1 Sheet #13 #Citric acid cycle made by zaid al-ghnaneem corrected by amer Al-salamat date 11/8/2016 Here we go.. Record #18
2 Three processes play central role in aerobic metabolism: 1) The citric acid cycle. 2) Electron transport. 3) 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. TCA cycle= Krebs cycle= citric acid cycle Kreps cycle is the linkage between catabolism of carbohydrate, lipids and proteins and electron transport system to generate ATP. Takes places in mitochondrial matrix, oxidative phosphorylation( electron transport system) in the inner membrane. Kreps cycle called tricarboxylic acid cycle because citrate and isocitrate contain 3 carboxyl groups. In eukaryotes, cycle occurs in the mitochondrial matrix
3 ** Colors is important. Start with acetyl CoA (2 carbon atom compound), it enters the cycle and leave the cycle as 2 molecules of carbon dioxide.
4 نصيحة هاي انصفحة ادرسها وانث حاطط انشكم قذامك وجحبع انخطىات اول باول** First reaction is pyruvate Acetyl CoA when the source of Acetyl CoA is carbohydrate (from glycolysis) from aerobic pathway (also in mitochondria). The ultimate aim of citric acid cycle is to generate electron carriers (NADH and FADH2) (3 molecules of NADH, one molecule of FADH2 and one molecule of substrate level phosphorylation (GTP) ). Each molecule of NADH generates 3 ATP and each FADH2 generate 2 ATP (TOTAL 11 ATP UNTIL NOW). Remember: GTP IS EQUIVALENT TO ATP, so the total becomes 12 ATP from one turn of cycle from one molecule of Acetyl CoA. Start molecule is Acetyl CoA means 12 ATP molecules.. Now! if we start from pyruvate there is another NADH so the total becomes 15 ATP. ( 30 from 2 pyruvate and so on, and 2 ATP from glycolysis, becomes 32, and remember.. there are 2 NADH in glycolysis also,, so the Total becomes 38 ). So! 38 ATP are produced when there is complete degradation of glucose using citric acid cycle and oxidative phosphorylation. First reaction is acetyl coa and oxaloacetate,, oxaloacetate 4 carbon molecules and acetyl coa 2 carbon molecules, they are condensation reaction, they release citrate with help of enzyme citrate synthase (citrate is 6 carbon molecules, 3 carboxyl group). So it is called citric acid cycle because the first product is citrate, tricarboxylic acid cycle because it contains 3 carboxyl groups (in citrate). Pyruvate dehydrogenase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, they are similar, and they are multiple enzyme subunits. Isocitrate dehydrogenase and a-ketoglutamate dehydrogenase by decarboxylation release carbon dioxide and by oxidation reduction they release NADH. Succinyl CoA (contains 4 carbons), the enzyme called succinyl coa synthase make the substrate level phosphorylation (which means convert ADP TO ATP or GDP TO GTP) and release succinate. Succinate to fumarate by succinate dehydrogenase and release FADH2 also, fumarate to malate by fumarase with hydration, finally malate to oxaloacete by malate dehydrogenase.
5 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 Pyruvate to acetyl coa is exergonic reaction. Structure of the pyruvate dehydrogenase complex E1, pyruvate dehydrogenase (yellow) E2, dihydrolipoyltransacetylase ;(green) E3, dihydrolipoyl dehydrogenase (red). The lipoyl domain of E2 (blue)
6 Exactly pyruvate structure. Thiamine pyrophosphate in pyruvate dehydrogenase. Deficiency in thiamine pyrophosphate or thiamine (vitamin B1) leads to Beriberi disease. 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." 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 Synthase means No ATP such as glycogen synthase and many others Acetyl CoA + oxaloacetate is condensation reaction. 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.
7 Step 3: oxidation of isocitrate followed by decarboxylation Isocitrate is similar to pyruvate dehydrogenase, because there are multi enzyme complexes and multiple subunits. Isocitrate dehydrogenase is an allosteric enzyme; it is inhibited by ATP and NADH, activated by ADP and NAD+ Step 4: oxidative decarboxylation of a-ketoglutarate to succinyl- CoA Like pyruvate dehydrogenase, this enzyme is a multienzyme complex and requires coenzyme A, thiamine pyrophosphate, lipoic acid, FAD, and NAD+ G = -33.4 kj mol-1 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
8 Step 6: oxidation of succinate to fumarate Note: succinate dehydrogenase is the only TCA enzyme that is located in the inner mitochondrial membrane and linked directly to ETC. Succinate dehydrogenase is the only enzyme in this cycle that attach to inner membrane of mitochondria, it contains iron and heme group so it is red. It connect citric acid cycle with oxidative phosphorylation ( electron transport system) It produce FADH2 not NADH! Step 7: hydration of fumarate Step 8: oxidation of malate Malate exits the mitochondria but the oxaloacetate does not leave the mitochondria because it is the starting point in KREPS.
9 From Pyruvate to CO 2 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 dehydrogenase 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
11 CONTROL OF CA CYCLE Occur on the main enzymes Three control points within the cycle: 1) Citrate synthase: inhibited by ATP, NADH, and succinyl CoA; also product inhibition by citrate. 2) Isocitrate dehydrogenase: activated by ADP and NAD+, inhibited by ATP and NADH. 3) 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. Conversion of pyruvate to acetyl CoA:
11 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.
12 Isocitrate lyase is NOT the isocitrate dehydrogenase. Citrate synthase is mutual in KREPS and glycoxalate cycle. Glycoxalate cycle Bacteria and plants can synthesize acetyl CoA from acetate and CoA by an ATP-driven reaction that is catalyzed by acetyl CoA synthetase. Very important When the glucogenesis is active, there is deficiency in oxaloacetate, so oxaloacetate will be released from pyruvate (NOT acetyl CoA because mammals can t do this) to initiate the KREPS.
13 Also when the oxaloacetate is involved in glucogenesis, it will be released from pyruvate, this is called fill up reaction or anaplerotic reaction. Oxaloacetate MUST be presented in KREPS. This is why KREPS is 8 steps, because the intermediates are very important in another metabolisms. Biosynthetic Roles of the Citric Acid Cycle. Intermediates drawn off for biosynthesis (shown by red arrows) are replenished by the formation of oxaloacetate from pyruvate. When You Want To Give Up, Remember Why You Started GOOD LUCK.