Biochemistry: The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

How electrons meet oxygen, how ATP is generated in the process, and other related phenomena.
Hemoproteins
Functions of hemoproteins

Heme is found in all organisms except for anaerobic clostridia and lactic acid Bacteria. Lactic acid bacteria are notable for apparently containing no iron at all, whereas clostridia are rich in non-heme iron compounds. Hemoproteins in the Blood reversibly bind oxygen, whereas in terminal oxidase systems, as well as in hydroxylases and oxygenases, hemoproteins "activate" oxygen, making it capable of reacting with carbon compounds or hydrogen. Other heme compounds catalyze reactions not with O2, but with H2O2. These include peroxidases and catalases. Another group consists of heme Enzymes whose sole function is electron transfer.

As Ingram noted [4], "bathing in 20% oxygen, we forget how reactive it is." From a thermodynamic standpoint, all Living matter is exceptionally unstable in terms of its potential to burn in oxygen. However, this requires a high Temperature, so with careful handling of fire we can avoid catastrophe. Yet 1 mole of copper (in the form of suitable chelates) can catalyze the consumption of atmospheric oxygen present in an average-sized room within a second [4]. Thus, as biochemists, we are interested in the kinetic stability and reaction inertness of O2 on the one hand, and the ability of oxidizing enzymes to carry out rapid oxygen-involving reactions on the other.

One might expect that two oxygen atoms, each containing six valence electrons, would form a double-bond Structure upon binding to each other, comprising one σ- and one π-bond:

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In fact, it has long been known that the O2 molecule is paramagnetic and contains two unpaired electrons. Based on this, we must assign the structure shown below on the left to the O2 molecule. However, Pauling suggested that the molecular structure also significantly features the state shown on the right, with two three-electron bonds (each comprising one unpaired electron):

The oxygen molecule is very stable. It is only with great difficulty that it accepts an additional electron to form the reactive superoxide anion radical O-2:

Therefore, oxidative attack by the O2 molecule proceeds slowly. However, once the first electron is attached, The addition of subsequent electrons is facilitated, and further reduction proceeds readily. In this regard, a crucial question arises in biochemistry: why are some hemoproteins capable of reversibly binding the O2 molecule without oxidizing the iron they contain, while others activate oxygen, enabling it to react with substrates?"



Last update: 06/08/2026

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