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5/22/06 FERMENTATION: an anaerobic biological reaction process in which a reduced organic compound (like glucose) acts as an electron donor and another organic compound acts as an electron acceptor What are the products of fermentation in yeast? 1 Commercially Valuable fermentation reactions: Alcoholic fermentation by yeast used in brewing and winemaking Bacteria also can carry out alcoholic fermentation under anaerobic conditions 2 The three common metabolic fates of pyruvate generated by glycolysis: 1. Under aerobic conditions, the pyruvate is completely oxidized via the citric acid cycle to CO2 and H2O [NADH acts as a high energy compound] 2. Under anaerobic conditions, alcoholic fermentation occurs in yeast: pyruvate is converted to ethanol + CO2 [free energy of NADH oxidation is dissipated as heat] 3. Under transient anaerobic conditions in muscle cells, pyruvate is reduced to lactate [free energy of NADH oxidation is dissipated as heat] 3 fermentation: catabolic reactions producing ATP in which organic compounds serve as both the primary electron donor and ultimate electron acceptor and ATP is produced by substrate level phosophorylation Note: fermentation is extremely inefficient compared to aerobic respiration. From first principles: Why? 4 The potential energy drop between glucose and an electron acceptor like pyruvate is a fraction of the potential energy drop that occurs during cellular respiration where O2 is the electron acceptor See ATP accounting table in previous lecture.... 5 Lactic acid fermentation: this type of fermentation occurs in some fungi and bacteria (used to make yogurt and cheese) and in human muscle cells when oxygen is limiting [Note, no CO2 is produced in this type of fermentation] What does your textbook say about lactic acid and muscle function? What did the New York Time's article say about lactic acid and muscle function? 6 * From a UC Berkeley web site -- caption is obviously written by someone not terribly familiar with basic chemistry Seems like there is some problems with the presentation and interpretation Think of parallels wih pyruvate dehydrogenase... This rat muscle cell has been labeled with a red dye (left) that highlights an enzyme, lactate dehydrogenase, that catalyzes the conversion of lactate to energy*, and a green dye (center) highlighting an enzyme, cytochrome oxidase, where oxygen is burned*. The colors overlap to glow yellow exactly at the membrane of the mitochondria, demonstrating that lactate is burned for energy on the inside of the mitochondrial membrane of muscle cells. (Brooks lab, UC Berkeley) 7 Control of Metabolic Processes in the Cell The cell has an elaborate interlocking system of feedback controls that coordinate the rates of glycolysis, fatty acid breakdown, the Krebs (citric acid cycle) and electron transport As a result of many control mechanisms, the body oxidizes fats and sugars 5-10 times more rapidly during a period of strenuous exercise than during a period of rest 8 How do cells regulate these complex reaction sequences? 9 The cell controls its metabolic activities by controling the • The activity of enzymes catalyzing key reactions • the transport of molecules from one cellular compartment to another How could the activity of an enzyme be modulated upwards or downwards? Many possibilites here 10 Allosteric Interaction: a regulatory mechanim in which a small molecule binds noncovalently to a protein and alters its activity by altering its shape Important ponts: • the small molecule is called an effector or a modulator • if the allosteric interaction involves an enzyme the modulator binds at a site other than the active site • the modulator can activate or inactivate a protein allo = other stere = solid or shape 11 End product inhibition is one example of allostery (see also pgs. 156-157 of Campbell) • Feedback inhibition regulates the flow through this metabolic pathway. The end product Z binds to and inhibits (lowers the activity of) the enzyme that catalyses the B to X reaction. In this way Z controls its own concentration. • Thus when quantities of the final product of a metabolic pathway begin to accumulate, the product binds to the first enzyme (unique to its synthesis) and slows down its catalytic action. • This is called negative regulation 12 Enzymes are also subject to positive regulation whereby the enzyme's catalytic rate is increased rather than decreased by the regulatory molecule Examine the Krebs cycle at this web site—which is labelled Detailed info on each Krebs cycle Reaction on 205 home page: http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/krebscyc/krebscyc.htm Which reactions are regulated? Look at isocitrate dehydrogenase and α ketoglutarate dehydrogenase What compounds act as negative regulators? What compounds act as positive regulators? 13 How else could the activity of an enzyme be regulated? • In bacteria, the trp repressor protein inhibits the transcription of a suite of genes coding for enyzmes required for the synthesis of the amino acid tryptophan • In the absence of tryptophan, the recognition helices are not in the proper orientation to contact the promoter DNA; no repressor binds and the genes coding for the enzymes are transcribed • If tryptophan levels are high, it binds to the repressor protein and causes a subtle shift in the orientation of the recognition helices; now the protein binds the promoter DNA with high affinity NOTE: this regulatory circuitry involves allosteric regulation of a regulatory protein (trp repressor) that controls tryptophan biosynthesis by controlling the level of production of the enzymes required for the biosynthesis of this amino acid 14