Tricarboxylic Acid Cycle TCA cycle Biochemistry Department Tricarboxylic Acid Cycle TCA cycle By Dr. Wael Elayat

Krebs cycle (TCA cycle) Steps: TCA cycle start by condensation reaction between acetyl CoA and Oxaloacetate.

Sources of oxaloacetate 1- From pyruvate

2- From aspartate

3- From malate

The steps of TCA cycle are 8 All are reversible except 1,3 and 4

Steps of TCA cycle cont,.

Step 1

Step 2

Step 3

Step 4 (another example of oxidation decarboxylation reaction of α keto acids)

Step 5

Step 6

Step 7

Step 8

Regulation of Krebs cycle (TCA cycle)

Regulation of Krebs cycle (TCA cycle) cont., 1- Enzymatic control Regulation of TCA cycle exerted on the 3 committed steps regulated by the 3 key regulatory enzymes: a- Citrate synthase b- Isocitrate dehydrogenase c- α ketoglutarate dehydrogenase complex

b- Isocitrate dehydrogense a-Citrate synthase It is not allosteric enzyme, yet it subjected to feed back inhibition by its product (Citrate) b- Isocitrate dehydrogense Allosterically activated by: Ca and ADP Allosterically inhibited by: NADH+H+ and ATP

c- α ketoglutarate dehydrogenase complex Allosterically activated by: Ca Allosterically inhibited by: NADH+H+ and succinyl CoA

2- Respiratory control - Krebs cycle produce a lot amount of reduced equivalent (NADH+H+ and FADH2), which must be reoxdized in order for TCA cycle to continue Reoxidation of reduced equivalent occur in electron transport chain (ETC) in mitochondria The rate of oxidation of reduced equivalent in ETC depend on availability of O2 So O2 can indirectly control rate of TCA cycle

Significance (Importance) of Krebs cycle TCA cycle has an amphibolic function (i.e both metabolic functions, anabolic and catabolic functions)

A- Anabolic function 1- Synthesis of ATP: TCA cycle produce 12 ATP from complete oxidation of one molecule of acetyl CoA. Mechanism of ATP production: - 1 ATP by substrate level phosphorylaion (step 5) 11 ATP by oxidative phosphorylation (3 molecules NADH+H+ and 1 molecule FADH2) (step 3,4,6 and 8)

2- Synthesis of fatty acids (FAs) FAs are synthesized from acetyl CoA in cytoplasm Acetyl CoA produced in mitochondria The mitochondrial membrane is impermeable to acetyl CoA Acetyl CoA converted to citrate in TCA cycle Citrate diffuse to cytoplasm In cytoplasm citrate hydrolysed to give acetyl CoA needed for FAs synthesis

3- Synthesis of amino acids (AAs) AAs can be synthesized from intermediate of TCA cycle: Α ketoglutrate Glutamic acid Oxaloacetate Aspartic acid

4- Synthesis of glucose from non carbohydrate source (Gluconeogenesis) See Gluconeogenesis (GNG)

5- Synthesis of heme part of Hb Succinyl CoA+ glycine ALA Heme

B- Catabolic function TCA cycle is the final common pathway of complete oxidation of any carbon skeleton 1- Catabolism of CHO Glucose Pyruvate acetyl CoA Co2+H2o 2- Catabolism of FAs: FAs acetyl CoA Co2+H2o

3- Oxidation of a.as.: Glutamic acid Α ketoglutrate Aspartic acid Oxaloacetate

Sources and fate of pyruvate A- Sources 1- From PEP (glycolysis): 2- From lactate (glycolysis)

3- From alanine a.a: 4- From malate

B- Fate 1- Pyruvate lactate 2- Pyruvate Acetyl CoA 3- Pyruvate Oxaloacetate 4- Pyruvate Alanine 5- Pyruvate Malate

Choose the best answer: 1- As regard regulation of TCA cycle: Citrate synthase is an allosteric enzyme Succinate thiokinase is one of the regulatory enzymes Isocitrate dehydrogenase is allosterically inhibited by Ca Level of O2 can affect indirectly the rate of the cycle All of the above

2- Succinate thiokinase: Catalyze synthesis of succinyl CoA from α ketoglutarate Catalyze synthesis of GTP from GDP Catalyze synthesis of Malate from fumarate Catalyze synthesis of oxaloacetate from malate Catalyze isocitrate from citrate