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Chapter 9. “Overview of metabolism” Chapter 10. “Glucose metabolism” (also known as glycolysis) The next few classes will be about metabolism. Next exam - Chapters 6 to 10

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Metabolism Metabolism is all the chemical reactions taking place in an organism Heterotrophs – use organic carbon as food (example = humans) Autotrophs – use inorganic carbon as food (examples = plants, some bacteria) Two subtypes of metabolism within cell Catabolism (complex ==> simple molecules) Anabolism (simple ==> complex molecules)

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Why would we want to make ATP? What’s it good for? One goal of metabolism is to make ATP. Useful energy is released when the phosphoanhydride bonds are cleaved.

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Hydrolysis of ATP releases energy. energy + +

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One thing that ATP is useful for is driving the conformational changes in the “motor protein” myosin that we met in Chapter 5.

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(the Na+/K+ transporter uses the energy released in ATP hydrolysis to move ions against a concentration gradient) Also, active transport requires energy, which may be provided by ATP hydrolysis. Active transport can work against a concentration gradient.

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There is something else that ATP can be used for: Energy released in ATP hydrolysis can be used to drive chemical reactions that would otherwise be thermodynamically unfavorable. Illustrate this important point with an example.

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(not spontaneous) Example: Cells need to produce glucose-6-phosphate from glucose. Why? Glucose-6-phosphate is a precursor for ribose, & other things.

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(not spontaneous) (spontaneous)

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+ ATP + (spontaneous) So the combined or “coupled” reaction is spontaneous, and cells are able to make glucose-6-phosphate by consuming ATP ! (not spontaneous)

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So inside cells, the enzyme “hexokinase” binds both glucose & ATP, and is able to produce glucose-6-phosphate by coupling two reactions. hexokinase

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Hopefully, I have convinced you that ATP is useful ! How do living things make ATP? One way to make ATP is through “glycolysis”.

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What are NAD+ and NADH ? And why are they in the reaction ??? Don’t be disturbed by the abbreviations. We know what these are: ADP, ATP Pi is inorganic phosphate: Glycolysis oxidizes glucose to pyruvate, and uses the energy released to make ATP. Overall glycolysis reaction:

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NAD+ NADH NAD+ is a “cofactor”

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Why is NAD+ needed ?

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Glucose gets oxidized to pyruvate. Something must get reduced! NAD+ gets reduced to NADH. Oxidation is the removal of electrons and/or removal of hydrogen and/or addition of oxygen. Reduction is the addition of electrons and/or addition of hydrogen and/or removal of oxygen. NAD+ is needed to accept the electrons and H liberated when glucose is oxidized. Glycolysis is an “oxidation-reduction” reaction:

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Why do some enzymes require cofactors ??? Amino acid side chains are NOT good at redox chemistry. So enzymes that catalyze oxidation/reduction reactions need cofactors that can be easily oxidized or reduced as needed.

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In NAD+, we just met a vitamin !

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Vitamin B3. Nicotinic acid (a precursor for NAD+). Also known as niacin. NAD+ is needed for glycolysis. nicotinic acid

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Glycolysis starts with a molecule with high energy content (glucose) and converts it to a molecule with lower energy content (pyruvate). The energy released by glucose converts ADP to ATP. Remember: ATP is a high energy molecule: energy can be released when needed elsewhere by converting ATP back to ADP. High energy content lower energy content Move muscles, move motor proteins, use in coupled reactions, etc.

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Where does glycolysis happen? In the cytosol ! cytosol Diagram of a prokaryotic cell

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Where does glycolysis happen? In the cytosol ! cytosol Diagram of a eukaryotic cell

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The glycolysis reaction:

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glucose 2 pyruvate Lots of steps means: Many opportunities for regulation. Plenty of intermediates (many of which are useful precursors for making cell components). 10 steps in glycolysis (sorry!). Each step is catalyzed by its own enzyme. Generate 2 ATPs for each glucose consumed. Why so many steps?

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Did I mention that glycolysis is regulated? This makes sense, it would have to be. Regulation is accomplished by slowing down or speeding up some of the steps, by inhibiting or activating the enzymes involved.

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Why study glycolysis? Almost all cells and almost all living things use glycolysis to make ATP. It’s the first complex metabolic pathway to be well understood. Mechanism of glycolysis was figured out between 1860 and 1940! 1860 - Pasteur figures out that microbes are responsible for fermentation. 1897 - Eduard Buchner found that extracts of cells can cause fermention. 1912 to 1940 - Otto Meyerhof (and many others) worked out the details of the glycolysis pathway (using yeast extracts, and good analytical chemistry to detect reaction intermediates). Meyerhof got 1922 Nobel in medicine for his work.

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A few words about glucose. Glucose is a carbohydrate.

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Hayworth projections of glucose: Open form alpha anomer beta anomer “Fischer projection” of glucose: Glucose is mostly “alpha” and “beta”, with just a little “open form”.

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Where do we get glucose for glycolysis ? Carbohydrates in our diet are hydrolyzed to glucose.

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What to do with other sugars that we may eat, that are not simply glucose? Lactase cleaves to glucose and galactose; galactose can be converted to glucose-6-phosphate (in 2 enzyme catalyzed steps).

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In many people, lactase production in the intestine decreases after childhood, so they can’t digest or use lactose. (their intenstinal bacteria can use the lactose) A mutation in chromosome 2 (which some of us have) delays the shutdown of lactase production.

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This is an example of recent human evolution. Mutant humans are able to consume milk products in adulthood. The mutation appears to have occurred independently in several population groups. The ability to digest lactose in adulthood is gradually entering the human population (99% of the Basque population can digest lactose, but less than 5% of Native Americans). The appearance of the mutation paralleled the domestication of milk producing animals. A big survival advantage to those who can digest milk in adulthood !

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Chapter 9. “Overview of metabolism” <==== today Chapter 10. “Glucose metabolism” Next exam - Chapters 6 to 10 Where we are going.

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