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Ce Lesson-6 Krebs cycle (Citric acid or TCA cycle) ch Dr. P. K. Agrawal M.Sc, Ph.D., CSIR NET (JRF), SRF, GATE, FIAZ Youtube Channel with over 1 million views 20 Years of Pre-medical teaching experience Ex- Faculty, Allen Career Institute, Kota Visit following link to follow my profile https:/lunacademy.com/user/biologybypkagrawal-6909
Krebs cycle or Tri-carboxylic acid Cycle or Citric Acid Cycle . The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a series of reactions in mitochondria that oxidize acetyl-CoA and reduce coenzymes, which upon oxidation are linked to the formation of ATP (in ETS) Krebs cycle strictly occurs under aerobic conditions in the mitochondria It causes complete oxidation of the acetyl coenzyme A Important to note Krebs cycle does not starts with pyruvic acid. In fact, first the pyruvic acid moves into the imtorchonieia and gets de-carborylated to produce acetyl coenzyme A. (This reaction is called link reaction). . After this, the acetyl coenzyme A, gets condenses with oxalo-acetic acid to start Krebs cycle . But in some books (including NCERT), the link reaction is given as a part of the Krebs cycle.
Link Reaction Pyruvic acid is produced in the cytoplasm through Glycolysis, but the process of Krebs' cycle occurs in mitochondria. Therefore the Pyruvic acid enters the mitochondria. In mitochondria it undergoes decarboxylation (removal of CO2) and forms a key compound, called acetyl coenzyme A (or Acetyl CoA). The acetyl CoA. This reaction is called Link reaction. The process of decarbosylation and acetylation of pyruvic acid occurs as follows Pyruvate dehydrogenase complex CH,-CO-COOH Pyruvic acid (3C) CoA-SH CH,-CO-SCoA Acetyl CoA (2C) + + CO2 CoA NAD NADH H This reaction requires a phosphorylated form of Vitamin-B1 which acts as a prosthetic group. In the reaction, one - NAD+ is reduced to [NADH H], per pyruvate. Since there are two pyruvic acid molecules from one glucose (in glycolysis), 2 CO2 are produced and two [NADH+ H ] are formed. Similarly two moles of acetyl CoA can execute two turns of Krebs' cycle.
PYRUVIC ACID (3C) CoA NAD-1 NADH H] Link Reaction ACETYL COA COA Condensation OXALO ACETIC ACID (4C CITRIC ACID (60) NADH+ H' INAD-1 CIS ACONITIC ACID (6C MALIC ACID (4C) ISO CITRIC ACID (6C) ACID (4C) co, Decarboxylation FADH NAD- FAD NADH H' a-KETOGLUTARIC ACID (SC) ACID (4C) ATP NAD CO Decarboxylation SUCCINYL COA (4C) ADPP GDPP [NADH H] CoA COA
Balance sheet of Krebs cycle Citric acid undergoes decarboxylation (removal of CO2) at two places- (a) during conversion of isocitric acid to alpha-ketoglutaric acid. (b) during the conversion of alpha-ketoglutaric acid to succinyl CoA. At four places a pair H+ (protons) and electrons is produced which are used to reduce 3NAD+ and one FAD to form three NADH+ H+ and one FADH2. . (a) during conversion of isocitric acid to a ketoglutaric acid.-NADH+H . (b) during the conversion of a ketoglutaric acid to succinyl CoA.-NADH+H (c) during the conversion of succinicacidto fumaric acid-FADH2 . (d) during the conversion of Malic acid to Oxaloacetic acid-NADH+ H . All these reduced coenzymes can undergo ETS to yield high amount of energy which can form ATP. (Each NADH + H in ETS can provide 3 ATPs while each FADH2 can provide 2 ATPs.). . Therefore, reduced coenzymes can give 11 ATPs when oxidized in ETS (9 from NADH H and 2 from FADH2.). One ATP is directly produced per acetyl CoA in Krebs' cycle. . Therefore, a total of 12 ATPs are formed from one acetyl CoA in one turn of Krebs cycle. . There is no use of oxygen in Krebs' cycle, but it occurs in aerobic conditions.
Functions and Significance of Krebs' Cycle It results in the complete oxidation of the foodstuff and produces a larger amount of energy. Krebs' cycle is the major means of ATP production in a cell. It gives rise to many intermediate compounds which are used by the cell for the synthesis of nucleotides, cytochromes, chlorophylls and amino acids etc. e.g., Succinyl CoA is used for the synthesis of chlorophyll molecules. Similarly, alpha-keto glutaric acid, pyruvic acid and oxaloacetic acids are used for the synthesis of amino acids.
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