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One of the most important aspects of animal metabolism involves the acquisition, storage and utilization of oxygen. Myoglobins (Mb ) and hemoglobins (Hb) are proteins designed to store oxygen and deliver O2 to cells as needed. In addition hemoglobin provdes a pathway for the removal of CO2.
Myoglobin is a protein found in virtually all higher animals whose function is oxygen storage, meaning that oxygen is bound revresibly and given up on demand. The three dimesional structure of myoglobin reveals a molecule ideally suited to the task of binding oxygen without itself undergoing irreversible oxidation. The tertiary structure of myoglobin has the following characteristics :
allows for an interactive display of the ogygenated form of sperm whale myoglobin.
The unique abilty of Mb and Hb to bind oxygen depends on the presence of a heme group. This prosthetic group consists of an Fe atom in its +2 oxidation state complexed with a protoporphyrin molecule. The central Fe(+2) atom and the four nitogen atoms of the porphyrin lie in what is termed the heme plane.
In myoglobin a histidine residue in the F helical segment, HIS 93, occupies a fifth cordination position. In oxymyoglobin the sixth cordination position is occupied by an oxygen molecule, giving an octahedral arrangement around the central iron atom. The oxygen atom is bent relative to the octahedral axis, and is positioned between the Fe atom and another histidine residue, HIS 64 (E helical section). In deoxymyoglobin this sixth coordination position is empty.
This geometric arrangement is shown below :
allows for an interactive display of the binding site and heme group of sperm whale oxymyoglobin.
A movie showing conformational change between the oxy- and deoxy- forms of the hemoglobin monomer can be viewed.
In larger animals the demands of aerobic metabolism require that the cells have a steady supply of oxygen and a means for removal of carbon dioxide, one of the metabolic waste products. Increased and more complex demands require a protein of greater complexity, and in erythrocytes this function is performed by hemoglobin.
Hemoglobin is a tetramer, comprised of four polypeptide subunits, two of one type (alpha) and two of another (beta). Thus hemoglobin exhibits quaternary strcture, with the subunits held together by non-covalent interactions. Each subunit has a primary, secondary and tertiary structure somewhat similar to that of myoglobin with each containing a heme group with a single oxygen-binding site.
The binding of oxygen in hemoglobin exhibits two features not seen in myoglobin and when taken together are said to be allosteric effects :
The mechanism of such allosteric effects involves conformational changes of the tetrameric structure, mediated by change in the non-covalent interactions between the subunits.
A movie showing conformational change between the oxy- and deoxy- forms of the hemoglobin tetramer can be viewed.
The immune system employs two main systems for antigen recognition :
as a call for help, from Thelper lymphocytes, if the protein is a class II MHC
or a signal for destruction, by Tkiller lymphocytes, if the protein is a class I MHC.
The following image is of the human class I histocompatibility antigen (HLA -A 0201) complexed with a nonameric peptide formed by the breakdown of HIV-1 GP120 envelope protein. Clicking on the peptide fragment will give a close-up image of the peptide and its binding by the MHC class 1 protein.
allows for an interactive display of the complex.
Note: This MHC class I protein represents a good example of quaternary structure. This protein is a dimeric complex with each monomer subunit composed of three domains. This complex is assymetric.
