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.


This class of enzymes does not incorporate dioxygen into an organic substrate. Rather it accepts electrons released from an organic substrate, through intemediate electron carriers (such as ubiquinone and cytochrome C) to form superoxide (as in NADPH-oxidase), hydrogen peroxide (as in xanthine oxidase) or water (as in cytochrome C oxidase). The mechanism of cytochrome C oxidase again supports our expectations about enzymes that use dioxygen.  Dioxygen binds metals in the enzyme. One oxygen atom binds a heme Fe2+ of cytochrome a3 which is bound to the enzyme, while the other binds a Cu1+ of Cu B.   All oxygyen reduction intermediates  remain bound to the enzyme.  Four electrons are added from four different cytochrome C molecules, which serve as mobile carriers of electrons.

Jmol: Updated Cytochrome C Oxidase   Jmol14 (Java) |  JSMol  (HTML5) 

Figure:  Oxidases:  Examples

Another example of an oxidase is monoamine oxidase.   Mitochondrial monoamine oxidase catalyzes the oxidative deamination of certain neurotransmitters after they have been taken up by post-synaptic neurons, in a process of inactivation.  A reaction is shown below.

Figure:  Monoamine oxidases

A Schiff base is formed which is then hydrolyzed, incorporating unlabeled oxygen into the oxidized molecule.


In the previous chapter section, we discussed the progressive oxidation of methane by 2 electron loses to form methanol, formaldehyde, formic acid and CO2, with a progressive increase in oxidation number for the C by +2 (from -4 in methane to +4 in CO2).

Methanotrophs are aerobic bacteria that use methane as a source of energy, converting it in a series of two electron oxidations as shown above,  to carbon dioxide. The enzymes involved in this sequential process are methane monooxygenase, methanol dehydrogenase, formaldehyde dehydrogenase, and formate dehydrogenase.   Methane monoxygenase exists in a soluble and membrane form, both of which are part of a larger complex.  Both have a hydoxylase (which uses dioxygen to add O to methane) and the membrane form has recently been shown to be associated with have methanol dehydrogenase in a larger complex consisting of trimers of each enzyme (the hydroxylase and the dehydrogenase).   


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