Download The Citric Acid Cycle: A Detailed Breakdown of Its Stages and Products and more Lecture notes Chemistry in PDF only on Docsity! Step 1 C C CH3 OO O - C CH3 O CoA CO2 NAD+ NADH CoA C C CH2 C O O - O O O - Oxaloacetate (4C) Citrate (6C) CoA C C CH2 C O O - O O - CH2OH C O O - C C CH C O O - O O - CH2 C O O - OH H Isocitrate (6C) NAD+ NADH Step 2 C CH2 CH2 C O O - O C O O - CO2 Step 3 α‐Ketoglutarate (5C) C CH2 CH2 C O O - O CoA C CH2 CH2 C O O - O O - C CH O O - CH C O O - C CH CH2 C O O - O O - OH NAD+ NADH CoA CO2 Step 4 Succinyl CoA (4C) GTP GDPCoA ATP ADP Pi Succinate (4C) FADH2 FAD Step 5 Furmate (4C) Step 6 H2O Malate (4C) Step 7 NAD+ NADH Step 8 The figure above represents the Citric Acid Cycle (also called the “Krebs Cycle”). The part of the molecule that becomes carbon dioxide is highlighted in a blue box. Notice that a 4 carbon molecule called Oxaloacetate picks up 2 more carbons when it is joined with an acetyl group from Acetyl CoA. Through the beginning steps of the cycle, 2 carbons are lost as carbon dioxide and the molecule is again restored to a 4 carbon state, ready to pick up another acetyl group. The details of the eight steps above are shown in the following pages. This time, the part of the molecule that undergoes a change is highlighted in blue and the name of the enzyme that catalyzes the reaction is in a green box. Citric Acid Cycle Step 1 Step 2 The CH3 end of the acetyl CoA loses a proton and becomes bonded to the second carbonyl carbon (C=O) of oxyloacetate. The coenzyme (CoA) is subsequently lost with the input of water. Citrate Synthase Acetyl CoA Oxaloacetate An isomerization reaction takes place. This involves the removal of a water molecule and then the insertion of a water molecule. The hydroxyl (OH) group changes position to a different carbon as a result. isocitrate Step 3 This is the first of 4 oxidation steps in the cycle. The carbon carrying the hydroxyl group (OH) is converted to a carbonyl group (C=O). CO2 is lost from the intermediate and alpha ketoglutarate is produced. NADH is produced. Isocitrate deyhdrogenase GDP Step 4 Another oxidation step that results in another loss of CO2. This reaction is very complex and is similar to the reaction that converts pyruvate to acetly CoA. NADH is produced. Step 5 CoA is displaced when an inorganic phosphate replaces CoA. Then the phosphate is used to phosphorylate GDP to make GTP. Later the high energy phosphate on GTP can be used to phosphorylate ADP to make ATP. + + citrate Aconitase Isocitrate α –Ketoglutarate dehydrogenase Succinyl‐CoA Succinate GTP C CH3 O CoA C C CH2 C O O - O O O - CoA H2O S‐Citryl‐CoA intermediate Citrate C C CH2 C O O - O O - CH2OH C O CoA C C CH2 C O O - O O - CH2OH C O O - C C CH C O O - O O - CH2OH C O O - H C C CH C O O - O O - CH2 C O O - OH H C C O O - CH2 C O O - C C O O - H H2O H2O Cis‐aconitate intermediate C C CH C O O - O O - CH2 C O O - OH H CO2 C C C C O O - O O - CH2 C O O - H O C CH2 CH2 C O O - O C O O - NAD+ NADH Oxylosuccinate intermediate α‐Ketoglutarate C CH2 CH2 C O O - O C O O - α‐Ketoglutarate + CoA C CH2 CH2 C O O - O CoA CO2 NAD+ NADH Pi C CH2 CH2 C O O - O CoA Succinyl‐CoA C CH2 CH2 C O O - O O - + CoA H2O Step 6 In this, the third oxidation reaction, two hydrogens are removed from succinate. FAD+ becomes reduced to FADH2. Furmate Succinate ATP ADP Succinyl ‐ CoA‐ synthase Succinate dehydrogenaseC C C C O O - O O - H H H H C C O O - C C O O - H H FADH2FAD
