Manganese-Oxygen Generation and Detoxification - Biological Inorganic Chemistry - page 311

Chapter 16

Manganese e Oxygen Generation
and Detoxification

Introduction: Mn Chemistry and Biochemistry 311

Photosynthetic Oxidation of Water e Oxygen Evolution 311

Mn 2D and Detoxification of Oxygen Free Radicals

314

Nonredox di-Mn Enzymes e Arginase 317

INTRODUCTION: MN CHEMISTRY AND BIOCHEMISTRY
Manganese has access to three oxidation states of relevance to biology, Mn(II), Mn(III), and Mn(IV). A major
differencewith other redox active metals, like iron, is that manganese has less reducing potential than iron under most
biological conditions. Whereas Fe 3 þ is stabilised with respect to Fe 2 þ ,Mn 2 þ is stabilised relative to Mn 3 þ e this is

because, in both cases, the half-filled d 5 shell of both Fe 3 þ andMn 2 þ confers thermodynamic stability. Two important
consequences of this redox chemistry are that, not surprisingly, Mn 2 þ can participate in useful redox catalysis on
many similar substrates to Fe 3 þ , whereas the higher redox potential of Mn 2 þ makes free Mn 2 þ innocuous under
conditions where free Fe 2 þ would wreak havoc through the generation of hydroxyl radicals. This means that cells
(notably bacterial cells) can tolerate very high cytoplasmic concentrations of Mn 2 þ with no negative consequences,
which is certainly not the case with other biologically important redox metal ions, like iron and copper.
The other property of Mn 2 þ which has important biochemical consequences is that it is a close, but not exact,
surrogate of Mg 2 þ . As we saw in Chapter 10, Mg 2 þ is confined to a strict octahedral coordination geometry, with
ligand bond angles close to 90 , making it an ideal 'structural' cation, particularly for phosphorylated biological
molecules. Mn 2 þ , with its relatively similar ionic radius, readily exchanges with Mg 2 þ in most structural envi-
ronments and exhibits much of the same labile, octahedral coordination chemistry. However, since Mn 2 þ -ligand
bonds are generally much more flexible than Mg 2 þ -ligand bonds, when Mn 2 þ replaces Mg 2 þ in a catalytic envi-
ronment, its flexibility is better at lowering the activation energy. It can more easily accommodate the distortions in
coordination geometry in progressing from the substrate-bound to the transition state and from there to the bound
product. Thus, substituting Mn 2 þ in the active site of a Mn 2 þ -enzyme often results in improved enzyme efficacy.
The major role of manganese in biology is in oxygen production by photosynthetic plants, algae, and
cyanobacteria. It is also involved in a number of mammalian enzymes like arginase and mitochondrial superoxide
dismutase and it also plays an important role in microbial metabolism. Most of manganese biochemistry can be
explained on the one hand by its redox activity and on the other by its analogy to Mg 2 þ .

PHOTOSYNTHETIC OXIDATION OF WATER e OXYGEN EVOLUTION
Somewhere around 2.5Ga 1 ago, an enzyme activity emerged which dramatically changed the chemical compo-
sition of the earth's atmosphere forever, resulting in a veritable explosion of biological activity. The enzyme,

1. Geologists use the designations of Ga for a billion years before present time (“G” stands for “giga”) and Ma for million years before.