The Citric Acid Cycle Hans Krebs, 1900–1981

Learning Objectives What Role Does the Citric Acid Cycle Play in Metabolism? What Is the Overall Pathway of the Citric Acid Cycle? How Is Pyruvate Converted to Acetyl-CoA? What Are the Individual Reactions of the Citric Acid Cycle? What Are the Energetics of the Citric Acid Cycle, and How Is It Controlled? What Is the Glyoxylate Cycle? What Role Does the Citric Acid Cycle Play in Catabolism? What Role Does the Citric Acid Cycle Play in Anabolism?

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

Mitochondrion

The Citric Acid Cycle TCA cycle= Krebs cycle= citric acid cycle In eukaryotes, cycle occurs in the mitochondrial matrix mitochondrion

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

Pyruvate to Acetyl-CoA Oxidative decarboxylation reaction Occurs in the mitochondria this reaction requires NAD+, FAD, Mg2+, thiamine pyrophosphate, coenzyme A, and lipoic acid G°’ = -33.4 kJ•mol-1

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)

Beriberi- A vitamin-deficiency disease first described in 1630 by Jacob Bonitus, a Dutch physician working in Java: "A certain very troublesome affliction, which attacks men, is called by the inhabitants Beriberi (which means sheep). I believe those, whom this same disease attacks, with their knees shaking and the legs raised up, walk like sheep. It is a kind of paralysis, or rather tremor: for it penetrates the motion and sensation of the hands and feet indeed sometimes of the whole body."

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

The Citric Acid Cycle Step 2: dehydration and rehydration gives isocitrate; catalyzed by aconitase citrate is achiral; it has no stereocenter 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 C - O H 2 Citrate Aconitate Isocitrate (3 carboxyl groups)

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

The Citric Acid Cycle Step 4: oxidative decarboxylation of -ketoglutarate to succinyl-CoA like pyruvate dehydrogenase, this enzyme is a multienzyme complex and requires coenzyme A, thiamine pyrophosphate, lipoic acid, FAD, and NAD+ DG0’ = -33.4 kJ•mol-1 C H 2 - O a -Ketoglutarate o A S N D + -ketoglutarate dehydrogenase complex Succinyl-CoA (1 carboxyl groups)

The Citric Acid Cycle Step 5: formation of succinate The two CH2-COO- groups of succinate are equivalent This is the first energy-yielding step of the cycle The overall reaction is slightly exergonic C H 2 - O S o A Succinyl-CoA G D P + i Succinate (2 carboxyl groups) T succinyl-CoA synthetase

The Citric Acid Cycle Step 6: oxidation of succinate to fumarate 2 C - O Succinate succinate Dehydrogenase +Fe, - heme 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

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

From Pyruvate to CO2

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 CO2

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 -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

Control of the CA Cycle Conversion of pyruvate to acetyl-CoA

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

Why Is the Oxidation of Acetate So Complicated?

Because …………… Citric Acid Cycle Components Are Important Biosynthetic Intermediates Besides its role in the oxidative catabolism of carbohydrates, fatty acids, and amino acids, the cycle provides precursors for many biosynthetic pathways. It is also important for plants and bacteria

Glyoxalate Cycle…..

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.

Biosynthetic Roles of the Citric Acid Cycle 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.

End Chapter 19