The Citric Acid Cycle 19-1

The Citric Acid Cycle 19-1

The Citric Acid Cycle Three processes play central role in aerobic metabolism the citric acid cycle electron transport oxidative phosphorylation Metabolism consists of catabolism: the oxidative breakdown of nutrients anabolism: the reductive synthesis of biomolecules The citric acid cycle is amphibolic; that is, it plays a role in both catabolism and anabolism It is the central metabolic pathway 19-2

Mitochondrion The Powerhouse of the Cell 19-3

The Citric Acid Cycle TCA cycle= Krebs cycle= Citric acid cycle In Eukaryotes, cycle occurs in the mitochondrial matrix In Prokaryotes CAC occurs in the cytosol mitochondrion 19-4

The Citric Acid Cycle Pyruvate CO2 N A D + N A D H Acetyl -CoA Coenzyme A N A D H N A D + F A D H 2 Citric acid cycle (8 steps) N A D + N A D H C O 2 F A D GTP GDP N A D H C O 2 N A D + 19-5

Summary Of Reactions Of CAC Can I Keep Selling Sex For Money Officer 19-6

19-7

Pyruvate to Acetyl-CoA Oxidative decarboxylation reaction Occurs in the mitochondria O CH 3 CCOO - + CoA-SH + NAD + Pyruvate Coenzyme A this reaction requires NAD +, FAD, Mg 2+, thiamine pyrophosphate, coenzyme A, and lipoic acid G = -33.4 kj mol -1 O pyruvate dehydrogenase complex CH 3 C-SCo A + CO 2 Acetyl-CoA + NADH 19-8

Structure of the pyruvate dehydrogenase complex E1, pyruvate dehydrogenase (yellow) (; E2, dihydrolipoyl transacetylase;(green) and E3,dihydrolipoyl dehydrogenase (red). The lipoyl domain of E2 (blue) 19-9

19-10

Congenital Lactic Acidosis Absence of pyruvate decarboxylase activity in man ( first enzyme ). characterised by progressive neuromuscular deterioration and accumulation of lactate and hydrogen ions in blood, urine and/or cerebrospinal fluid, frequently resulting in early death. It is an x-linked dominant disease affecting both sexes. 19-11

Arsenic poisoning It inhibits lipoic acid. Epidemiological evidence shows an association between inorganic arsenic in drinking water and increased risk of skin, lung and bladder cancers 19-12

Beriberi A vitamin-deficiency disease first described in 1630 by Jacob Bonitus, a Dutch physician working in Java The term beriberi is derived from the Sinhalese word meaning extreme weakness. 19-13

Beriberi It is a nutritional disorder caused by a deficiency of thiamin (vitamin B 1 ) and characterized by impairment of the nerves and heart. In the form known as dry beriberi, there is a gradual degeneration of the long nerves, first of the legs and then of the arms, with associated atrophy of muscle and loss of reflexes. In wet beriberi, a more acute form, there is edema resulting largely from cardiac failure and poor circulation. In infants breast-fed by mothers who are deficient in thiamin, beriberi may lead to rapidly progressive heart failure. 19-14

Wernicke Korsakoff syndrome This is found predominantly in alcoholics. Chronic alcohol consumption can result in thiamine deficiency by causing inadequate nutritional thiamine intake, decreased absorption of thiamine from the gastrointestinal tract, and impaired thiamine utilization in the cells. People differ in their susceptibility to thiamine deficiency, however, and different brain regions also may be more or less sensitive to this condition. 19-15

The Citric Acid Cycle Step 1: Formation of citrate by condensation of acetyl-coa with oxaloacetate; G = -32.8 kj mol -1 O C H 3 C - S C o A Acetyl-CoA c itrate C H 2 - C O O - synthase + H O C - C O O - + O C - C O O - C H 2 - C O O - C H 2 - C O O - Oxaloacetate Citrate (3 carboxyl groups) C o A - S H Coenzyme A citrate synthase (condensing E) is an allosteric enzyme, inhibited by NADH, ATP, and succinyl-coa 19-16

The Citric Acid Cycle Step 2: dehydration and rehydration gives isocitrate; catalyzed by aconitase(-by flouroacetate rat poison). H O C H 2 - C O O - C - C O O - C H 2 - C O O - Citrate C H 2 - C O O - C H 2 - C O O - C - C O O - H C - C O O - C H - C O O - Aconitate H O citrate is achiral; it has no stereocenter C H - C O O - Isocitrate (3 carboxyl groups) isocitrate is chiral; it has 2 stereocenters and 4 stereoisomers are possible only one of the 4 stereoisomers of isocitrate is formed in the cycle 19-17

The Citric Acid Cycle Step 3: oxidation of isocitrate followed by decarboxylation H C H 2 - C O O - C - C O O - H O C H - C O O - Isocitrate N A D + H N A D H isocitrate dehydrogenase O C H 2 - C O O - C - C O O - C - C O O - Oxalosuccinate C O 2 C H 2 - C O O - H C - H O C - C O O - a -Ketoglutarate (2 carboxyl groups) isocitrate dehydrogenase is an allosteric enzyme; it is inhibited by ATP and NADH, activated by ADP and NAD + 19-18

The Citric Acid Cycle Step 4: oxidative decarboxylation of a-ketoglutarate to succinyl-coa C o A - S H C H 2 - C O O - N A D + N A D H C H 2 - C O O - C H 2 O C - C O O - a -Ketoglutarate a -ketoglutarate dehydrogenase complex O C H 2 C S C o A Succinyl-CoA (1 carboxyl groups) + C O 2 like pyruvate dehydrogenase, this enzyme is a multienzyme complex and requires coenzyme A, thiamine pyrophosphate, lipoic acid, FAD, and NAD + G 0 = -33.4 kj mol -1 19-19

