E.g. Acetyl choline esterase

4. Lyases Add water, ammonia or carbon dioxide across double bonds, or remove these elements to produce double bonds. e.g. Aldolase.

5. Isomerases Carry out many kinds of isomerization: L to D isomerizations, mutase reactions (shifts of chemical groups) and others. e.g. Triose phosphate isomerase

6. Ligases Catalyze reactions in which two chemical groups are joined (or ligated) with the use of energy from ATP. e.g. Acetyl CoA carboxylase

Glycogen Storage Diseases are genetic enzyme deficiencies associated with excessive glycogen accumulation within cells.

• When an enzyme defect affects mainly glycogen storage in liver, a common symptom is hypoglycemia (low blood glucose), relating to impaired mobilization of glucose for release to the blood during fasting.
• When the defect is in muscle tissue, weakness and difficulty with exercise result from inability to increase glucose entry into Glycolysis during exercise.

Various type of Glycogen storage disease are