NEET MDS Lessons
Biochemistry
Ampholytes, Polyampholytes, pI and Zwitterion
Many substances in nature contain both acidic and basic groups as well as many different types of these groups in the same molecule. (e.g. proteins). These are called ampholytes (one acidic and one basic group) or polyampholytes (many acidic and basic groups). Proteins contains many different amino acids some of which contain ionizable side groups, both acidic and basic. Therefore, a useful term for dealing with the titration of ampholytes and polyampholytes (e.g. proteins) is the isoelectric point, pI. This is described as the pH at which the effective net charge on a molecule is zero.
For the case of a simple ampholyte like the amino acid glycine the pI, when calculated from the Henderson-Hasselbalch equation, is shown to be the average of the pK for the a-COOH group and the pK for the a-NH2 group:
pI = [pKa-(COOH) + pKa-(NH3+)]/2
For more complex molecules such as polyampholytes the pI is the average of the pKa values that represent the boundaries of the zwitterionic form of the molecule. The pI value, like that of pK, is very informative as to the nature of different molecules. A molecule with a low pI would contain a predominance of acidic groups, whereas a high pI indicates predominance of basic groups.
FATTY ACIDS
Fatty acids consist of a hydrocarbon chain with a carboxylic acid at one end.
• are usually in esterified form as major components of other lipids
• are often complexed in triacylglycerols (TAGs)
• most have an even number of carbon atoms (usually 14 to 24)
• are synthesized by concatenation of C2 units.
• C16 & C18 FAs are the most common FAs in higher plants and animals
• Are either:
—saturated (all C-C bonds are single bonds) or
—unsaturated (with one or more double bonds in the chain)
—monounsaturated (a single double bond)
1.Example of monounsaturated FA: Oleic acid 18:1(9) (the number in unsaturated FA parentheses indicates that the double bond is between carbons 9 & 10)
2. Double bonds are almost all in the cis conformation
—polyunsaturated (more then one double bond)
Polyunsaturated fatty acids contain 2 or more double bonds. They usually occur at every third carbon atom towards the methyl terminus (-CH3 ) of the molecule. Example of polyunsaturated FA: Linoleic acid 18:2(9,12)
• the number of double bonds in FAs varies from 1 to 4 (usually), but in most bacteria it is rarely more than 1
Saturated FAs are highly flexible molecules that can assume a wide range of conformations because there is relatively free rotation about their C-C bonds.
Weak Acids and pKa
• The strength of an acid can be determined by its dissociation constant, Ka.
• Acids that do not dissociate significantly in water are weak acids.
• The dissociation of an acid is expressed by the following reaction: HA = H+ + A- and the dissociation constant Ka = [H+ ][A- ] / [HA]
• When Ka < 1, [HA] > [H+ ][A- ] and HA is not significantly dissociated. Thus, HA is a weak acid when ka < 1.
• The lesser the value of Ka, the weaker the acid.
• Similar to pH, the value of Ka can also be represented as pKa.
• pKa = -log Ka.
• The larger the pKa, the weaker the acid.
• pKa is a constant for each conjugate acid and its conjugate base pair.
• Most biological compounds are weak acids or weak bases.
Glucagon
Glucagon, a peptide hormone synthesized and secreted from the α-cells of the islets of Langerhans of pancreas, raises blood glucose levels. The pancreas releases glucagon when blood sugar (glucose) levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is released into the bloodstream. Glucagon and insulin are part of a feedback system that keeps blood glucose levels at a stable level.
Regulation and function
Secretion of glucagon is stimulated by hypoglycemia, epinephrine, arginine, alanine, acetylcholine, and cholecystokinin.
Secretion of glucagon is inhibited by somatostatin, insulin, increased free fatty acids and keto acids into the blood, and increased urea production.
Classification of Fatty Acids and Triglycerides
Short-chain: 2-4 carbon atoms
Medium-chain: 6-12 carbon atoms
Long-chain: 14-20 carbon atoms
Very long-chain: >20 carbon atoms
• are usually in esterified form as major components of other lipids
A16-carbon fatty acid, with one cis double bond between carbon atoms 9 and 10 may be represented as 16:1 cisD9.
Double bonds in fatty acids usually have the cis configuration. Most naturally occurring fatty acids have an even number of carbon atoms
Examples of fatty acids
|
18:0 |
stearic acid |
|
18:1 cisD9 |
oleic acid |
|
18:2 cisD9,12 |
linoleic acid |
|
18:3 cisD9,12,15 |
linonenic acid |
|
20:4 cisD5,8,11,14 |
arachidonic acid |
There is free rotation about C-C bonds in the fatty acid hydrocarbon, except where there is a double bond. Each cis double bond causes a kink in the chain,
Vitamin B6: Pyridoxine, Pyridoxal, Pyridoxamine
Aids in protein metabolism and red blood cell formation. It is also involved in the body’s production of chemicals such as insulin and hemoglobin.
Vitamin B6 Deficiency Deficiency symptoms include skin disorders, dermatitis, cracks at corners of mouth, anemia, kidney stones, and nausea. A vitamin B6 deficiency in infants can cause mental confusion.
The Hemoglobin Buffer Systems
These buffer systems are involved in buffering CO2 inside erythrocytes. The buffering capacity of hemoglobin depends on its oxygenation and deoxygenation. Inside the erythrocytes, CO2 combines with H2O to form carbonic acid (H2CO3) under the action of carbonic anhydrase.
At the blood pH 7.4, H2CO3 dissociates into H+ and HCO3 − and needs immediate buffering.