KREBS CYCLE BDD 1 STEPS OF A KREBS

KREB’S CYCLE BDD 1

STEPS OF A KREB’S CYCLE • A) FORMATION OF ACETYL COA • B) FORMATION OF CITRIC ACID • F) DECARBOXYLATION • G) OXIDATIVE DECARBOXYLATION • H) OXIDATION • C) DEHYDRATION • I) HYDRATION I • D) HYDRATION I • J) DEHYDROGENATION II • E) DEHYDROGENATION I 2

• THE RAW MATERIAL FOR CITRIC ACID CYCLE IS ACETYL COA. • IT IS FORMED FROM FATTY ACIDS BY BOXIDATION AND FROM PYRUVIC COA TO FORM ACETYL CO A. • THIS REACTION IS CATALYZED BY A SET OF ENZYMES CALLED PYRUVIC ACID DEHYDROGENASE. • IN THIS REACTION, 2 HYDROGEN ATOMS AND ONE CO 2 MOLECULE ARE REMOVED. HENCE THIS REACTION IS CALLED OXIDATIVE DECARBOXYLATION. • THE 2 HYDROGEN ATOMS ARE ACCEPTED BY NAD AND NAD IS CONVERTED INTO 3 NADH.

Formation of citric acid The acetyl- Co. A combines with oxaloacetic acid to form citric acid. It contains 6 carbon atoms. This reaction is catalyzed by an enzyme called citric acid synthetase. 4

Dehydration Citric acid undergoes dehydration and forms cis-aconitic acid. This reaction is catalyzed by the enzyme aconitase. 5

Hydration The aconitic acid is hydrated and it forms isocitric acid. This reaction is catalyzed by the enzyme aconitase. 6

Dehydration Isocitric acid undergoes dehydrogenation in the presence of isocitric acid dehydrogenase to form oxalo succinic acid. In this reaction 2 hydrogen atoms are released. They are accepted by NAD+ to form NADH. 7

Oxidative decarboxylation During oxidative decarboxylation aketoglutaric acid is converted into succinyl Co. A. This reaction is catalyzed by aketoglutaric acid dehydrogenase. Two hydrogen atoms are released and they are transferred to NAD. The NAD is converted into NADH. In the next step, the succinyl Co. A is decarboxylated to succinic acid. This step is catalyzed by succinic acid thiokinase. Co. A is liberated 8

Oxidation Succinic acid is oxidized to fumaric acid by the removal of 2 hydrogen atoms. The reaction is catalyzed by succinic acid dehydrogenase. The hydrogen atoms are accepted by FAD and it forms FADH 2. 9

Hydration Fumaric acid undergoes hydration to form malic acid. This reaction is catalyzed by fumarase. 10

Dehydrogenation It is the final step in Krebs cycle. Oxalo acetic acid is regenerated from malic acid by a process of dehydrogenation. This reaction is catalyzed by malic acid dehydrogenase in the presence of NAD. The 2 hydrogen atoms removed are accepted by NAD and it forms NADH 2. The oxaloacetic acid formed in the reaction condenses with the acetyl Co. A to form citric acid again and thus 11