Biochemistry, 4th Edition P62. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Two CO2 Molecules Produced from Acetyl-CoA 573 a H2C COO- I C COO E- b H- c-H COO- Citrate Aconitase removes the pro-A H of the pro-A arm of citrate pro-5 arm H2O T pro-A arm H2O h2c coo- 2I h2o h2c coo 2i 5 C COO- II l V H C COO HC COO- HC COO h2o lA OH Isocitrate FIGURE a The aconitase reaction converts citrate to cis-aconitate and then to isocitrate. Aconitase is stereospecific and removes the pro-R hydrogen from the pro-P arm of citrate. b The active site of aconitase. The iron-sulfur cluster pink is coordinated by cysteines orange and isocitrate purple pdb id 1B0J . b Aconitase Utilizes an Iron-Sulfur Cluster Aconitase contains an iron-sulfur cluster consisting of three iron atoms and four sulfur atoms in a near-cubic arrangement Figure . Cysteine residues from the enzyme coordinate the three iron atoms. In the inactive state of the enzyme one corner of the cube is vacant. Binding of iron as Fe2 to this position activates aconitase. The iron atom in this position can coordinate the C-3 carboxyl and hydroxyl groups of citrate. This iron atom thus acts as a Lewis acid accepting an unshared pair of electrons from the hydroxyl making it a better leaving group. The equilibrium for the aconitase reaction favors citrate and an equilibrium mixture typically contains about 90 citrate 4 cfs-aconitate and 6 isocitrate. The AGis kJ mol. Fluoroacetate Blocks the TCA Cycle Fluoroacetate is an extremely poisonous agent that blocks the TCA cycle in vivo although it has no apparent effect on any of the isolated enzymes. Its LD50 the lethal dose for 50 of animals consuming it is mg per kilogram of body weight it has been used as a rodent poison. The action of fluoroacetate has been traced to aconitase which is inhibited in vivo by fluorocitrate which is formed from fluoroacetate in two steps. F . . COO- I Acetyl-CoA O Citrate HO C COO synthetase 11 synthase FCH2COO- ----------------- FCH2 C SCoA -------------------- H2C COO- .
