NEET MDS Lessons
Biochemistry
Enzyme Kinetics
Enzymes are protein catalysts that, like all catalysts, speed up the rate of a chemical reaction without being used up in the process. They achieve their effect by temporarily binding to the substrate and, in doing so, lowering the activation energy needed to convert it to a product.
The rate at which an enzyme works is influenced by several factors, e.g.,
- the concentration of substrate molecules (the more of them available, the quicker the enzyme molecules collide and bind with them). The concentration of substrate is designated [S] and is expressed in unit of molarity.
- the temperature. As the temperature rises, molecular motion - and hence collisions between enzyme and substrate - speed up. But as enzymes are proteins, there is an upper limit beyond which the enzyme becomes denatured and ineffective.
- the presence of inhibitors.
- competitive inhibitors are molecules that bind to the same site as the substrate - preventing the substrate from binding as they do so - but are not changed by the enzyme.
- noncompetitive inhibitors are molecules that bind to some other site on the enzyme reducing its catalytic power.
- pH. The conformation of a protein is influenced by pH and as enzyme activity is crucially dependent on its conformation, its activity is likewise affected.
The study of the rate at which an enzyme works is called enzyme kinetics.
Sphingosine is an amino alcohol present in sphingomyelins (sphingophospholipids). They do not contain glycerol at all.
Sphingosine is attached by an amide linkage to a fatty acid to produce ceramide. The alcohol group of sphingosine is bound to phosphorylcholine in sphingomyelin structure. .
Sphingomyelins are important constituents of myelin and are found in good quantity in brain and nervous tissues.
VITAMINS
Based on solubility Vitamins are classified as either fat-soluble (lipid soluble) or water-soluble. Vitamins A, D, E and K are fat-soluble
Vitamin C and B is water soluble.
B-COMPLEX VITAMINS
Eight of the water-soluble vitamins are known as the vitamin B-complex group: thiamin (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), vitamin B6 (pyridoxine), folate (folic acid), vitamin B12, biotin and pantothenic acid.
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.
Role of Coenzymes
The functional role of coenzymes is to act as transporters of chemical groups from one reactant to another.
Ex. The hydride ion (H+ + 2e-) carried by NAD or the mole of hydrogen carried by FAD;
The amine (-NH2) carried by pyridoxal phosphate
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
Clinical significance
Primary hyperparathyroidism is due to autonomous, abnormal hypersecretion of PTH in the parathyroid gland
Secondary hyperparathyroidism is an appropriately high PTH level seen as a physiological response to hypocalcemia.
A low level of PTH in the blood is known as hypoparathyroidism and is most commonly due to damage to or removal of parathyroid glands during thyroid surgery.