NEET MDS Lessons
Biochemistry
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.
Cholesterol synthesis:
Hydroxymethylglutaryl-coenzyme A (HMG-CoA) is the precursor for cholesterol synthesis.
HMG-CoA is also an intermediate on the pathway for synthesis of ketone bodies from acetyl-CoA. The enzymes for ketone body production are located in the mitochondrial matrix. HMG-CoA destined for cholesterol synthesis is made by equivalent, but different, enzymes in the cytosol.
HMG-CoA is formed by condensation of acetyl-CoA and acetoacetyl-CoA, catalyzed by HMG-CoA Synthase.
HMG-CoA Reductase, the rate-determining step on the pathway for synthesis of cholesterol.
ESSENTIAL FATTY ACIDS (EFAs) Polyunsaturated FAs,such as Linoleic acid and g(gamma)- Linolenic acid, are ESSENTIAL FATTY ACIDS — we cannot make them, and we need them, so we must get them in our diets mostly from plant sources.
The amino acids buffer system
Amino acids contain in their molecule both an acidic (− COOH) and a basic (− NH2) group. They can be visualized as existing in the form of a neutral zwitterion in which a hydrogen atom can pass between the carboxyl and amino groups.
By the addition or subtraction of a hydrogen ion to or from the zwitterion, either the cation or anion form will be produced
Thus, when OH− ions are added to the solution of amino acid, they take up H+ from it to form water, and the anion is produced. If H+ ions are added, they are taken up by the zwitterion to produce the cation form. In practice, if NaOH is added, the salt H2N - CH2 - COONa would be formed. and the addition of HCl would result in the formation of amino acid hydrochloride.
CALCIUM
Total calcium in the human body is 1 to 1.5kg, out of which 99% is seen in bone and 1% in extracellular fluid. The main source of calcium is milk.
The daily requirement of calcium for child is 1200mg/day and for adult it is 500mg/day. During pregnancy /lactation the calcium requirement is 1500mg/day.
The absorption of calcium takes place in 1st and 2nd part of deuodenum. Calcium absorption requires carrier protein, helped by Ca2+ - dependent ATpase.
Factors responsible for increase in calcium absorption include Vitamin D, Parathyroid hormone, acidity and amino acids. Factors such as phytic acid,oxalates, malabsorption syndromes and Phosphates decreases calcium absorption. The normal calcium level in blood is 9-11mg/dl.
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
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.