NEET MDS Lessons
Biochemistry
Step 1. Acyl-CoA Dehydrogenase catalyzes oxidation of the fatty acid moiety of acyl-CoA, to produce a double bond between carbon atoms 2 and 3.
There are different Acyl-CoA Dehydrogenases for short (4-6 C), medium (6-10 C), long and very long (12-18 C) chain fatty acids. Very Long Chain Acyl-CoA Dehydrogenase is bound to the inner mitochondrial membrane. The others are soluble enzymes located in the mitochondrial matrix.
FAD is the prosthetic group that functions as electron acceptor for Acyl-CoA Dehydrogenase.
A glutamate side-chain carboxyl extracts a proton from the a-carbon of the substrate, facilitating transfer of 2 e- with H+ (a hydride) from the b position to FAD. The reduced FAD accepts a second H+, yielding FADH2
The carbonyl oxygen of the thioester substrate is hydrogen bonded to the 2'-OH of the ribityl moiety of FAD, giving this part of FAD a role in positioning the substrate and increasing acidity of the substrate a-proton
The reactive glutamate and FAD are on opposite sides of the substrate at the active site. Thus the reaction is stereospecific, yielding a trans double bond in enoyl-CoA.
FADH2 of Acyl CoA Dehydrogenase is reoxidized by transfer of 2 electrons to an Electron Transfer Flavoprotein (ETF), which in turn passes the electrons to coenzyme Q of the respiratory chain.
Step 2. Enoyl-CoA Hydratase catalyzes stereospecific hydration of the trans double bond produced in the 1st step of the pathway, yielding L-hydroxyacyl-Coenzyme A
Step 3. Hydroxyacyl-CoA Dehydrogenase catalyzes oxidation of the hydroxyl in the b position (C3) to a ketone. NAD+ is the electron acceptor.
Step 4. b-Ketothiolase (b-Ketoacyl-CoA Thiolase) catalyzes thiolytic cleavage.
A cysteine S attacks the b-keto C. Acetyl-CoA is released, leaving the fatty acyl moiety in thioester linkage to the cysteine thiol. The thiol of HSCoA displaces the cysteine thiol, yielding fatty acyl-CoA (2 C shorter).
A membrane-bound trifunctional protein complex with two subunit types expresses the enzyme activities for steps 2-4 of the b-oxidation pathway for long chain fatty acids. Equivalent enzymes for shorter chain fatty acids are soluble proteins of the mitochondrial matrix.
Summary of one round of the b-oxidation pathway:
fatty acyl-CoA + FAD + NAD+ + HS-CoA →
fatty acyl-CoA (2 C shorter) + FADH2 + NADH + H+ + acetyl-CoA
The b-oxidation pathway is cyclic. The product, 2 carbons shorter, is the input to another round of the pathway. If, as is usually the case, the fatty acid contains an even number of C atoms, in the final reaction cycle butyryl-CoA is converted to 2 copies of acetyl-CoA
ATP production:
- FADH2 of Acyl CoA Dehydrogenase is reoxidized by transfer of 2 e- via ETF to coenzyme Q of the respiratory chain. H+ ejection from the mitochondrial matrix that accompanies transfer of 2 e- from CoQ to oxygen, leads via chemiosmotic coupling to production of approximately 1.5 ATP. (Approx. 4 H+ enter the mitochondrial matrix per ATP synthesized.)
- NADH is reoxidized by transfer of 2 e- to the respiratory chain complex I. Transfer of 2 e- from complex I to oxygen yields approximately 2.5 ATP.
- Acetyl-CoA can enter Krebs cycle, where the acetate is oxidized to CO2, yielding additional NADH, FADH2, and ATP.
- Fatty acid oxidation is a major source of cellular ATP
b-Oxidation of very long chain fatty acids also occurs within peroxisomes
FAD is electron acceptor for peroxisomal Acyl-CoA Oxidase, which catalyzes the first oxidative step of the pathway. The resulting FADH2 is reoxidized in the peroxisome producing hydrogen peroxide FADH2 + O2 à FAD + H2O2
The peroxisomal enzyme Catalase degrades H2O2 by the reaction:
2 H2O2 → 2 H2O + O2
These reactions produce no ATP
Once fatty acids are reduced in length within the peroxisomes they may shift to the mitochondria to be catabolized all the way to CO2. Carnitine is also involved in transfer of fatty acids into and out of peroxisomes
Factors regulating blood calcium level
(i) Vitamin D
(a) Vitamin D and absorption of calcium: Active form of calcium is calcitriol. Calcitriol enters intestinal wall and binds to cytoplasmic receptor and then binds with DNA causes depression and consequent transcription of gene code for calbindin. Due to increased availability of calbindin, absorption of calcium increases leading to increased blood calcium level.
(b) Vitamin D and Bone: Vitamin D activates osteoblast, bone forming cells & also stimulates secretion of alkaline phosphatase. Due to this enzyme, calcium and phosphorus increase.