The Citric Acid Cycle Step 5: formation of succinate O C H 2 - C O O - C H 2 C S C o A Succinyl-CoA + G D P + P i The two CH 2 -COO - groups of succinate are equivalent This is the first energy-yielding step of the cycle The overall reaction is slightly exergonic Su ccin yl-coa+ H 2 O GDP + Pi Su ccin yl-coa+ GDP + Pi succinyl-coa synthetase C H - C O O - 2 + G T P C H 2 - C O O - + Succinate (2 carboxyl groups) Su ccin ate + CoA-SH GTP + H 2 O Su ccin ate+ CoA-SH + GTP G 0' (k J m ol -1 ) -33.4 +30.1-3.3 C o A - S H 19-20

The Citric Acid Cycle Step 6: oxidation of succinate to fumarate C H 2 - C O O - C H 2 - C O O - Succinate F A D F A D H 2 succinate Dehydrogenase +Fe, - heme H - O O C C C C O O - H Fumarate (2 carboxyl groups) Note: succinate dehydrogenase is the only TCA enzyme that is located in the inner mitochondrial membrane and linked directly to ETC 19-21

The Citric Acid Cycle Step 7: hydration of fumarate H - O O C C C C O O - H Fumarate H 2 O H O C H - C O O - fumarase C H 2 - C O O - L-Malate (2 carboxyl groups) Step 8: oxidation of malate H O C H - C O O - C H 2 - C O O - L-Malate N A D + N A D H malate dehydrogenase O C - C O O - C H 2 - C O O - Oxaloacetate (2 carboxyl groups) 19-22

From Pyruvate to CO 2 Pyruvate dehydrogenase complex Pyruvate + CoA-SH + NAD + Acetyl-CoA + NADH + CO 2 + H + Citric acid cycle Acetyl-CoA + 3NAD + + FAD + G DP + P i + 2 H 2 O 2 CO 2 + CoA-SH + 3NADH + 3H + + FADH 2 + G TP Pyruvate + 4NAD + + FAD + G DP + P i + 2 H 2 O 3 CO 2 + 4NADH + FADH 2 + G TP + 4H + 19-23

Summary The two-carbon unit needed at the start of the citric acid cycle is obtained by converting pyruvate to acetyl-coa This conversion requires the three primary enzymes of the pyruvate dehydogenase complex, as well as, the cofactors TPP, FAD, NAD +, and lipoic acid The overall reaction of the pyruvate dehydogenase complex is the conversion of pyruvate, NAD +, and CoA-SH to acetyl-coa, NADH + H +, and CO 2 19-24

1. 2. 3. 4. 5. Pyruvate + CoA-SH + NAD + Acetyl-CoA + NADH + CO 2 + H + Acetyl-CoA + Oxaloacetate + H 2 O Citrate + CoA-SH + H + Citrate Is ocitrate Is ocitrate + NAD + a-ketoglutarate + NADH + CO 2 a-ketoglutarate + NAD + + CoA-SH Succinyl-CoA + NADH + CO 2 + H + Succinyl-CoA Succinate + G DP + P i + G TP + CoA-SH 6. Succinate + FAD Fumarate + FADH 2 7. Fumarate + H 2 O Malate 8. Malate + NAD + Oxaloacetate + N AD H G ' (kj mol -1 ) -33.4-32.2 +6.3-7.1-33.4-3.3 ~0-3.8 +29.2 Pyruvate + 4 NAD + + FAD + G DP + P i 3 CO 2 + 4 NADH + FADH 2 + G TP + 4 H + -77.7 19-25

Control of the CA Cycle Three control points within the cycle citrate synthase: inhibited by ATP, NADH, and succinyl CoA; also product inhibition by citrate isocitrate dehydrogenase: activated by ADP and NAD +, inhibited by ATP and NADH a-ketoglutarate dehydrogenase complex: inhibited by ATP, NADH, and succinyl CoA; activated by ADP and NAD + One control point outside the cycle pyruvate dehydrogenase: inhibited by ATP and NADH; also product inhibition by acetyl-coa 19-26

Control of the CA Cycle Conversion of pyruvate to acetyl-coa 19-27

Cells in a resting metabolic state Cells in an active metabolic state need and use comparatively little energy need and use more energy than resting cells high ATP, low ADP imply high ATP/ADP ratio low ATP, high ADP imply low ATP/ADP ratio high NADH, low NAD + imply high NADH/NAD + ratio low NADH, high NAD + imply low NAHDH/NAD + ratio 19-28

Why Is the Oxidation of Acetate So Complicated? 19-29

Because 1. Besides its role in the oxidative catabolism of carbohydrates, fatty acids, and amino acids, the cycle provides precursors for many biosynthetic pathways. 2. It is also important for plants and bacteria 19-30

Glyoxalate Cycle.. 19-31

Glyoxalate Cycle Bacteria and plants can synthesize acetyl CoA from acetate and CoA by an ATP-driven reaction that is catalyzed by acetyl CoA synthetase. 19-32

19-33

Biosynthetic Roles of the Citric Acid Cycle. Intermediates drawn off for biosyntheses (shown by red arrows) are replenished by the formation of oxaloacetate from pyruvate. 19-34

End Chapter 19 19-35