NEET MDS Lessons
Biochemistry
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.
By rearranging the above equation we arrive at the Henderson-Hasselbalch equation:
pH = pKa + log[A-]/[HA]
It should be obvious now that the pH of a solution of any acid (for which the equilibrium constant is known, and there are numerous tables with this information) can be calculated knowing the concentration of the acid, HA, and its conjugate base [A-].
At the point of the dissociation where the concentration of the conjugate base [A-] = to that of the acid [HA]:
pH = pKa + log[1]
The log of 1 = 0. Thus, at the mid-point of a titration of a weak acid:
pKa = pH
In other words, the term pKa is that pH at which an equivalent distribution of acid and conjugate base (or base and conjugate acid) exists in solution.
Glycogen Metabolism
The formation of glycogen from glucose is called Glycogenesis
Glycogen is a polymer of glucose residues linked mainly by a(1→ 4) glycosidic linkages. There are a(1→6) linkages at branch points. The chains and branches are longer than shown. Glucose is stored as glycogen predominantly in liver and muscle cells
Glycogen Synthesis
Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis. As glucose residues are added to glycogen, UDP-glucose is the substrate and UDP is released as a reaction product. Nucleotide diphosphate sugars are precursors also for synthesis of other complex carbohydrates, including oligosaccharide chains of glycoproteins, etc.
UDP-glucose is formed from glucose-1-phosphate and uridine triphosphate (UTP)
glucose-1-phosphate + UTP → UDP-glucose + 2 Pi
Cleavage of PPi is the only energy cost for glycogen synthesis (1P bond per glucose residue)
Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes glycosylation of one of its own tyrosine residues.
Physiological regulation of glycogen metabolism
Both synthesis and breakdown of glycogen are spontaneous. If glycogen synthesis and phosphorolysis were active simultaneously in a cell, there would be a futile cycle with cleavage of 1 P bond per cycle
To prevent such a futile cycle, Glycogen Synthase and Glycogen Phosphorylase are reciprocally regulated, both by allosteric effectors and by covalent modification (phosphorylation)
Glycogen catabolism (breakdown)
Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the →(1→4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as the reaction product.
Glycogen (n residues) + Pi → glycogen (n-1 residues) + glucose-1-phosphate
The Major product of glycogen breakdown is glucose -1-phosphate
Fate of glucose-1-phosphate in relation to other pathways:
Phosphoglucomutase catalyzes the reversible reaction:
Glucose-1-phosphate → Glucose-6-phosphate
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.
Niacin: Vitamin B3, Nicotinamide, Nicotinic Acid Niacin, or vitamin B3,
is involved in energy production, normal enzyme function, digestion, promoting normal appetite, healthy skin, and nerves.
RDA Males: 16 mg/day; Females: 14 mg/day
Niacin Deficiency : Pellagra is the disease state that occurs as a result of severe niacin deficiency. Symptoms include cramps, nausea, mental confusion, and skin problems.
Thiamin: Vitamin B1
Thiamin, or vitamin B1, helps to release energy from foods, promotes normal appetite, and is important in maintaining proper nervous system function.
RDA (Required Daily allowance) Males: 1.2 mg/day; Females: 1.1 mg/day
Thiamin Deficiency
Symptoms of thiamin deficiency include: mental confusion, muscle weakness, wasting, water retention (edema), impaired growth, and the disease known as beriberi.
PROPERTIES OF TRIACYLGTYCEROLS
1. Hydrolysis : Triacylglycerols undergo stepwise enzymatic hydrolysis to finally liberate free fatty acids and glycerol.
The process of hydrolysis, catalysed by lipases is important for digestion of fat in the gastrointestinal tract and fat mobilization from the adipose tissues.
2. Saponification : The hydrolysis of triacylglycerols by alkali to produce glycerol and soaps is known as saponification.
3.Rancidity: Rancidity is the term used to represent the deterioration of fats and oils resulting in an unpleasant taste. Fats containing unsaturated fatty acids are more susceptible to rancidity.
Hydrolytic rancidity occurs due to partial hydrolysis of triacylglycerols by bacterial enzymes.
Oxidative rancidity is due to oxidation of unsaturated fatty acids.
This results in the formation of unpleasant products such as dicarboxylic acids, aldehydes, ketones etc.
Antioxidants : The substances which can prevent the occurrence of oxidative rancidity are known as antioxidants.
Trace amounts of antioxidants such as tocopherols (vitamin E), hydroquinone, gallic acid and c,-naphthol are added to the commercial preparations of fats and oils to prevent rancidity. Propylgallate, butylatedhydroxyanisole (BHA) and butylated hydroxytoluene (BHT) are the antioxidants used in food preservation.
Lipid peroxidation in vivo: In the living cells, lipids undergo oxidation to produce peroxides and free radicals which can damage the tissue. .
The free radicals are believed to cause inflammatory diseases, ageing, cancer , atherosclerosis etc
Iodine number : lt is defined as the grams (number) of iodine absorbed by 100 g of fat or oil. lodine number is useful to know the relative
unsaturation of fats, and is directly proportional to the content of unsaturated fatty acids
Determination of iodine number will help to know the degree of adulteration of a given oil
Saponification number : lt is defined as the mg (number) of KOH required to hydrolyse (saponify) one gram of fat or oiL
Reichert-Meissl (RM) number: lt is defined as the number of ml 0.1 N KOH required to completely neutralize the soluble volatile fatty acids distilled from 5 g fat. RM number is useful in testing the purity of butter since it contains a good concentration of volatile fatty acids (butyric acid, caproic acid and caprylic acid).
Acid number : lt is defined as the number of mg of KOH required to completely neutralize free fatty acids present in one gram fat or oil. In normal circumstances, refined oils should be free from any free fatty acids.