NEET MDS Lessons
Biochemistry
Enzymes are protein catalyst produced by a cell and responsible ‘for the high rate’ and specificity of one or more intracellular or extracellular biochemical reactions.
Enzymes are biological catalysts responsible for supporting almost all of the chemical reactions that maintain animal homeostasis. Enzyme reactions are always reversible.
The substance, upon which an enzyme acts, is called as substrate. Enzymes are involved in conversion of substrate into product.
Almost all enzymes are globular proteins consisting either of a single polypeptide or of two or more polypeptides held together (in quaternary structure) by non-covalent bonds. Enzymes do nothing but speed up the rates at which the equilibrium positions of reversible reactions are attained.
In terms of thermodynamics, enzymes reduce the activation energies of reactions, enabling them to occur much more readily at low temperatures - essential for biological systems.
Growth hormone
Growth hormone (GH or HGH), also known as somatotropin or somatropin, is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans.
Growth hormone is a single-chain polypeptide that is synthesized, stored, and secreted by somatotropic cells within the lateral wings of the anterior pituitary gland.
Regulation of growth hormone secretion
Secretion of growth hormone (GH) in the pituitary is regulated by the neurosecretory nuclei of the hypothalamus. These cells release the peptides Growth hormone-releasing hormone (GHRH or somatocrinin) and Growth hormone-inhibiting hormone (GHIH or somatostatin) into the hypophyseal portal venous blood surrounding the pituitary.
GH release in the pituitary is primarily determined by the balance of these two peptides, which in turn is affected by many physiological stimulators (e.g., exercise, nutrition, sleep) and inhibitors (e.g., free fatty acids) of GH secretion.
Regulation
Stimulators of growth hormone (GH) secretion include peptide hormones, ghrelin, sex hormones, hypoglycemia, deep sleep, niacin, fasting, and vigorous exercise.
Inhibitors of GH secretion include somatostatin, circulating concentrations of GH and IGF-1 (negative feedback on the pituitary and hypothalamus), hyperglycemia, glucocorticoids, and dihydrotestosterone.
Clinical significance
The most common disease of GH excess is a pituitary tumor composed of somatotroph cells of the anterior pituitary. These somatotroph adenomas are benign and grow slowly, gradually producing more and more GH excess. The adenoma may become large enough to cause headaches, impair vision by pressure on the optic nerves, or cause deficiency of other pituitary hormones by displacement.
FAT-SOLUBLE VITAMINS
The fat-soluble vitamins, A, D, E, and K, are stored in the body for long periods of time and generally pose a greater risk for toxicity when consumed in excess than water-soluble vitamins.
VITAMIN A: RETINOL
Vitamin A, also called retinol, has many functions in the body. In addition to helping the eyes adjust to light changes, vitamin A plays an important role in bone growth, tooth development, reproduction, cell division, gene expression, and regulation of the immune system.
The skin, eyes, and mucous membranes of the mouth, nose, throat and lungs depend on vitamin A to remain moist. Vitamin A is also an important antioxidant that may play a role in the prevention of certain cancers.
One RAE equals 1 mcg of retinol or 12 mcg of beta-carotene. The Recommended Dietary Allowance (RDA) for vitamin A is 900 mcg/ day for adult males and 700 mcg/ day for adult females.
Vitamin A Deficiency
Vitamin A deficiency is rare, but the disease that results is known as xerophthalmia.
Other signs of possible vitamin A deficiency include decreased resistance to infections, faulty tooth development, and slower bone growth.
Vitamin A toxicity The Tolerable Upper Intake Level (UL) for adults is 3,000 mcg RAE.
VITAMIN D
Vitamin D plays a critical role in the body’s use of calcium and phosphorous. It works by increasing the amount of calcium absorbed from the small intestine, helping to form and maintain bones.
Vitamin D benefits the body by playing a role in immunity and controlling cell growth. Children especially need adequate amounts of vitamin D to develop strong bones and healthy teeth.
RDA From 12 months to age fifty, the RDA is set at 15 mcg.
20 mcg of cholecalciferol equals 800 International Units (IU), which is the recommendation for maintenance of healthy bone for adults over fifty.
Vitamin D Deficiency
Symptoms of vitamin D deficiency in growing children include rickets (long, soft bowed legs) and flattening of the back of the skull. Vitamin D deficiency in adults may result in osteomalacia (muscle and bone weakness), and osteoporosis (loss of bone mass).
