NEET MDS Lessons
Biochemistry
LIPOPROTIENS
Lipoproteins Consist of a Nonpolar Core & a Single Surface Layer of Amphipathic Lipids
The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules .These are oriented so that their polar groups face outward to the aqueous medium. The protein moiety of a lipoprotein is known as an apolipoprotein or apoprotein,constituting nearly 70% of some HDL and as little as 1% of Chylomicons. Some apolipoproteins are integral and cannot be removed, whereas others can be freely transferred to other lipoproteins.
There re five types of lipoproteins, namely chylomicrons, very low density lipoproteins(VLDL) low density lipoproteins (LDL), high density Lipoproteins (HDL) and free fatty acid-albumin complexes.
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.
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.
The input to fatty acid synthesis is acetyl-CoA, which is carboxylated to malonyl-CoA.
The ATP-dependent carboxylation provides energy input. The CO2 is lost later during condensation with the growing fatty acid. The spontaneous decarboxylation drives the condensation.
fatty acid synthesis
acetyl-CoA + 7 malonyl-CoA + 14 NADPH → palmitate + 7 CO2 + 14 NADP+ + 8 CoA
ATP-dependent synthesis of malonate:
8 acetyl-CoA + 14 NADPH + 7 ATP → palmitate + 14 NADP+ + 8 CoA + 7 ADP + 7 Pi
Fatty acid synthesis occurs in the cytosol. Acetyl-CoA generated in the mitochondria is transported to the cytosol via a shuttle mechanism involving citrate
Glycogenolysis
Breakdown of glycogen to glucose is called glycogenolysis. The Breakdown of glycogen takes place in liver and muscle. In Liver , the end product of glycodgen breakdown is glucose where as in muscles the end product is Lactic acid Under the combined action of Phosphorylase (breaks only –α-(1,4) linkage )and Debranching enzymes (breaks only α-(1,6) linkage )glycogen is broken down to glucose.
Acids and bases can be classified as proton donors and proton acceptors, respectively. This means that the conjugate base of a given acid will carry a net charge that is more negative than the corresponding acid. In biologically relavent compounds various weak acids and bases are encountered, e.g. the acidic and basic amino acids, nucleotides, phospholipids etc.
Weak acids and bases in solution do not fully dissociate and, therefore, there is an equilibrium between the acid and its conjugate base. This equilibrium can be calculated and is termed the equilibrium constant = Ka. This is also referred to as the dissociation constant as it pertains to the dissociation of protons from acids and bases.
In the reaction of a weak acid:
HA <-----> A- + H+
the equlibrium constant can be calculated from the following equation:
Ka = [H+][A-]/[HA]
As in the case of the ion product:
pKa = -logKa
Therefore, in obtaining the -log of both sides of the equation describing the dissociation of a weak acid we arrive at the following equation:
-logKa = -log[H+][A-]/[HA]
Since as indicated above -logKa = pKa and taking into account the laws of logrithms:
pKa = -log[H+] -log[A-]/[HA]
pKa = pH -log[A-]/[HA]
From this equation it can be seen that the smaller the pKa value the stronger is the acid. This is due to the fact that the stronger an acid the more readily it will give up H+ and, therefore, the value of [HA] in the above equation will be relatively small.
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.