Kevin Ahern's BB 350 (Photosynthesis & Nitrogen I) 2014 - #36

1. Contact me at [email protected] / Friend me on Facebook (kevin.g.ahern) 2. Download my free biochemistry book at http://biochem.science.oregonstate.ed... 3. Take my free iTunes U course at https://itunes.apple.com/us/course/bi... 4. Check out my free book for pre-meds at http://biochem.science.oregonstate.ed... 5. Lecturio videos for medical students - https://www.lecturio.com/medical-cour... 6. Course video channel at http://www.youtube.com/user/oharow/vi... 7. Check out all of my free workshops at http://oregonstate.edu/dept/biochem/a... 8. Check out my Metabolic Melodies at http://www.davincipress.com/ 9. My courses can be taken for credit (wherever you live) via OSU's ecampus. For details, see http://ecampus.oregonstate.edu/soc/ec... 10. Course materials at http://oregonstate.edu/instruct/bb350 1. The dark reactions of photosynthesis are also known as the Calvin cycle in honor of their discoverer, Melvin Calvin. 2. In the dark reactions, carbon dioxide is removed from the atmosphere in a process called fixation. Carbon dioxide is first covalently attached to ribulose1,5bisphosphate (Ri1,5BP) to form a six carbon intermediate that immediately breaks down to from two molecules of 3-phosphoglycerate (3-PG) for each molecule of carbon dioxide combined with Ru1,5BP. This reaction is catalyzed by the enzyme with the acronym RuBisco. 3. 3-PG is an intermediate in glycolysis and gluconeogenesis. Phosphorylation and reduction of it leads to glyceraldehyde-3-phosphate (G3P). 4. You do NOT need to know the individual reactions of the Calvin cycle, except for the first one I described where carbon dioxide is attached to Ri1,5BP. 5. Rubisco is a relatively inefficient enzyme. When it encounters molecular oxygen instead of CO2, it catalyzes a reaction that ultimately breaks ribulose 1,5 bisphosphate into 3-PG and glyoxylate. Thus, the glyoxylate cycle in plants makes a lot of sense. 6. The pathway described to this point is that taken by plants known as C3 plants, by virtue of the fact that the first molecule made after fixation of carbon dioxide has three carbons. Another group of plants, known as C4 plants, fixes carbon dioxide to PEP and forms (surprise!) a four carbon molecule, oxaloacetate. Ultimately, this four carbon molecule is decarboxylated in another part of the plant, releasing carbon dioxide once again. This newly released carbon dioxide enters the Calvin cycle (same reactions here as C3 plants). This unique delivery system allows C4 plants to efficiently deliver CO2 to where it is used and allows them to avoid water loss. It may also allow them to be more efficient in that the rubisco reaction is occurring in the plant at a location where the oxygen concentration is lower than it is where the rubisco reaction occurs in C3 plants. Highlights Nitrogen Metabolism 1. Reduction of nitrogen to a form useful for organisms (called nitrogen fixation) is a difficult and energetically costly process. Nitrogen fixation is made possible by bacteria that have an enzyme known as the nitrogenase complex. 2. In nitrogen fixation, N2 is reduced to ammonium ion (NH4+). The process theoretically requires six electrons and 12 ATPs to make two ammonia molecules (NH3), but hydrogen gas is always produced, so two additional electrons and 4ATPs are also needed (giving a total of 8 electrons and 16 ATPs) . 3. Once reduced to ammonium ion, nitrogen can readily be incorporated into the amino acids glutamate, and glutamine. 4. Amino acids are grouped into families corresponding to the precursors they are made from. You should know the amino acids that are simple transamination products - glutamate (made from alpha-ketoglutarate), glutamine (made from glutamate), aspartate (made from oxaloacetate), alanine (made from pyruvate), asparagine (made from aspartate). 5. Transamination reactions are catalyzed by enzyme that use pyridoxal phosphate as a coenzyme. As a class, they are called 'transaminases.' Note that transamination reactions require an amine donor and an amine recipient (equivalent to reduction/oxidation reactions that require electron donors and recipients). 6. Synthesis of amino acids occurs in "families" in which groups of amino acids have common synthesis routes. 7. Amino acid metabolism has intimate ties to the citric acid cycle.