NAD+ NADH CoA O O - O CCCH 3 CoA O CO2 CoA C CH3 - - O O C O C O - HO C CH2 C O

NADH C O CH2 O CH2 Step 1 C O NAD+ - C O O - O - O Oxaloacetate (4C) Citrate (6C) Step 2 C O - H C CH2 C O O Step 8 HO CH NAD+ - O Isocitrate (6C) C O - C O O NADH HC OH CH Step 3 2 Malate (4C)

C O CO2 - O Citric Acid Cycle - O C O

Step 7 α‐Ketoglutarate (5C) CH2 CH2 - O Furmate (4C) C O C O C O H O - 2 CH O Succinyl CoA (4C) CH Succinate (4C) - Step 4 CoA Step 6 - O CO O - C O O C O NAD+ Pi CH2 CH2 Step 5 CH2 CH2 C O FADH2 O C CO FAD - CoA 2 NADH O

CoA GTP GDP

ADP ATP

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. - - Step 1 O - O Citrate O H2O C O C O Synthase C O - The CH end of the acetyl CoA loses a O 3 C O HO C CH C CoA HO C CH2 C O proton and becomes bonded to the 2 CoA CoA C CH3 + CH + 2 O CH2 O second carbonyl carbon (C=O) of CH2 oxyloacetate. The coenzyme (CoA) is C O C O C O - subsequently lost with the input of - - O O O water. Acetyl CoA Oxaloacetate Citrate S‐Citryl‐CoA intermediate

- O - - Step 2 O O C O H2O H2O C O C O - HO C CH2 C O - - An isomerization reaction takes place. This C CH2 C O H C CH C O O 2 involves the removal of a water molecule H CH O H C HO CH O and then the insertion of a water molecule. C O Aconitase The hydroxyl (OH) group changes position - C O C O O - - to a different carbon as a result. O O citrate isocitrate Cis‐aconitate intermediate

- - - O Step 3 O Isocitrate O C O C O C O This is the first of 4 oxidation steps in the - deyhdrogenase H C CH C O - CH2 cycle. The carbon carrying the hydroxyl 2 H C CH2 C O CH O CH2 group (OH) is converted to a carbonyl HO O C O CO C O group (C=O). CO2 is lost from the C O 2 - C O intermediate and alpha ketoglutarate is O - C O O - produced. NADH is produced. Isocitrate NAD+ NADH O Oxylosuccinate intermediate α‐Ketoglutarate

- - Step 4 O O C O C O Another oxidation step that results in α –Ketoglutarate dehydrogenase CH2 another loss of CO2. This reaction is CH2 very complex and is similar to the CH2 CoA + CH2 reaction that converts pyruvate to C O CO 2 C O acetly CoA. NADH is produced. C O - O NAD+ NADH CoA α‐Ketoglutarate Succinyl‐CoA

- - Step 5 O O C O C O CoA is displaced when an inorganic Succinyl ‐ CoA‐ synthase CH2 CH phosphate replaces CoA. Then the 2 CH CoA phosphate is used to phosphorylate 2 CH2 + GDP to make GTP. Later the high C O C O energy phosphate on GTP can be used Pi H2O - to phosphorylate ADP to make ATP. CoA O GDP GTP Succinyl‐CoA Succinate

ADP ATP

- - Step 6 O O C O In this, the third oxidation reaction, C O Succinate dehydrogenase two hydrogens are removed from HHC H C succinate. FAD+ becomes reduced to HHC C H FADH2. C O CO - O - FAD FADH2 O Succinate Furmate - - Step 7 O O C O Furmase C O Water is used in this reaction to place H C HHO C a hydroxyl (OH) group on a carbon. C H HHC C O CO H O - - 2 O O Malate Furmate

- - O Step 8 O C O C O In this the final of the four oxidation Malate dehydrogenase reactions, the carbon carrying the HHO C C O hydroxyl group (OH) is converted to a HHC H C H carbonyl group (C=O). NADH is created C O C O as NAD+ accepts the proton and - O - electrons. Oxaloacetate is regenerated O Malate NAD+ NADH and ready to begin step 1 again. Oxaloacetate

Net Results of the Citric Acid Cycle CoA O CoA C CH3

3‐NAD+ 1‐FADH2

3‐NADH

1‐FAD+

2‐CO2

1‐ATP 1‐ADP

The citric acid cycle requires the input of an acetyl CoA. This acetyl CoA generally comes from the break down of glucose (Glycolysis) or from the break down of fatty acids (Beta‐oxidation). One turn of the citric acid cycle releases two carbons as carbon dioxide and produces 3 NADHs, 1 ATP, and 1 FADH2. Ultimately the NADHs and the FADH2 will help with ATP synthesis in the electron transport chain.