(c) Vitamin D and Kidney: Calcitriol increase reabsorption of calcium and phosphorus by renal tubules.
(ii) Parathyroid hormone (PTH)
Normal PTH level in serum is 10-60ng/l.
(a) PTH and bones: In bone, PTH causes demineralization. It also causes recreation of collagenase from osteoclast leads to loss of matrix and bone resorption. As a result, mucopolysacharides and hydroxyproline are excreted in urine.
(b) PTH and Kidney: In kidney, PTH causes increased reabsorption of calcium but decreases reabsorption of phosphorus from kidney tubules.
(iii) Calcitonin Calcitonin decreases serum calcium level. It inhibits resorption of bone. It decreases the activity of osteoclasts and increases osteoblasts.
Hyper Calcemia When plasma Ca2+ level is more than 11mg/dl is called Hypercalcemia. It is due to parathyroid adenoma or ectopic PTH secreting tumor. calcium excreted in urine decreases excretion of chloride causing hyperchloremic acidosis.
Hypocalcemia Plasma calcium level less than 8mg/dl is called hypocalcemia. Tetany due to accidental surgical removal of parathyroid glands or by autoimmune disease. In tetany, neuromuscular irritability is increased. Increased Q-7 internal in ECG is seen. Main manifestation is carpopedal spasm. Laryngismus and stridor are also observed.
Erythrocytes and the Pentose Phosphate Pathway
The predominant pathways of carbohydrate metabolism in the red blood cell (RBC) are glycolysis, the PPP and 2,3-bisphosphoglycerate (2,3-BPG) metabolism (refer to discussion of hemoglobin for review of the synthesis and role role of 2,3-BPG).
Glycolysis provides ATP for membrane ion pumps and NADH for re-oxidation of methemoglobin. The PPP supplies the RBC with NADPH to maintain the reduced state of glutathione.
The inability to maintain reduced glutathione in RBCs leads to increased accumulation of peroxides, predominantly H2O2, that in turn results in a weakening of the cell wall and concomitant hemolysis.
Accumulation of H2O2 also leads to increased rates of oxidation of hemoglobin to methemoglobin that also weakens the cell wall.
Glutathione removes peroxides via the action of glutathione peroxidase.
The PPP in erythrocytes is essentially the only pathway for these cells to produce NADPH.
Any defect in the production of NADPH could, therefore, have profound effects on erythrocyte survival.
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.
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
Vitamin B12: Cobalamin
Vitamin B12, also known as cobalamin, aids in the building of genetic material, production of normal red blood cells, and maintenance of the nervous system.
RDA The Recommended Dietary Allowance (RDA) for vitamin B12 is 2.4 mcg/day for adult males and females
Vitamin B12 Deficiency
Vitamin B12 deficiency most commonly affects strict vegetarians (those who eat no animal products), infants of vegan mothers, and the elderly. Symptoms of deficiency include anemia, fatigue, neurological disorders, and degeneration of nerves resulting in numbness and tingling.
Glycolysis enzymes are located in the cytosol of cells. Pyruvate enters the mitochondrion to be metabolized further
Mitochondrial compartments: The mitochondrial matrix contains Pyruvate Dehydrogenase and enzymes of Krebs Cycle, plus other pathways such as fatty acid oxidation.
Pyruvate Dehydrogenase catalyzes oxidative decarboxylation of pyruvate, to form acetyl-CoA
FAD (Flavin Adenine Dinucleotide) is a derivative of the B-vitamin riboflavin (dimethylisoalloxazine-ribitol). The flavin ring system undergoes oxidation/reduction as shown below. Whereas NAD+ is a coenzyme that reversibly binds to enzymes, FAD is a prosthetic group, that is permanently part of the complex.
FAD accepts and donates 2 electrons with 2 protons (2 H):
Thiamine pyrophosphate (TPP) is a derivative of thiamine (vitamin B1). Nutritional deficiency of thiamine leads to the disease beriberi. Beriberi affects especially the brain, because TPP is required for carbohydrate metabolism, and the brain depends on glucose metabolism for energy
Acetyl CoA, a product of the Pyruvate Dehydrogenase reaction, is a central compound in metabolism. The "high energy" thioester linkage makes it an excellent donor of the acetate moiety
For example, acetyl CoA functions as:
- input to the Krebs Cycle, where the acetate moiety is further degraded to CO2.
- donor of acetate for synthesis of fatty acids, ketone bodies, and cholesterol.
ATPs formed in TCA cycle from one molecule of Pyruvate
1. 3ATP 7. 3ATP 5. 3 ATP
8. 1 ATP 9. 2 ATP 11.3 ATP Total =15 ATP.
ATPS formed from one molecule of Acetyl CoA =12ATP
ATPs formed from one molecule of glucose after complete oxidation
One molecule of glucose -->2 molecules of pyruvate
['By glycolysis] ->8 ATP
2 molecules of pyruvate [By TCA cycle] -> 30 ATP
Total = 38 ATP