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    Respiratory Physiology, Myoglobin and Hemoglobin Structure and Function

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    Hemoglobin & Transport of O2 & CO2

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    1. Define the following: prosthetic group, fractional saturation, allosterism, cooperativity, Bohr effect, isohydric carriage, respiratory acidosis & alkalosis, metabolic acidosis & alkalosis. Please provide examples.

    2. Describe the interactions (e.g., H-bonding) that are involved in the tight binding of heme to apomyoglobin?

    3. Describe Myoglobin's structure (include comments on its primary, secondary, tertiary and quaternary structure).

    4. What are the major similarities and differences in structure and properties between myoglobin and hemoglobin? Provide descriptions.

    5. What are the structural & functional aspects of the effect of H+ on the O2 binding of hemoglobin? Include any pKa differences, at least one example of a specific functional group involved, etc. What are the consequences of this binding? How does it effect the body in the lungs and in the periphery?

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    SOLUTION This solution is FREE courtesy of BrainMass!

    Hello, here are the responses to your assignment. I don't have access to your exact text book, but you might want to throw some references in there that site your text book.

    1. Prosthetic group: these are tightly bound molecules that attach themselves to proteins that are required for some proteins to 'work' within the body. For example, vitamins are prosthetic groups that attach themselves through covalent bonds to some enzymes at the active sites to promote function of the enzyme. Fractional saturation: the amount of available sites on a protein molecule that are actually saturated with a ligand. For example, if a protein has X for the total potential ligand bonds, and Y for the actual sites that are bonded, y/x= the fractional saturation of the protein. Allosterism is when a molecule binds to a site on an enzyme other than the active site, thus changing the shape of the enzyme and disallowing it to bind to its ligand, potentially altering the function of this enzyme. Cooperativity is when an enzyme with multiple ligand binding sites has one ligand bind to it, and then increases or decreases the enzymes affinity for other ligand bonding. The Bohr effect can be defined as the relationship of acidity and concentration of carbon dioxide to hemoglobin's affinity to oxygen. Increasing the bloods concentration with carbon dioxide (lowering the pH and thus increasing acidity) will result in the loss of oxygen from the hemoglobin molecule. The inverse is also true. Isohydric carriage refers to hemoglobins ability to bond with positively charged hydrogen atoms without a change in pH occurring. This typically occurs when hemoglobin carries carbonic anhydrase without changing the pH of blood. Respiratory acidosis occurs when the lungs are incapable of getting rid of the carbon dioxide produced by the body. This causes the blood to become acidic, and is respiratory in origin. An example of a condition that causes this is chronic obstructive pulmonary disease. Respiratory alkalosis is caused by a high respiratory rate which decreases the level of carbon dioxide, rising the pH and causing an alkalotic state. Any disease state that results in a high respiratory rate, even anxiety, can cause respiratory alkalosis. Metabolic acidosis is when the body produces too much carbon dioxide, or acids, resulting in a decreased pH in the blood. One example of this is diabetic ketoacidosis, where the body produces an acidic substance called ketones, which make the blood acidotic. Metabolic alkalosis is caused by having too much bicarbonate (or alkaline) substance within the blood. Usually caused by vomiting or kidney disease, metabolic alkalosis is also the result of severe electrolyte disturbances for example having a loss of chloride or potassium.

    2. Apomyoglobin is a myoglobin molecule, without a heme unit. The heme unit is responsible for oxygen binding, and without the heme unit, a myoglobin is functionless in terms of oxygen transport. The heme unit is bonded to the apomyoglobin through several hydrogen bonds with nearby surface amino acid chains. These several hydrogen bonds result in the folding mechanism that stabilizes the myoglobin unit. These bonds are hydrophobic and contribute to the stabilization of the heme-myoglobin unit. In addition, a nitrogen atom from a histidine R group located above the plane of the heme ring is coordinated with the iron atom further stabilizing the bond between the heme and the protein.

    3. Myoglobin is a protein with eight alpha helices connected by loops. Human myoglobin contains 154 different chains of amino acids. At the center is a polymorphin ring with an iron. Each myoglobin unit contains one heme prosthetic group, and is folded in several places around its heme group. The tertiary structure of myoglobin is that of a typical water soluble globular protein. Its secondary structure is unusual in that it contains a very high proportion (75%) of α-helical secondary structure. A myoglobin polypeptide is comprised of 8 separate right handed α-helices, designated A through H, that are connected by short non helical regions. Amino acid R-groups packed into the interior of the molecule are predominantly hydrophobic in character while those exposed on the surface of the molecule are generally hydrophilic, thus making the molecule relatively water soluble. ****PLEASE NOTE THIS IS DIRECTLY FROM THE INTERNET AND WILL NEED A SOURCE, LIKELY YOUR TEXTBOOK***

    4. Similarities between hemoglobin and myoglobin: both have a prosthetic group for heme, both have an alpha helical secondary structure and they are both made of proteins with hydrogen bonds existing throughout. Hemoglobin has four sites for a heme and iron, which bind to oxygen and myoglobin only has one. Both myoglobin and hemoglobin are responsible for oxygen transport, however myoglobin is saturated with oxygen much faster than hemoglobin. Myoglobin can be fully oxygen saturated in the tissues, but hemoglobin requires much higher oxygen tension to become fully saturated which only occurs in the lungs.

    5. H+ has negative effect on the affinity of hemoglobin binding to oxygen. The proton helps hemoglobin release its oxygen in the capillaries, thus delivering oxygen to tissues. In general the lower a pKa value, the lower the pH. This is extremely important in understanding the relationship between acidosis and oxygen saturation. For example, bicarb (HCO3-) has a pKa value of 10.2. Consequences of this binding result in an acidotic state which decreases oxygens ability to bind to hemoglobin in the lungs and periphery. Decreased oxygen in the periphery and in the lungs can cause permanent tissue and organ damage.

    I really hope this doesn't confuse you, I tried to be as clear as possible. Good luck!!

    Here is a really good reference with pictures too: http://themedicalbiochemistrypage.org/hemoglobin-myoglobin.php
    This too: http://home.sandiego.edu/~josephprovost/Chem331%20Lect%208_9%20Myo%20Hemoglobin.pdf

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