Citric Acid CycleKrebs Cycle citric acid cycle is

Citric Acid Cycle/Krebs Cycle citric acid cycle is used to harvest high energy electrons from carbon fuel. the central metabolic hub of the cell produces intermediates which are precursors for fatty acids, amino acids, nucleotide bases, and cholesterol named after Hans Krebs who was largely responsible for elucidating its pathways in the 1930 s.

Summary of the citric acid cycle acetyl Co. A combines with oxaloacetate to form citrate. Ultimately, the oxaloacetate is recycled and the acetate is broken down to CO 2. Each cycle produces 1 ATP, 3 NADH, and 1 FADH, per acetyl Co. A. For each acetyl Co. A which enters the cycle: (1) Two molecules of CO 2 are released (2) Coenzymes NAD+ and FAD+ are reduced (3) One GDP is phosphorylated (4) The initial acceptor molecule (oxaloacetate) is reformed

“All roads lead to Rome” To extract energy from nutrients: break down of large molecules into smaller ones Who? degradation by final products Polysaccarides Specific enzymes ex amylases sugar monomers Proteins proteases amino acids Fats (triacylglycerols) lipases glycerol to fatty acids then eventually acetyl-Co. A

“All roads lead to Rome” Mille viae ducunt homines per saecula Romam: A thousand roads lead men forever to Rome To extract energy from nutrients: break down of large molecules into smaller ones final products sugar monomers, amino acids fatty acids, acetyl-Co. A “delivered” into CA cycle!! Catabolic pathways feed into CA cycle cross into mitochondria Just as “All roads lead to Rome”. . All pathways lead to CA cycle! PS opposite is true!!!

“All roads lead to Rome” Catabolic pathways feed into CA cycle cross into mitochondria Just as “All roads lead to Rome”. . All pathways lead to CA cycle! ALL METABOLIC PATHWAYS ARE RELATED, AND ALL OPERATE SIMULTANEOUSLY! CA cycle. . . Generate energy from our food, and generate intermediates for our cellular processes. . Metabolic “hub” of our cells!!

gluconeogenesis GLUCOSE glycolysis alanine aminotransferase lactate PYRUVATE dehydrogenase pyruvate carboxylase Alanine pyruvate dehydrogenase lipogenesis Oxaloacetate Acetyl Co. A Fatty acids -oxidation citric acid cycle CO 2 Lactate ketogenesis (liver only) ketone oxidation Ketone bodies cholesterol synthesis Cholesterol steroid hormones (endocrine glands)

The Citric Acid Cycle Can Be a Multistep-Catalyst Oxaloacetate is regenerated The cycle is a mechanism for oxidizing acetyl Co. A to CO 2 by NAD+ and FAD The cycle itself is not a pathway for a net degradation of any cycle intermediates Cycle intermediates can be shared with other pathways, which may lead to a re-supply or net decrease in cycle intermediates

Fates of carbon atoms in the cycle Carbon atoms from acetyl Co. A (red) are not lost in the first turn of the cycle

Citric Acid Cycle/Krebs Cycle citric acid cycle is used to harvest high energy electrons from carbon fuel. the central metabolic hub of the cell produces intermediates which are precursors for fatty acids, amino acids, nucleotide bases, and cholesterol The citric acid cycle may seem like an elaborate way to oxidize acetate into carbon dioxide, but there is chemical logic to the cycle.

Citric Acid Cycle/Krebs Cycle The citric acid cycle may seem like an elaborate way to oxidize acetate into carbon dioxide, but there is chemical logic to the cycle. In order to directly oxidize acetate into two molecules of CO 2 a C —C bond must be broken. Under the mild conditions found in cells, there is insufficient energy to break the bond. Cells often break C—C bonds between carbon atoms α and β to a carbonyl called b cleavage Enzymes that catalyze: aldolases and transaldolases

Another common type of C—C cleavage is α-cleavage of an αhydroxy-ketone carried out by ketolases and decarboxylases. Since neither of these common strategies for cleavage of C—C bonds is available to acetate, the acetate is activated in the form acetyl-Co. A, condensed with oxaloacetate to form citrate and then a β-cleavage is carried out in subsequent steps.

1. Citrate Synthase Citrate formed from acetyl Co. A and oxaloacetate Only cycle reaction with C-C bond formation a synthase is an enzyme which catalyzes a synthesis process; makes covalent bonds

Stereo views of citrate synthase (a) Open conformation (b) Closed conformation Product citrate (red)

2. Aconitase catalyses the stereo-specific isomerization of citrate to isocitrate via cis-aconitate Elimination of H 2 O from citrate to form C=C bond of cis-aconitate Stereospecific addition of H 2 O to cis-aconitate to form 2 R, 3 SIsocitrate

3. Isocitrate Dehydrogenase dehydrogenase is an enzyme that oxidizes a substrate by transferring one or more hydrides to an acceptor, NAD or FAD Oxidative decarboxylation of isocitrate to a-ketoglutarate (a-kg) (a metabolically irreversible reaction) One of four oxidation-reduction reactions of the cycle Hydride ion from the C-2 of isocitrate is transferred to NAD+ to form NADH Oxalosuccinate is decarboxylated to a-kg

Isocitrate dehydrogenase reaction

4. The a-Ketoglutarate Dehydrogenase Complex

5. Succinyl-Co. A Synthetase Free energy in thioester bond of succinyl Co. A is conserved as GTP (or ATP in plants, some bacteria) GDP + Pi GTP Synthetase is an enzyme that catalyzes the linking together of two molecules especially by using the energy derived from the concurrent splitting off of a pyrophosphate group from a triphosphate (as ATP)— now called ligase

6. The Succinate Dehydrogenase (SDH) Complex Located on the inner mitochondrial membrane (other components are dissolved in the matrix) Dehydrogenation is stereospecific; only the trans isomer is formed Substrate analog malonate is a competitive inhibitor of the SDH complex

Reaction of the succinate dehydrogenase complex

7. Fumarase Stereospecific trans addition of water to the double bond of fumarate to form L-malate

8. Malate Dehydrogenase

Citric acid cycle inputs and outputs per glucose molecule Inputs: Outputs: 2 acetyl groups 6 NAD+ 2 FAD 2 ADP + 2 P 4 CO 2 6 NADH 2 FADH 2 2 ATP

Energy conservation by the cycle Energy is conserved in the reduced coenzymes NADH, QH 2 and one GTP NADH, QH 2 can be oxidized to produce ATP by oxidative phosphorylation

Reduced Coenzymes Fuel the Production of ATP Each acetyl Co. A entering the cycle nets: (1) 3 NADH (2) 1 QH 2 (3) 1 GTP (or 1 ATP) Oxidation of each NADH yields 2. 5 ATP Oxidation of each QH 2 yields 1. 5 ATP Complete oxidation of 1 acetyl Co. A = 10 ATP

Fig. 19 -8, p. 526

3 control points in CA cycle PDH. . Covered Isocitrate dehydronase ADP/NAD+ allosteric activators a-ketogluterate dehydrogenase complex Guess who inhibits? ? (same as always: ATP/NADH say No!)