Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online





Learning Goals/Objectives for Chapter 8B:  After class and this reading, students will be able to

  • state the type of oxidizing reagent used and the products forms on oxidation reactions catalyzed by dehydrogenases, monoxygenases (hydroxylases), dioxygenases, and oxidases;
  • draw the reactive end of NAD+ and mechanisms showing it's reactions with substrates in enzyme-catalyzed two electron oxidation reactions;
  • explain differences in chemical reactivity of NAD+ and FAD in one and two electrons oxidations and with dioxygen;
  • describe the stereochemistry of the alcohol dehydrogenase-catalyzed oxidation of prochiral ethanol by NAD+;
  • explain why FAD/FADH2 are often tightly bound to dehydrogenases in contrast to NAD+/NADH where are freely diffusable substrates;
  • given standard reduction potentials, determine the ΔGo' for given redox reactions;
  • explain why different FAD and other flavin containing dehydrogenases have varying standard reduction potentials for the flavin but NAD+ dependent dehydrogenase have only one;
  • describe the role of heme in mono- and dioxygenases in activating dioxygen and minimizing side reactions of ROSs;
  • describe the biological role of cytochrome P450s;
  • define and give examples of oxidases;
  • compare the contrast the role of the heme in carrying hemoglobin and myoglobin, monoxygenases, and in oxidases.

B5.  Hydrogenases (not dehydrogenases): A break from oxidation reactions

Our world desperately needs an energy efficient way to produce H2 for energy production without producing waste pollutants.  Catalytic cracking of molecules and newly developed fuel cells offer two possibilities. Wouldn't it be great if a reactant like water could be used for H2 production (without the use of electrolysis) or expensive metal catalysts?  Nature may show the way.  Bacteria (even E. Coli found in our GI system) can use simple metals like iron to produce H2 from H+ with electrons for the reduction of H+ coming from a donor (such as a reduced heme in proteins): 

Dred+ H+ <=> Dox + H2

The reaction is also reversible in the presence of an acceptor of electrons from H2 as it gets oxidized:

Aox+ H2  <=> Ared + H+

The enzymes that catalyze hydrogen production are hydrogenases (not dehydrogenases).  Note that the name hydrogenases best reflects the reverse reaction when a molecule (P) in an oxidized state gets reduced (to S) and H2 gets oxided to H+. 

Crystal structures of hydrogenases show them to be unique among metal-containing enzymes. They contain two metals bonded to each other.  The metal centers can either be both iron or one each of iron and nickel.  The ligands interacting with the metals are two classical metabolic poisons, carbon monoxide and cyanide.  Passages for flow of electrons and H2 connect the buried metals and the remaining enzymes. The metals are also bound to sulfhydryl groups of cysteine side chains.  It appears that two electrons are added to a single proton making a hydride anion which accepts a proton to form H2.   In the two Fe hydrogenases, the geometry of the coordinating ligands distorts the bond between the two iron centers, leading to irons with different oxidation numbers.  Electrons appear to flow from one center to the other, as does carbon monoxide as well.  Ultimately, hydrogenases or small inorganic mimetics of the active site could be coated on electrodes and used to general H2 when placed in water in electrolytic experiments.


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