Vitamin D toxicity
The Tolerable Upper Intake Level (UL) for vitamin D is set at 100 mcg for people 9 years of age and older. High doses of vitamin D supplements coupled with large amounts of fortified foods may cause accumulations in the liver and produce signs of poisoning.
VITAMIN E: TOCOPHEROL
Vitamin E benefits the body by acting as an antioxidant, and protecting vitamins A and C, red blood cells, and essential fatty acids from destruction.
RDA One milligram of alpha-tocopherol equals to 1.5 International Units (IU). RDA guidelines state that males and females over the age of 14 should receive 15 mcg of alpha-tocopherol per day.
Vitamin E Deficiency Vitamin E deficiency is rare. Cases of vitamin E deficiency usually only occur in premature infants and in those unable to absorb fats.
VITAMIN K
Vitamin K is naturally produced by the bacteria in the intestines, and plays an essential role in normal blood clotting, promoting bone health, and helping to produce proteins for blood, bones, and kidneys.
RDA
Males and females age 14 - 18: 75 mcg/day; Males and females age 19 and older: 90 mcg/day
Vitamin K Deficiency
Hemorrhage can occur due to sufficient amounts of vitamin K.
Vitamin K deficiency may appear in infants or in people who take anticoagulants, such as Coumadin (warfarin), or antibiotic drugs.
Newborn babies lack the intestinal bacteria to produce vitamin K and need a supplement for the first week.
Polyprotic Acids
• Some acids are polyprotic acids; they can lose more than one proton.
• In this case, the conjugate base is also a weak acid.
• For example: Carbonic acid (H2CO3 ) can lose two protons sequentially.
• Each dissociation has a unique Ka and pKa value.
Ka1 = [H+ ][HCO3 - ] / [H2CO3]
Ka2 = [H+ ][CO3 -2 ] / [HCO3- ]
Note: (The difference between a weak acid and its conjugate base differ is one hydrogen)
IRON
The normal limit for iron consumption is 20 mg/day for adults, 20-30 mg/day for children and 40 mg/day for pregnant women.
Milk is considered as a poor source of iron.
Factors influencing absorption of iron Iron is absorbed by upper part of duodenum and is affected by various factors
(a) Only reduced form of iron (ferrous) is absorbed and ferric form are not absorbed
(b) Ascorbic acid (Vitamin C) increases the absorption of iron (c) The interfering substances such as phytic acid and oxalic acid decreases absorption of iron
Regulation of absorption of Iron
Absorption of iron is regulated by three main mechanisms, which includes
(a) Mucosal Regulation
(b) Storer regulation
(c) Erythropoietic regulation
In mucosal regulation absorption of iron requires DM-1 and ferroportin. Both the proteins are down regulated by hepcidin secreted by liver. The above regulation occurs when the body irons reserves are adequate. When the body iron content gets felled, storer regulation takes place. In storer regulation the mucosal is signaled for increase in iron absorption. The erythropoietic regulation occurs in response to anemia. Here the erythroid cells will signal the mucosa to increase the iron absorption.
Iron transport in blood
The transport form of iron in blood is transferin. Transferin are glycoprotein secreted by liver. In blood, the ceruloplasmin is the ferroxidase which oxidizes ferrous to ferric state.
Storage form of iron is ferritin. Almost no iron is excreted through urine.
Anemia
Anemia is the most common nutritional deficiency disease. The microscopic appearance of anemia is characterized by microcytic hypochromic anemia
The abnormal gene responsible for hemosiderosis is located on the short arm of chromosome No.6.
The main causes of iron deficiency or anemia are
(a) Nutritional deficiency of iron (b) Lack of iron absorption (c) Hook worm infection (d) Repeated pregnancy (e) Chronic blood loss (f) Nephrosis (g) Lead poisoning
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
BIOLOGICAL ROLES OF LIPID
Lipids have the common property of being relatively insoluble in water and soluble in nonpolar solvents such as ether and chloroform. They are important dietary constituents not only because of their high energy value but also because of the fat-soluble vitamins and the essential fatty acids contained in the fat of natural foods
Nonpolar lipids act as electrical insulators, allowing rapid propagation of depolarization waves along myelinated nerves
Combinations of lipid and protein (lipoproteins) are important cellular constituents, occurring both in the cell membrane and in the mitochondria, and serving also as the means of transporting lipids in the blood.