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HEME:
You have recognized heme (or haeme), a
metalloprotein, as an important biochemical structure. It is, indeed,
present in hemoglobin, chlorophyll and cytochromes. It can be found in
mitochondria, chloroplasts, ER and other places in the cell. The
electrochemistry of the metal present in the protein is critical to its
function. The tremendous value is the ability to interact in
oxidation-reduction reactions and transport oxygen, both between tissues
and within cells. Redox reactions and the biological transfer of electrons
are essential for energy production in living organisms. Interest in the
structure of heme is the reason that myoglobin was the first protein whose
structure was described and the finding of tertiary and quaternary
structures. These configurations give the molecule three-dimensional shape
and allow the molecule to twist and turn to provide access to the heme
groups.
Hemes are present in every Kingdom, bacteria,
protists, fungi and plants and animal. The prevalence of this pathway
suggests that the gene that encodes the protein is ancient. The different
functions are probably new uses for a protein structure whose gene was
already present.
Biosynthesis of the heme of hemoglobin and
chlorophyll both start with the porphyrin ring structure. The pathways
diverge where the metal ion is added, iron for hemoglobin, magnesium for
chlorophyll. A mutation in the pathway to hemoglobin results in the
genetic disease porphyria
http://www.madsci.org/posts/archives/apr2000/956010537.Bc.r.html
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 The
molecule called heme consists of a complex of protoporphyrin IX
with Fe(II). Heme is the part of the
hemoglobin
and myoglobin
molecules that binds to oxygen.
Iron-containing proteins can hold Fe(II) in a number
of possible ways. Throughout the myoglobin-hemoglobin family, the iron is
chelated by a tetrapyrrole ring system called protoporphyrin IX, one of a
large class of porphyrin compounds. The basic ring structure of a
porphyrin is shown in
Figure 7.4a,
and protoporphyrin IX is shown in
Figure 7.4b.
Porphoryrins are also components of chlorophyll, the cytochrome proteins,
and some natural pigments. Like most compounds with large conjugated ring
systems, the porphyrins are strongly colored. The iron-porphyrin in
hemoglobin accounts for the red color of blood, and the
magnesium-porphyrin in
chlorophyll
is responsible for the green of plants.
Heme is noncovalently bonded in a hydrophobic
crevice in the myoglobin or hemoglobin molecule (see
Figure 7.3).
The binding of oxygen to heme is illustrated in
Figure 7.5,
which shows the oxygenated form of myoglobin. Ferrous iron is normally
octahedrally coordinated, which means it should have six ligands, or
binding groups, attached to it. As shown in
Figure 7.5a,
the nitrogen atoms of the porphyrin ring account for only four of these
ligands. Two remaining coordination sites are available, and they lie
along an axis perpendicular to the plane of the ring. In both the
deoxygenated and the oxygenated forms of myoglobin, one of these sites is
occupied by the nitrogen of
histidine
residue number 93.
http://www.aw-bc.com/mathews/ch07/heme.htm
CHLOROPHYLL
 Chlorophyll
a and chlorophyll b are the most abundant plant pigments.
Chlorophylls a and b are found in higher plants and algae.
Bacteriochlorophylls differ slightly in structure. Chlorophylls a
and b are related to the protoporphyrin IX found in
hemoglobin
and myoglobin.
However, the bound metal in the chlorophylls is Mg2+
rather than Fe2+. In
Figure 17.7c,
the accessory pigments
 -carotene
and phycocyanin are also shown.
All of these molecules absorb light in the visible
region of the spectrum because they have large conjugated double-bond
systems. Because chlorophylls a and b absorb strongly in
both the deep blue and red, the light that is not absorbed but reflected
from chloroplasts is green, the color we associate with most growing
plants. The other observed colors, such as the red, brown, or purple of
algae and photosynthetic bacteria, are accounted for by differing amounts
of accessory pigments. Loss of chlorophylls in autumn leaves allows the
colors of the accessory pigments, as well as nonphotosynthetic pigments,
to become evident. Some photosynthetic bacteria use pigments that absorb
wavelengths up to about 1000 nm, in the near infrared.
http://www.aw-bc.com/mathews/ch17/chloropa.htm
To capture the available light energy,
photosynthetic organisms have evolved a set of pigments that efficiently
absorb visible and near-infrared light. These pigments are sometimes
referred to as chromophores - compounds that absorb light of specific
wavelength. Structures of a few of the most important photosynthetic
chromophores are shown in
Figure 17.7.
Chlorophyll and some of the accessory
pigments are contained in the
thylakoid
membranes of the
chloroplast.
The assemblies of light-harvesting pigments in the thylakoid membrane,
together with their associated proteins, are organized into well-defined
photosystems, structural units dedicated to the task of absorbing light
photons and recovering some of their energy in a chemical form. The first
part of this process takes place in what are referred to as
light-harvesting complexes. Each is a multisubunit protein complex
containing multiple antenna pigment molecules (chlorophylls and
some accessory pigments) and a pair of chlorophyll molecules that
act as the reaction center, trapping energy quanta excited by the
absorption of light.
