Page 36 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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

124 Bioinorganic Chemistry

Porphyrins have two properties that are essential for the
proper functioning of the cofactor. First, the four pyrrole-
type nitrogen donors are perfectly designed to bind iron
either in high-spin or low-spin electronic states. High-spin
Fe(II) has unpaired electrons that can interact favorably
with paramagnetic molecules such as O 2 to form bonds.
Low-spinFe(II)oreitherspinstateforFe(III)willnotreact
with O 2 . In contrast, electron transfer reactions occur most
easily when iron is in the low-spin electronic configura-
tion. Second, the electron donor capacity of the porphyrin,
in conjunction with the types of proximal and distal li-
gands, specifies whether the heme cofactor will be used for
oxygen transport or electron transfer, or to form a cation
radical. In iron(III) porphyrins, substrates such as hydro-
gen peroxide can simultaneously oxidize both the metal
+•
and the porphyrin to form an Fe(IV) O(porphyrin ).
This highly oxidizing state can insert an oxygen atom into
a carbon–hydrogen bond to form epoxides and alcohols.
In modified porphyrins such as chlorophylls, this ox-
idation can be driven by the absorption of solar energy.
Chlorophylls harvest light energy and channel it for use
in photosynthesis. Chlorophylls are closely related to por-
phyrins. In the case of chlorophyll, however, the metal is
magnesium and the ligand includes a reduced and modi-
FIGURE 4 Amino acid ligands.
fied porphyrin. In photohsystems I and II, chlorophylls form
weak dimers, which is one way plants control absorption
iron–sulfur, and iron–alternate metal–sulfur clusters, of the proper solar radiation. The chlorophyll shown in
the nickel-containing factor F 430 , chlorophylls, and the Fig. 5 contains a long alkane chain that helps it associate
cobalt–corrin structure of vitamin B 12 . In addition, for with the membrane within a chloroplast.
structures of significant importance, an independent pre- The cobalt center of vitamin B 12 and coenzyme B 12 is
sentation of the specifics of their structures is warranted. also similar to a heme. In this case, cobalt is the metal and
Among these are the iron–molybdenum cofactor respon- a corrin is the aromatic ligand. A corrin differs from a por-
sible for nitrogen fixation and the cobalt–corrin structure phyrin in two important respects. First, one “meso carbon”
of vitamin B 12 and coenzyme B 12 . Several of the more that joins the A and D rings of the porphyrin is removed.
important bioinorganic cofactors (Fig. 5) will first be dis- This alters both the aromaticity of the ring and the size
cussed independently of the specific proteins or enzymes of the metal-binding cavity. Second, a benzimidazole nu-
that contain them. cleotide linked to the corrin ring can act as the proximal
A heme cofactor contains iron bound to an aromatic ligand (in another enzyme, methionine synthase, the prox-
organic molecule called a porphyrin. This cofactor is the imal ligand is replaced by a histidine). Cobalt-containing
most ubiquitous of the metal cofactors. Heme function in corrinsaredesignatedcobalamins.OneformsvitaminB 12 ,
proteins ranges from electron transfer, to oxygen binding cyanocobalamine, when cyanide binds as the distal ligand
and transport, to oxygen activation and oxidation of or- (R group in Fig. 5). Other important forms of cobalamins
ganic molecules; functions also include sensing O 2 and are methylcobalamin (R = CH 3 ), which is used to trans-
CO levels in certain microorganisms. Hemes have been fer methyl groups (e.g., in methionine biosynthesis), and
known to contain iron in the +2, +3, and +4 oxidation adenosylcobalamin, which uses a radical mechanism to
states. The porphyrin moiety binds the iron in a four- isomerize small organic substrates (e.g., in glutamate mu-
coordinate fashion, leaving available two open coordina- tase). The B 12 cofactors were the first biological molecules
tion sites. One of these sites is almost always bound by an recognized to form metal–carbon bonds.
amino acid ligand (histidine, serine, cysteine, etc.), which Iron–sulfur centers are second in the list of most-diverse
is designated the proximal ligand. The sixth site, known inorganic cofactors. Iron–sulfur centers are used in elec-
as the distal ligand, is either bound to another amino acid tron transfer and to carry out chemical modifications. They
ligand or an exogenous ligand such as water, or left open canalsobeemployedasstructuralelementsthathelpstabi-
to bind substrate. lize protein structure. The four simplest structures of these