Bioinorganic chemistry
Bioinorganic chemistry, the study of the functions of metals in biological systems,
Bioinorganic chemistry includes the study of both natural phenomena such as the behavior
of metalloproteins as well as artificially introduced metals, including those that are non-essential, in medicine and toxicology.
The following list shows typical bioactive substances containing metals.
1. Electron carriers. Fe: cytochrome, iron-sulfur protein. Cu: blue copper protein.
2. Metal storage compound. Fe: ferritin, transferrin. Zn: metallothionein.
3. Oxygen transportation agent. Fe: hemoglobin, myoglobin. Cu: hemocyanin.
4. Photosynthesis. Mg: chlorophyll.
5. Hydrolase. Zn: carboxypeptidase. Mg: aminopeptidase.
6. Oxidoreductase. Fe: oxygenase, hydrogenase. Fe, Mo: nitrogenase.
7. Isomerase. Fe: aconitase. Co: vitamin B12 coenzyme.
The basis of chemical reactions of metalloenzymes are
1. Coordinative activation (coordination form, electronic donating, steric effect),
2. Redox (metal oxidation state),
3. Information communication: biopolymers such as proteins,
Examples of actions of metals other than by metalloenzymes include
1. Mg: Mg-ATP energy transfer
2. Na/K: ion pumping,
3. Ca: transfer of hormone functions, muscle contraction, nerve transfer, blood coagulation,
are some of the important roles of metals.
Metalloporphyrin
Any compound, such as heme, formed by a combination of a porphyrin and a metal, often
iron, copper, silver, zinc, or magnesium.
Metalloporphyrins:
Porphyrins which are combined with a metal ion. The metal is bound equally to all four
nitrogen atoms of the pyrrole rings. They possess characteristic absorption spectra which can be utilized for identification or quantitative estimation of porphyrins and porphyrin-bound compounds.
Typically, M(Por) containing proteins have one metal coordination site (the proximal side
of the M(Por) plane) occupied by an electron donor ligand (N-, S- and O-) from an amino acid residue.
It is at the other axial coordination site (the distal side) where biologically important transformations involving small molecules such as NO or nitrite occur.
The important roles these tetrapyrrolic macrocycles play in vital biological processes, in particular photosynthesis (chlorophyll), oxygen transport (hemoglobin), oxygen activation (cytochrome), have led to their characterization as ‘pigments of life’.
Porphyrin
A porphyrin is a large ring molecule consisting of 4 pyrroles, which are smaller rings made
from 4 carbons and 1 nitrogen. These pyrrole molecules are connected together through a series of single and double bonds which forms the molecule into a large ring. The technical name for 4 pyrroles connected together is a tetrapyrrole. The ring is flat in space, and the distribution of electrons is fairly equal around the circumference of the ring. For this reason, a porphyrin is
considered an aromatic compound. This means that a porphyrin molecule is very stable.
The model of a general porphyrin is called porphin. This molecule is only rarely found in nature as an intermediate, but it is the base of all porphyrin molecules. Porphin can be seen below.
The blue parts of the molecule represent the aromatic ring which forms the base of
all porphyrin molecules. The black molecules and bonds will eventually be substituted for
complex side chains. These molecules will allow the cellular machinery to attach to and use the porphyrin. Porphyrins are also capable of absorbing certain wavelengths of light, especially when associated with different ions. Porphyrins cause both the red color of blood and the green color of plants, as discussed below.Porphyrin molecules serve a number of purposes in animals, plants, and even bacteria. A major use of porphyrin molecules in animals is in the construction of heme groups. These molecules are simply a porphyrin molecule with various side-chains substituted around the main ring. In a heme, the porphyrin ring serves an important function. The nitrogen molecules at the
center of the ring are capable of “hosting” an iron molecule. It is this porphyrin structure, holding iron, which gives blood its red color. While the nitrogen does not technically bind to the iron molecule, it is nonetheless held in place by the influence of the nitrogen molecules and their distribution in space. A general heme can be seen below.
Porphyrins in Animals
The general purpose of a heme is to transport oxygen. This can be seen in the above image.
When the oxygen is bound to the heme, it can be transported quickly around the body and through the cells. There is a specific protein associated with each part of the body which uses hemes to transport oxygen. The red blood cells have the protein hemoglobin, which holds the heme in place. Therefore, when carbon dioxide is high, cells need more oxygen. This mechanism of hemoglobin
allows the oxygen to be released in the correct parts of the body.
Porphyrins in Plants
Because the porphyrin molecules are evolutionarily conserved, we see the same iron-
holding heme porphyrins in plants as we do in animals. Plants also use the electron transport
chain to produce ATP, and similar porphyrins are used as those in animals.
However, plants have also mastered a different configuration of porphyrin molecule, which
allows them to capture the energy in sunlight. Chlorophyll is a special molecule designed around
a porphyrin base. Seen below, the chlorophyll molecule has several unique side-chains off of the
porphyrin molecule. It also has a really long side chain, seen coming off the bottom. These side-
chains slightly change the shape and configuration of the base porphyrin.
Other Porphyrins
Porphyrins have complex cyclic structures. All porphyrin compounds absorb light
intensely at or close to 410 nanometres.
Structurally, porphyrin consists of four pyrrole rings (five-membered closed structures containing one nitrogen and four carbon atoms) linked to each other by methine groups (―CH=). The iron atom is kept in the centre of the porphyrin ring by interaction with the four nitrogen atoms. The iron atom can combine with two other substituents; in oxyhemoglobin, one substituent is a histidine of the protein carrier, and the other is an oxygen molecule. In some heme proteins, the protein is also bound covalently to the side chains of porphyrin
Good effort
ReplyDelete