Most of the chlorophyll molecules are not
directly engaged in the photochemical process itself but act, instead, as
antenna molecules of the light-harvesting complexes. Antenna molecules
absorb photons, and the energy is passed by resonance transfer to specific
chlorophyll molecules in a relatively few reaction centers. In
other words, the energy of a photon absorbed by any antenna molecule in a
photosystem wanders about the system randomly (Figure
17.11). Eventually (meaning in about 10-10 s),
the energy finds its way to a chlorophyll molecule in the reaction
center. This molecule is like the other chlorophylls, but it is in
a somewhat different environment, so that its excited state energy level
is a bit lower. Thus, it acts as a trap for quanta of energy absorbed by
any of the other pigment molecules. It is the excitation of this reaction
center that begins the actual photochemistry of the light reactions, for
it starts a series of electron transfers.
http://www.aw-bc.com/mathews/ch17/chloroph.htm
HEMOCYANINS
Hemocyanins are the oxygen-transporting proteins in
arthropods and molluscs, the oxygen is bound by two copper atoms.
Spectroscopic studies on the active site show similarities to the active
site of a further group of copper-containing proteins, the tyrosinases.
Arthropodan and molluscan hemocyanins form high-molecular aggregates which
are markedly different in size and quaternary structure. There is only one
tertiary structure of an arthropodan hemocyanin available, but from
comparison of all amino acid sequences known so far from arthropodan
hemocyanins, a common tertiary structure for all arthropodan hemocyanins
can be deduced. Again, sequence comparison allows the construction of an
evolutionary tree for some oxygen-binding copper proteins.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2664531&dopt=Abstract
HEMERYTHRIN
Hemerythrin is not, as the name might suggest, a
heme. The
word "heme" was at first applied to any oxygen-carrying proteins, such as
hemoglobin, and when hemoglobin was analyzed, the heme structure was
named for the iron-porphyrin cofactor in the transport protein (globin
being a general term for a globular protein). Later, after hemerythrin was
named, it was discovered that not every oxygen-carrying protein has a heme
prosthetic group.
As it turns out the oxygen binding site is a
binuclear iron center. The iron atoms are coordinated to the protein
through the carboxylate side chains of a glutamate and aspartate, and five
histidine peptides. When oxygenated the iron atoms are bridged by a μ-oxo
bridge. When deoxygenated the iron atoms are bridged by a hydroxyl group.
When binding a oxygen molecule the hydrogen atom from the hydroxyl bridged
moves over onto the bound ligand. When releasing the oxygen molecule, the
mu-oxo bridge retrieves the hydrogen atom and returns to a hydroxyl group.
Thus the following:
Fe-OH-Fe + O2 <-> Fe-O-Fe-O2 +
H+
Hemerythrin affinity for
Carbon Monoxide (CO) is actually lower then its affinity for Oxygen (O2)
(unlike hemoglobin which has a very high affinity for CO) making
Hemerythrin immune to CO poisoning.
http://en.wikipedia.org/wiki/Hemerythrin
CHLOROCRUORIN
Accurate oxygen equilibrium curves of chlorocruorin
of a marine polychaete annelid, Potamilla leptochaeta, were
determined under a variety of experimental conditions. Like
chlorocruorins from other species Potamilla chlorocruorin
exhibited a low oxygen affinity, a large Bohr effect, and high
cooperativity compared to those of human hemoglobin. However, in
contrast to chlorocruorins from other species, the shape of the
oxygen equilibrium curve for Potamilla chlorocruorin varied
dramatically upon changes of pH or temperature. As observed in
hemocyanins and annelid hemoglobins, cations, especially
divalent ones such as Mg2+ and Ca2+, caused marked increase in
oxygen affinity and cooperativity of Potamilla chlorocruorin.
This finding together with the determination of cations in
Potamilla blood has made clear the physiological role of chlorocruorin as
an oxygen carrier. A graphical analysis based on the
Monod-Wyman-Changeux allosteric model indicated that the number
of sites for oxygen binding involved in heme-heme interactions
is six, defining the functional unit of chlorocruorin molecule.
http://content.febsjournal.org/cgi/content/abstract/147/3/453
TUNICATES (SEA SQUIRTS) AND
VANADIUM
The tunicate Styela plicata is, in the
eyes of most people, a singularly unattractive organism. They are often
found on fouling assemblages on the pilings of piers (and, in some
instances, perhaps, even on peers). In the raw, that is, when not covered
over by all sorts of surrounding organisms, they look like warty, pink
chunks of brain ranging in size from about an inch to the size of a human
fist. Styela is sessile organism, that is, it spends its life
attached to a substrate, except for a brief period as a larva.
Some ascidians perform a remarkable bit of chemistry
and as a consequence have green blood (just like all those aliens in the
old sci-fi movies). These tunicates concentrate vanadium, a rather
rare element, which they extract from the surrounding water.
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjan03/rhstyela.html
The use of Vanabins and Vanadium for oxygen
transport in Ascidians and Tunicates is doubtful. Another hypothesized
reason for these organisms collecting vanadium is to make themselves toxic
to predators, parasites and microorganisms. At present there is no
conclusive understanding of why these organisms collect Vanadium and it
remains a biological mystery.
http://en.wikipedia.org/wiki/Vanabins
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All original material copyright John J.
Emerson
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