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Encyclopedia of Physical Science and Technology EN002F-55 May 22, 2001 21:6

Bioinorganic Chemistry 133

I. Nucleotide Metabolism teine closer to the substrate in much the same way as Type I
and II enzymes.
The reduction of ribonucleic acids (RNAs) to form de-
Because RNA is a precursor in the formation of DNA,
oxyribonucleic acids (DNAs) is the first committed step
RRs seemingly are a prerequisite of DNA evolution. On
in the biosynthesis of DNA:
early earth, oxygen was sparse. Therefore, aerobic reduc-
tases are not good candidates for this role. An understand-
HO B HO B
5′ ing of how Type III reductases work will lead to an under-
O O standing of how DNA evolved.
4′ 3′ 2′ 1′
OH OH OH H III. BIOMINERALIZATION
RNA DNA
A. Overview
The process involves the removal of a single oxygen atom Biomineralization is the most glaring example of the mis-
from the ribose ring. The mechanism by which this occurs nomer of “bioinorganic” chemistry. It encompasses the
is initiated by the removal of a hydrogen atom from the 3 formation of largely inorganic minerals by the processes
position of the ring. This mechanism requires the forma- of life. Examples of biomineralization are the formation of
tion of a radical (an unpaired electron) in the interior of calcium phosphate to create bones for structure, calcium
a protein. Radicals are very unstable and require care in carbonate as protective shells, iron oxide to store iron in
their formation. The enzymes that carry out this reaction animal cells, and the formation of magnetite as orienta-
are the ribonucleotide reductases (RRs). There are three tional materials in magnetobacterial cells (Table III). All
classes of RRs. There are two components to the struc- of these examples demonstrate the precise control of min-
ture of RRs in all three classes. The first component is eral size, structure, shape, orientation, and organization
a metal-containing unit responsible for the generation of that chemists strive for in the development of novel meth-
radicals. The second is responsible for substrate binding ods for material syntheses.
and catalyzing the reaction.
Type I RRs are found in all eukaryotes and in some B. Four Steps to Biominerals
prokaryotes and require oxygen. Their radical-generating
component consists of a di-iron center and a tyrosine. The Organisms generally produce biominerals following a ba-
di-iron center is similar but not identical to that found sic four-step process. This process includes supramolec-
ular preorganization, interfacial molecular recognition,
in hemerythrin. In the resting state, the two irons are both
vectorial regulation, and cellular processing.
Fe(II). One of the irons is coordinated to a histidine, an as-
partate, and a water molecule. A glutamate and oxo bridge
1. Supramolecular preorganization requires the con-
both iron atoms. The remaining ligands around the second
struction of an organized reaction environment prior to
iron are a histidine and two glutamates. In the presence of
the actual mineralization event. In general this involves
oxygen, the di-iron center is oxidized to an Fe(III)–Fe(III)
the self-assembly of lipid vesicles that provide an enclosed
center and a tyrosine radical is generated. This tyrosine
space for mineralization. Sometimes a protein construct is
can oxidize a cysteine near the substrate. The resulting
made for encapsulating the mineral. The latter is the case
thiyl radical is directly responsible for the abstraction of a
of ferritin, the iron storage protein in mammals. Several
hydrogen atom from the 3 position of the ribose ring that

ultimately leads to the formation of DNA.
The remaining two types of RRs are less well under- TABLE III Examples of Biominerals
stood than the Type I reductases. Type II RRs require an Mineral Formula Function
adenosylcobalamine cofactor (R is adenosyl in Fig. 5),
coenzyme B 12 , for catalysis. They are found in both aer- Calcium carbonate CaCO 3 Algae exoskeletons,
obic and anaerobic bacteria and archae. In an analogous calcium storage in plants
way to the Type I class, a radical is generated on the adeno- Calcium phosphate Ca 10 (PO 4 ) 6 (OH) 2 Endoskeletons, teeth,
calcium storage
syl moiety on the cofactor that is used to oxidize a cys-
Calcium oxalate CaC 2 O 4 Calcium storage
teine residue close to the 3 position on the ribose ring.

Barite BaSO 4 Gravity device
Type III RRs are found in strictly anaerobic microorgan-
Silica SiO 2 •nH 2 O Algae exoskeletons
isms. They contain an iron–sulfur cluster. This iron–sulfur
Magnetite Fe 3 O 4 Magnetotaxis
cluster along with adenosylmethionine aid in forming a
Ferrihydrite 5Fe 2 O 3 •9H 2 O Iron storage
glycine radical that is presumably used to oxidize a cys-