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38 - Monday Lecture 38 1 Reading problems as in LG 2 Last...

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Monday, November 29, 2010 Lecture 38 1. Reading & problems as in LG. 2. Last PyMOL assignment: G-proteins (the only PyMOL exercise for the final exam) 3. Please do fill out an online evaluation for this course. We appreciate the effort you make to do this. The prof reads every comment carefully, and takes your advice seriously. Monday's lecture: lipid biosynthesis: . synthesis of fat and phospholipid . cholesterol synthesis and transport Today The atherosclerotic "plaque" is actually an inflammatory state, a growth of cells caused by the body’s system for responding to inflammation. Medical doctors refer to it as a “lesion”. It is a misconception that cholesterol just builds up in a heap on the arterial wall, like deposits inside a water pipe. Instead, the arterial “lesion” is a kind of inflammation: white blood cells migrate to the site in the artery where LDL have been taken up. The white blood cells engulf the LDL, becoming so-called "foam cells" when they have taken in a lot of LDL. The foam cells send out signals to other cells in the immediate vicinity, such as muscle cells, to grow, to divide, and to migrate to this area. The result is a growing mass in the artery. It is a benign, and cannot metastasize. However, the most deadly aspect is that the surface can "crack" or otherwise become damaged, then a blood clot will form at that spot. Pieces of the clot can break off and travel in the blood until they lodge in a smaller artery, cutting off further blood flow. This is actually the most deadly aspect of atherosclerosis. We will talk about synthesis of nitrogen-containing molecules when we return from the break. Next we consider how living systems can "fix" N 2 gas into ammonium ions, then into N- containing compounds, LG page 272: Nitrogen Fixation : Only a few prokaryotes (termed diazotrophs ) can take atmospheric N 2 (gas) and reduce it ("fix" it) to NH 3 . Although few, these few species are abundant: 1. In the soil: Azotobacter, Klebsiella, Clostridium 2. In fresh and salt water: Cyanobacteria 3. Symbiotic with legumes: Rhizobia The overall reaction that occurs is the following: N 2 + 3H 2 2NH 3 G o ' = -34kJ/mol This reaction can be displayed on a free energy diagram, with the reactants at a higher free energy than the products. The negative value of G o ' tells us that the reaction is favorable, under standard conditions. But the transition state between the reactants and the product is incredibly high. This is because the bond between the two nitrogen atoms is a strong, triple bond. To break this takes a lot of energy: N N N + N H o = +942kJ/mol
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Compare this to H o for a typical C-C bond, about +350kJ/mol. The enzyme-catalyzed reaction of nitrogen fixation does NOT proceed along a pathway that involves "putting in" 942 kJ/mol to break the triple bond, and then adding H 2 .
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